rooting your own phone: android security Feb 08 2009 10:22AM
Pavel Machek (pavel ucw cz)
Hi!

Note that I'm not giving google much advance warning: I mentioned few
of those issues on irc, but they seemed not welcome there. And purpose
of security system of t-mobile g1 is not to protect owner from evil
attacker; rather it prevents owner from using his phone. g1, as
shipped, will not for example allow connecting your notebook to the
internet.

(I have czech version of g1; you can't simply downgrade it to
rc8/rc29, as it is prevented by CID check).

Yes, I want to get root on my shiny new t-mobile g1. I tried
exploiting dnotify hole that was fixed in 2.6.25.1... only to find out
that CONFIG_DNOTIFY is off in g1 kernel. So I made sure that
CONFIG_INOTIFY is on, and tried exploiting
6ee5a399d6a92a52646836a6e10faf255c16393e. It triggers very
reliably... with SLAB debugging on. With debugging off, it took 2+
hours to reproduce on PC. Given that I'd have to manually
insert/remove SD card for each try, that is not an option. I thought
that rooting 2.6.25 would be easy, but it turns out it is lot harder
than I expected.

I discovered few problems (see below), most are security-relevant, but
none of them is enough to root the phone.

If you have any ideas how to get root on 2.6.25 running on ARM, or
where weak spots in android security are, let me know. (Perhaps by
private email).
Pavel

Security problems in dynamic linker
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Hmmm, linker fails to check for setgid, and fails to close descriptors
in such cases. It should be possible to write arbitrary files with gid
3003/3004 permissions.

diff --git a/linker/linker.c b/linker/linker.c
index 8f15f62..5e963b4 100644
--- a/linker/linker.c
+++ b/linker/linker.c
@@ -1563,13 +1563,13 @@ static int link_image(soinfo *si, unsigned wr_offset)
}
#endif

- /* If this is a SETUID programme, dup /dev/null to openned stdin,
+ /* If this is a SET?ID program, dup /dev/null to openned stdin,
stdout and stderr to close a security hole described in:

ftp://ftp.freebsd.org/pub/FreeBSD/CERT/advisories/FreeBSD-SA-02:23.stdio
.asc

*/
- if (getuid() != geteuid())
+ if (getuid() != geteuid() || getgid() != getegid())
nullify_closed_stdio ();
call_constructors(si);
notify_gdb_of_load(si);

Unfortunately, their linker does not support LD_PRELOAD or
LD_LIBRARY_PATH, so nothing to play with there. Interestingly, their
linker they still set it LD_LIBRARY_PATH on system startup.

Integer overflows in *calloc
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

chk_calloc is vulnerable to integer overflows. dlcalloc() _is_
protected. It is controlled by
system_property_get("libc.debug.malloc"). Unfortunately, AFAICT debug
version is not used on production G1.

diff --git a/libc/bionic/malloc_leak.c b/libc/bionic/malloc_leak.c
index 5ddc913..dd42fe3 100644
--- a/libc/bionic/malloc_leak.c
+++ b/libc/bionic/malloc_leak.c
@@ -608,6 +608,7 @@ void chk_free(void* mem)

void* chk_calloc(size_t n_elements, size_t elem_size)
{
+ /* FIXME: fails to check overflow -> security hole */
size_t size = n_elements * elem_size;
void* ptr = chk_malloc(size);
if (ptr != NULL) {
@@ -763,6 +764,7 @@ void leak_free(void* mem)

void* leak_calloc(size_t n_elements, size_t elem_size)
{
+ /* FIXME: fails to check overflow -> security hole */
size_t size = n_elements * elem_size;
void* ptr = leak_malloc(size);
if (ptr != NULL) {

integer overflow in libcutils/strdup8to16()
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

(Header files probably should not be executable.)

diff --git a/adb/history.h b/adb/history.h
old mode 100755
new mode 100644

strdup8to16 contains integer overflow. Unfortunately, strdup8to16 does
not seem to be used in security-relevant context.

diff --git a/libcutils/strdup8to16.c b/libcutils/strdup8to16.c
index 8654b04..13a6430 100644
--- a/libcutils/strdup8to16.c
+++ b/libcutils/strdup8to16.c
@@ -49,6 +49,7 @@ extern char16_t * strdup8to16 (const char* s, size_t *out_len)
len = strlen8to16(s);

// no plus-one here. UTF-16 strings are not null terminated
+ /* Integer overflow here; pass 2.1GB string here and see .... */
ret = (char16_t *) malloc (sizeof(char16_t) * len);

return strcpy8to16 (ret, s, out_len);

Integer overflow in liblog/showLog()
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

showLog() fails to check for integer overflow. If too many lines are
sent to it, it will overflow its buffers. I'm not sure if this code is
used on G1, or if it is only used on emulator.

diff --git a/liblog/fake_log_device.c b/liblog/fake_log_device.c
index d9d67b4..e6775a4 100644
--- a/liblog/fake_log_device.c
+++ b/liblog/fake_log_device.c
@@ -343,6 +343,8 @@ static ssize_t fake_writev(int fd, const struct iovec *iov, int iovcnt) {
*
* Log format parsing taken from the long-dead utils/Log.cpp.
*/
+
+/* If we can call this with too many lines of input, it will buffer overrun its buffers...*/
static void showLog(LogState *state,
int logPrio, const char* tag, const char* msg)
{
@@ -449,6 +451,7 @@ static void showLog(LogState *state,

numLines *= 3; // 3 iovecs per line.
if (numLines > INLINE_VECS) {
+ /* Integer overflow here, use 2G lines to exploit... */
vec = (struct iovec*)malloc(sizeof(struct iovec)*numLines);
if (vec == NULL) {
msg = "LOG: write failed, no memory";
@@ -670,6 +673,7 @@ int fakeLogClose(int fd)
return redirectClose(fd);
}

+/* logWritev is vulnerable to buffer overflow */
ssize_t fakeLogWritev(int fd, const struct iovec* vector, int count)
{
/* Assume that open() was called first. */

(Sidenote; passing arrays by value is probably not good idea).

diff --git a/vold/ProcessKiller.c b/vold/ProcessKiller.c
index eeaae04..b8856ac 100644
--- a/vold/ProcessKiller.c
+++ b/vold/ProcessKiller.c
@@ -66,6 +66,7 @@ static boolean PathMatchesMountPoint(const char* path, const char* mountPoint)
return false;
}

+/* Ouch, this si going to eat a lot of stack. Does it work at all? */
static void GetProcessName(int pid, char buffer[PATH_MAX])
{
int fd;

Buffer overflow in vold
~~~~~~~~~~~~~~~~~~~~~~~

Unfortunately you can only exploit it by putting overrly long
filenames to /dev/block.

diff --git a/vold/inotify.c b/vold/inotify.c
index a7b789c..db0a0f7 100644
--- a/vold/inotify.c
+++ b/vold/inotify.c
@@ -85,6 +85,7 @@ int inotify_bootstrap(void)
if (de->d_name[0] == '.')
continue;

+ /* Filename in /dev/block longer than 250-or-so characters, and boom you go */
sprintf(filename, "%s/%s", DEVPATH, de->d_name);

if (stat(filename, &sbuf) < 0) {

(sidenote: drop_caches does not do what you expect it to. It is debug
interface, and racy. And it takes '3', not 3, to do cache flushing).

static void DropSystemCaches(void)
{
int fd;

LOG_MOUNT("Dropping system caches\n");
fd = open("/proc/sys/vm/drop_caches", O_WRONLY);

if (fd > 0) {
char ch = 3;
int rc;

rc = write(fd, &ch, 1);

--
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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