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CVE-2014-7817: wordexp fails to honour WRDE_NOCMD.

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The function wordexp() fails to properly handle the WRDE_NOCMD
flag when processing arithmetic inputs in the form of "$((... ``))"
where "..." can be anything valid. The backticks in the arithmetic
epxression are evaluated by in a shell even if WRDE_NOCMD forbade
command substitution. This allows an attacker to attempt to pass
dangerous commands via constructs of the above form, and bypass
the WRDE_NOCMD flag. This patch fixes this by checking for WRDE_NOCMD
in exec_comm(), the only place that can execute a shell. All other
checks for WRDE_NOCMD are superfluous and removed.

We expand the testsuite and add 3 new regression tests of roughly
the same form but with a couple of nested levels.

On top of the 3 new tests we add fork validation to the WRDE_NOCMD
testing. If any forks are detected during the execution of a wordexp()
call with WRDE_NOCMD, the test is marked as failed. This is slightly
heuristic since vfork might be used in the future, but it provides a
higher level of assurance that no shells were executed as part of
command substitution with WRDE_NOCMD in effect. In addition it doesn't
require libpthread or libdl, instead we use the public implementation
namespace function __register_atfork (already part of the public ABI
for libpthread).

Tested on x86_64 with no regressions.
This commit is contained in:
Carlos O'Donell
2014-11-19 11:44:12 -05:00
parent 130ac68ca2
commit a39208bd7f
4 changed files with 77 additions and 13 deletions
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44
posix/wordexp-test.c
View File

@ -27,6 +27,25 @@
#define IFS " \n\t"
extern void *__dso_handle __attribute__ ((__weak__, __visibility__ ("hidden")));
extern int __register_atfork (void (*) (void), void (*) (void), void (*) (void), void *);
static int __app_register_atfork (void (*prepare) (void), void (*parent) (void), void (*child) (void))
{
return __register_atfork (prepare, parent, child,
&__dso_handle == NULL ? NULL : __dso_handle);
}
/* Number of forks seen. */
static int registered_forks;
/* For each fork increment the fork count. */
static void
register_fork (void)
{
registered_forks++;
}
struct test_case_struct
{
int retval;
@ -206,6 +225,12 @@ struct test_case_struct
{ WRDE_SYNTAX, NULL, "$((2+))", 0, 0, { NULL, }, IFS },
{ WRDE_SYNTAX, NULL, "`", 0, 0, { NULL, }, IFS },
{ WRDE_SYNTAX, NULL, "$((010+4+))", 0, 0, { NULL }, IFS },
/* Test for CVE-2014-7817. We test 3 combinations of command
substitution inside an arithmetic expression to make sure that
no commands are executed and error is returned. */
{ WRDE_CMDSUB, NULL, "$((`echo 1`))", WRDE_NOCMD, 0, { NULL, }, IFS },
{ WRDE_CMDSUB, NULL, "$((1+`echo 1`))", WRDE_NOCMD, 0, { NULL, }, IFS },
{ WRDE_CMDSUB, NULL, "$((1+$((`echo 1`))))", WRDE_NOCMD, 0, { NULL, }, IFS },
{ -1, NULL, NULL, 0, 0, { NULL, }, IFS },
};
@ -258,6 +283,15 @@ main (int argc, char *argv[])
return -1;
}
/* If we are not allowed to do command substitution, we install
fork handlers to verify that no forks happened. No forks should
happen at all if command substitution is disabled. */
if (__app_register_atfork (register_fork, NULL, NULL) != 0)
{
printf ("Failed to register fork handler.\n");
return -1;
}
for (test = 0; test_case[test].retval != -1; test++)
if (testit (&test_case[test]))
++fail;
@ -367,6 +401,9 @@ testit (struct test_case_struct *tc)
printf ("Test %d (%s): ", ++tests, tc->words);
if (tc->flags & WRDE_NOCMD)
registered_forks = 0;
if (tc->flags & WRDE_APPEND)
{
/* initial wordexp() call, to be appended to */
@ -378,6 +415,13 @@ testit (struct test_case_struct *tc)
}
retval = wordexp (tc->words, &we, tc->flags);
if ((tc->flags & WRDE_NOCMD)
&& (registered_forks > 0))
{
printf ("FAILED fork called for WRDE_NOCMD\n");
return 1;
}
if (tc->flags & WRDE_DOOFFS)
start_offs = sav_we.we_offs;