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Fix CVE-2014-7817: wordexp fails to honour WRDE_NOCMD.
Note: Here the ChangeLog and NEWS updates are removed from Carlos's
patch, since they depend on other earlier commits.
From: Carlos O'Donell <carlos@redhat.com>
Date: Wed, 19 Nov 2014 16:44:12 +0000 (-0500)
Subject: CVE-2014-7817: wordexp fails to honour WRDE_NOCMD.
X-Git-Url: https://sourceware.org/git/gitweb.cgi?p=glibc.git;a=commitdiff_plain;h=33ceaf6187b31ea15284ac65131749e1cb68d2ae
CVE-2014-7817: wordexp fails to honour WRDE_NOCMD.
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.
(cherry picked from commit a39208bd7fb76c1b01c127b4c61f9bfd915bfe7c)
---
diff --git a/posix/wordexp-test.c b/posix/wordexp-test.c
index 4957006..bdd65e4 100644
--- a/posix/wordexp-test.c
+++ b/posix/wordexp-test.c
@@ -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;
diff --git a/posix/wordexp.c b/posix/wordexp.c
index b6b65dd..26f3a26 100644
--- a/posix/wordexp.c
+++ b/posix/wordexp.c
@@ -893,6 +893,10 @@ exec_comm (char *comm, char **word, size_t *word_length, size_t *max_length,
pid_t pid;
int noexec = 0;
+ /* Do nothing if command substitution should not succeed. */
+ if (flags & WRDE_NOCMD)
+ return WRDE_CMDSUB;
+
/* Don't fork() unless necessary */
if (!comm || !*comm)
return 0;
@@ -2082,9 +2086,6 @@ parse_dollars (char **word, size_t *word_length, size_t *max_length,
}
}
- if (flags & WRDE_NOCMD)
- return WRDE_CMDSUB;
-
(*offset) += 2;
return parse_comm (word, word_length, max_length, words, offset, flags,
quoted? NULL : pwordexp, ifs, ifs_white);
@@ -2196,9 +2197,6 @@ parse_dquote (char **word, size_t *word_length, size_t *max_length,
break;
case '`':
- if (flags & WRDE_NOCMD)
- return WRDE_CMDSUB;
-
++(*offset);
error = parse_backtick (word, word_length, max_length, words,
offset, flags, NULL, NULL, NULL);
@@ -2357,12 +2355,6 @@ wordexp (const char *words, wordexp_t *pwordexp, int flags)
break;
case '`':
- if (flags & WRDE_NOCMD)
- {
- error = WRDE_CMDSUB;
- goto do_error;
- }
-
++words_offset;
error = parse_backtick (&word, &word_length, &max_length, words,
&words_offset, flags, pwordexp, ifs,
|