| From a39208bd7fb76c1b01c127b4c61f9bfd915bfe7c Mon Sep 17 00:00:00 2001 |
| From: Carlos O'Donell <carlos@redhat.com> |
| Date: Wed, 19 Nov 2014 11:44:12 -0500 |
| Subject: [PATCH 1/1] 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. |
| --- |
| posix/wordexp-test.c | 44 ++++++++++++++++++++++++++++++++++++++++++++ |
| posix/wordexp.c | 16 ++++------------ |
| |
| 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, |
| -- |
| 1.7.1 |
| |