| 1 | From b9b6e3f01655942891bf4c66a2c5e8246cdad7e1 Mon Sep 17 00:00:00 2001 |
| 2 | From: Carlos O'Donell <carlos@redhat.com> |
| 3 | Date: Wed, 19 Nov 2014 11:44:12 -0500 |
| 4 | Subject: [PATCH 1/2] CVE-2014-7817: wordexp fails to honour WRDE_NOCMD. |
| 5 | |
| 6 | The function wordexp() fails to properly handle the WRDE_NOCMD |
| 7 | flag when processing arithmetic inputs in the form of "$((... ``))" |
| 8 | where "..." can be anything valid. The backticks in the arithmetic |
| 9 | epxression are evaluated by in a shell even if WRDE_NOCMD forbade |
| 10 | command substitution. This allows an attacker to attempt to pass |
| 11 | dangerous commands via constructs of the above form, and bypass |
| 12 | the WRDE_NOCMD flag. This patch fixes this by checking for WRDE_NOCMD |
| 13 | in exec_comm(), the only place that can execute a shell. All other |
| 14 | checks for WRDE_NOCMD are superfluous and removed. |
| 15 | |
| 16 | We expand the testsuite and add 3 new regression tests of roughly |
| 17 | the same form but with a couple of nested levels. |
| 18 | |
| 19 | On top of the 3 new tests we add fork validation to the WRDE_NOCMD |
| 20 | testing. If any forks are detected during the execution of a wordexp() |
| 21 | call with WRDE_NOCMD, the test is marked as failed. This is slightly |
| 22 | heuristic since vfork might be used in the future, but it provides a |
| 23 | higher level of assurance that no shells were executed as part of |
| 24 | command substitution with WRDE_NOCMD in effect. In addition it doesn't |
| 25 | require libpthread or libdl, instead we use the public implementation |
| 26 | namespace function __register_atfork (already part of the public ABI |
| 27 | for libpthread). |
| 28 | |
| 29 | Tested on x86_64 with no regressions. |
| 30 | |
| 31 | Conflicts: |
| 32 | ChangeLog |
| 33 | NEWS |
| 34 | --- |
| 35 | ChangeLog | 266 +++++++++++++++++++++++++++++++++++++++++++++++++++ |
| 36 | NEWS | 10 ++ |
| 37 | posix/wordexp-test.c | 44 +++++++++ |
| 38 | posix/wordexp.c | 16 +--- |
| 39 | 4 files changed, 324 insertions(+), 12 deletions(-) |
| 40 | |
| 41 | diff --git a/posix/wordexp-test.c b/posix/wordexp-test.c |
| 42 | index 4957006..bdd65e4 100644 |
| 43 | --- a/posix/wordexp-test.c |
| 44 | +++ b/posix/wordexp-test.c |
| 45 | @@ -27,6 +27,25 @@ |
| 46 | |
| 47 | #define IFS " \n\t" |
| 48 | |
| 49 | +extern void *__dso_handle __attribute__ ((__weak__, __visibility__ ("hidden"))); |
| 50 | +extern int __register_atfork (void (*) (void), void (*) (void), void (*) (void), void *); |
| 51 | + |
| 52 | +static int __app_register_atfork (void (*prepare) (void), void (*parent) (void), void (*child) (void)) |
| 53 | +{ |
| 54 | + return __register_atfork (prepare, parent, child, |
| 55 | + &__dso_handle == NULL ? NULL : __dso_handle); |
| 56 | +} |
| 57 | + |
| 58 | +/* Number of forks seen. */ |
| 59 | +static int registered_forks; |
| 60 | + |
| 61 | +/* For each fork increment the fork count. */ |
| 62 | +static void |
| 63 | +register_fork (void) |
| 64 | +{ |
| 65 | + registered_forks++; |
| 66 | +} |
| 67 | + |
| 68 | struct test_case_struct |
| 69 | { |
| 70 | int retval; |
| 71 | @@ -206,6 +225,12 @@ struct test_case_struct |
| 72 | { WRDE_SYNTAX, NULL, "$((2+))", 0, 0, { NULL, }, IFS }, |
| 73 | { WRDE_SYNTAX, NULL, "`", 0, 0, { NULL, }, IFS }, |
| 74 | { WRDE_SYNTAX, NULL, "$((010+4+))", 0, 0, { NULL }, IFS }, |
| 75 | + /* Test for CVE-2014-7817. We test 3 combinations of command |
| 76 | + substitution inside an arithmetic expression to make sure that |
| 77 | + no commands are executed and error is returned. */ |
| 78 | + { WRDE_CMDSUB, NULL, "$((`echo 1`))", WRDE_NOCMD, 0, { NULL, }, IFS }, |
| 79 | + { WRDE_CMDSUB, NULL, "$((1+`echo 1`))", WRDE_NOCMD, 0, { NULL, }, IFS }, |
| 80 | + { WRDE_CMDSUB, NULL, "$((1+$((`echo 1`))))", WRDE_NOCMD, 0, { NULL, }, IFS }, |
| 81 | |
| 82 | { -1, NULL, NULL, 0, 0, { NULL, }, IFS }, |
| 83 | }; |
| 84 | @@ -258,6 +283,15 @@ main (int argc, char *argv[]) |
| 85 | return -1; |
| 86 | } |
| 87 | |
| 88 | + /* If we are not allowed to do command substitution, we install |
| 89 | + fork handlers to verify that no forks happened. No forks should |
| 90 | + happen at all if command substitution is disabled. */ |
| 91 | + if (__app_register_atfork (register_fork, NULL, NULL) != 0) |
| 92 | + { |
| 93 | + printf ("Failed to register fork handler.\n"); |
| 94 | + return -1; |
| 95 | + } |
| 96 | + |
| 97 | for (test = 0; test_case[test].retval != -1; test++) |
| 98 | if (testit (&test_case[test])) |
| 99 | ++fail; |
| 100 | @@ -367,6 +401,9 @@ testit (struct test_case_struct *tc) |
| 101 | |
| 102 | printf ("Test %d (%s): ", ++tests, tc->words); |
| 103 | |
| 104 | + if (tc->flags & WRDE_NOCMD) |
| 105 | + registered_forks = 0; |
| 106 | + |
| 107 | if (tc->flags & WRDE_APPEND) |
| 108 | { |
| 109 | /* initial wordexp() call, to be appended to */ |
| 110 | @@ -378,6 +415,13 @@ testit (struct test_case_struct *tc) |
| 111 | } |
| 112 | retval = wordexp (tc->words, &we, tc->flags); |
| 113 | |
| 114 | + if ((tc->flags & WRDE_NOCMD) |
| 115 | + && (registered_forks > 0)) |
| 116 | + { |
| 117 | + printf ("FAILED fork called for WRDE_NOCMD\n"); |
| 118 | + return 1; |
| 119 | + } |
| 120 | + |
| 121 | if (tc->flags & WRDE_DOOFFS) |
| 122 | start_offs = sav_we.we_offs; |
| 123 | |
| 124 | diff --git a/posix/wordexp.c b/posix/wordexp.c |
| 125 | index 366ec18..36a1367 100644 |
| 126 | --- a/posix/wordexp.c |
| 127 | +++ b/posix/wordexp.c |
| 128 | @@ -893,6 +893,10 @@ exec_comm (char *comm, char **word, size_t *word_length, size_t *max_length, |
| 129 | pid_t pid; |
| 130 | int noexec = 0; |
| 131 | |
| 132 | + /* Do nothing if command substitution should not succeed. */ |
| 133 | + if (flags & WRDE_NOCMD) |
| 134 | + return WRDE_CMDSUB; |
| 135 | + |
| 136 | /* Don't fork() unless necessary */ |
| 137 | if (!comm || !*comm) |
| 138 | return 0; |
| 139 | @@ -2082,9 +2086,6 @@ parse_dollars (char **word, size_t *word_length, size_t *max_length, |
| 140 | } |
| 141 | } |
| 142 | |
| 143 | - if (flags & WRDE_NOCMD) |
| 144 | - return WRDE_CMDSUB; |
| 145 | - |
| 146 | (*offset) += 2; |
| 147 | return parse_comm (word, word_length, max_length, words, offset, flags, |
| 148 | quoted? NULL : pwordexp, ifs, ifs_white); |
| 149 | @@ -2196,9 +2197,6 @@ parse_dquote (char **word, size_t *word_length, size_t *max_length, |
| 150 | break; |
| 151 | |
| 152 | case '`': |
| 153 | - if (flags & WRDE_NOCMD) |
| 154 | - return WRDE_CMDSUB; |
| 155 | - |
| 156 | ++(*offset); |
| 157 | error = parse_backtick (word, word_length, max_length, words, |
| 158 | offset, flags, NULL, NULL, NULL); |
| 159 | @@ -2357,12 +2355,6 @@ wordexp (const char *words, wordexp_t *pwordexp, int flags) |
| 160 | break; |
| 161 | |
| 162 | case '`': |
| 163 | - if (flags & WRDE_NOCMD) |
| 164 | - { |
| 165 | - error = WRDE_CMDSUB; |
| 166 | - goto do_error; |
| 167 | - } |
| 168 | - |
| 169 | ++words_offset; |
| 170 | error = parse_backtick (&word, &word_length, &max_length, words, |
| 171 | &words_offset, flags, pwordexp, ifs, |
| 172 | -- |
| 173 | 2.2.1 |
| 174 | |