[PATCH] Avoid accessing corrupted stack from __stack_chk_fail [BZ #21752]

[Carlos O'Donell]

On 07/24/2017 08:50 AM, H.J. Lu wrote:
> On Wed, Jul 19, 2017 at 11:50 AM, H.J. Lu <hongjiu.lu@intel.com> wrote:
>> __libc_argv[0] points to address on stack and __libc_secure_getenv
>> accesses environment variables which are on stack.  We should avoid
>> accessing stack when stack is corrupted.
>>
>> This patch also renames function argument in __fortify_fail_abort
>> from do_backtrace to need_backtrace to avoid confusion with do_backtrace
>> from enum __libc_message_action.
>>
>> Tested on x86-64 and i686.  OK for master?
>>
>> H.J.
>> ---
>>         [BZ #21752]
>>         * debug/fortify_fail.c (__fortify_fail_abort): Don't pass down
>>         __libc_argv[0] if we aren't doing backtrace.  Rename do_backtrace
>>         to need_backtrace.
>>         * sysdeps/posix/libc_fatal.c (__libc_message): Don't call
>>         __libc_secure_getenv if we aren't doing backtrace.
>> ---
>>  debug/fortify_fail.c       | 12 ++++++++----
>>  sysdeps/posix/libc_fatal.c | 15 ++++++++++-----
>>  2 files changed, 18 insertions(+), 9 deletions(-)
>>
>> diff --git a/debug/fortify_fail.c b/debug/fortify_fail.c
>> index c90d384daf..a0777ae570 100644
>> --- a/debug/fortify_fail.c
>> +++ b/debug/fortify_fail.c
>> @@ -24,13 +24,17 @@ extern char **__libc_argv attribute_hidden;
>>
>>  void
>>  __attribute__ ((noreturn)) internal_function
>> -__fortify_fail_abort (_Bool do_backtrace, const char *msg)
>> +__fortify_fail_abort (_Bool need_backtrace, const char *msg)
>>  {
>> -  /* The loop is added only to keep gcc happy.  */
>> +  /* The loop is added only to keep gcc happy.  Don't pass down
>> +     __libc_argv[0] if we aren't doing backtrace since __libc_argv[0]
>> +     may point to the corrupted stack.  */
>>    while (1)
>> -    __libc_message (do_backtrace ? (do_abort | do_backtrace) : do_abort,
>> +    __libc_message (need_backtrace ? (do_abort | do_backtrace) : do_abort,
>>                     "*** %s ***: %s terminated\n",
>> -                   msg, __libc_argv[0] ?: "<unknown>");
>> +                   msg,
>> +                   (need_backtrace && __libc_argv[0] != NULL
>> +                    ? __libc_argv[0] : "<unknown>"));
>>  }
>>
>>  void
>> diff --git a/sysdeps/posix/libc_fatal.c b/sysdeps/posix/libc_fatal.c
>> index 25af8bd413..c9189194dd 100644
>> --- a/sysdeps/posix/libc_fatal.c
>> +++ b/sysdeps/posix/libc_fatal.c
>> @@ -75,11 +75,16 @@ __libc_message (enum __libc_message_action action, const char *fmt, ...)
>>    FATAL_PREPARE;
>>  #endif
>>
>> -  /* Open a descriptor for /dev/tty unless the user explicitly
>> -     requests errors on standard error.  */
>> -  const char *on_2 = __libc_secure_getenv ("LIBC_FATAL_STDERR_");
>> -  if (on_2 == NULL || *on_2 == '\0')
>> -    fd = open_not_cancel_2 (_PATH_TTY, O_RDWR | O_NOCTTY | O_NDELAY);
>> +  /* Don't call __libc_secure_getenv if we aren't doing backtrace, which
>> +     may access the corrupted stack.  */
>> +  if ((action & do_backtrace))
>> +    {
>> +      /* Open a descriptor for /dev/tty unless the user explicitly
>> +        requests errors on standard error.  */
>> +      const char *on_2 = __libc_secure_getenv ("LIBC_FATAL_STDERR_");
>> +      if (on_2 == NULL || *on_2 == '\0')
>> +       fd = open_not_cancel_2 (_PATH_TTY, O_RDWR | O_NOCTTY | O_NDELAY);
>> +    }
>>
>>    if (fd == -1)
>>      fd = STDERR_FILENO;
>> --
>> 2.13.3
>>
> 
> Any comments, objections?
> 
> Glibc should avoid accessing corrupted stack from __fortify_fail_abort,
> which somewhat defeats the purpose of -fstack-protector.

I agree. This looks good to me.

-- 
Cheers,
Carlos.