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misc: Fix rare fortify crash on wchar funcs. [BZ 29030] 

If `__glibc_objsize (__o) == (size_t) -1` (i.e. `__o` is unknown size), fortify
checks should pass, and `__whatever_alias` should be called.

Previously, `__glibc_objsize (__o) == (size_t) -1` was explicitly checked, but
on commit a643f60c53, this was moved into `__glibc_safe_or_unknown_len`.

A comment says the -1 case should work as: "The -1 check is redundant because
since it implies that __glibc_safe_len_cond is true.". But this fails when:
* `__s > 1`
* `__osz == -1` (i.e. unknown size at compile time)
* `__l` is big enough
* `__l * __s <= __osz` can be folded to a constant
(I only found this to be true for `mbsrtowcs` and other functions in wchar2.h)

In this case `__l * __s <= __osz` is false, and `__whatever_chk_warn` will be
called by `__glibc_fortify` or `__glibc_fortify_n` and crash the program.

This commit adds the explicit `__osz == -1` check again.
moc crashes on startup due to this, see: https://bugs.archlinux.org/task/74041

Minimal test case (test.c):
    #include <wchar.h>

    int main (void)
    {
        const char *hw = "HelloWorld";
        mbsrtowcs (NULL, &hw, (size_t)-1, NULL);
        return 0;
    }

Build with:
    gcc -O2 -Wp,-D_FORTIFY_SOURCE=2 test.c -o test && ./test

Output:
    *** buffer overflow detected ***: terminated

Fixes: BZ #29030
Signed-off-by: Joan Bruguera <joanbrugueram@gmail.com>
Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
This commit is contained in:
Joan Bruguera 2022-04-11 19:49:56 +02:00 committed by Siddhesh Poyarekar
parent 693517b922
commit 33e03f9cd2
2 changed files with 11 additions and 6 deletions
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5
debug/tst-fortify.c
View File

@ -1504,6 +1504,11 @@ do_test (void)
CHK_FAIL_END
#endif
/* Bug 29030 regresion check */
cp = "HelloWorld";
if (mbsrtowcs (NULL, &cp, (size_t)-1, &s) != 10)
FAIL ();
cp = "A";
if (mbstowcs (wenough, cp, 10) != 1
|| wcscmp (wenough, L"A") != 0)

12
misc/sys/cdefs.h
View File

@ -162,13 +162,13 @@
|| (__builtin_constant_p (__l) && (__l) > 0))
/* Length is known to be safe at compile time if the __L * __S <= __OBJSZ
condition can be folded to a constant and if it is true. The -1 check is
redundant because since it implies that __glibc_safe_len_cond is true. */
condition can be folded to a constant and if it is true, or unknown (-1) */
#define __glibc_safe_or_unknown_len(__l, __s, __osz) \
(__glibc_unsigned_or_positive (__l) \
&& __builtin_constant_p (__glibc_safe_len_cond ((__SIZE_TYPE__) (__l), \
__s, __osz)) \
&& __glibc_safe_len_cond ((__SIZE_TYPE__) (__l), __s, __osz))
((__osz) == (__SIZE_TYPE__) -1 \
|| (__glibc_unsigned_or_positive (__l) \
&& __builtin_constant_p (__glibc_safe_len_cond ((__SIZE_TYPE__) (__l), \
(__s), (__osz))) \
&& __glibc_safe_len_cond ((__SIZE_TYPE__) (__l), (__s), (__osz))))
/* Conversely, we know at compile time that the length is unsafe if the
__L * __S <= __OBJSZ condition can be folded to a constant and if it is