[Bug libc/18032] New: buffer overflow (read past end of buffer) in internal_fnmatch

["konstantin.s.serebryany at gmail dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=18032

            Bug ID: 18032
           Summary: buffer overflow (read past end of buffer) in
                    internal_fnmatch
           Product: glibc
           Version: 2.21
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: konstantin.s.serebryany at gmail dot com
                CC: drepper.fsp at gmail dot com

#include <fnmatch.h>
#include <string.h>
int main(int argc, const char* argv[]) {
  const char text[] = {44, 92, 91, 44, 91, 46, 0};
  const char p[] = {91, 44, 91, 46, 0};
  const char *Pat = strdup(p);
  fnmatch(Pat, text, 0);
}

gcc -g fn2.c && valgrind ./a.out # 2.19

==32342== Invalid read of size 1
==32342==    at 0x4EFEF92: internal_fnmatch (fnmatch_loop.c:965)
==32342==    by 0x4EFFF71: fnmatch@@GLIBC_2.2.5 (fnmatch.c:458)
==32342==    by 0x400663: main (fn2.c:7)
==32342==  Address 0x51fc045 is 0 bytes after a block of size 5 alloc'd
==32342==    at 0x4C2ABBD: malloc (vg_replace_malloc.c:296)
==32342==    by 0x4EBF3C9: strdup (strdup.c:42)
==32342==    by 0x400647: main (fn2.c:6)

Reproduced on 2.19 and on fresh trunk; 
initially found with an experimental AddressSanitizer build of glibc and
a coverage guided fuzzer. 

# trunk

==32737==ERROR: AddressSanitizer: heap-buffer-overflow on address
0x60200000eff5 at pc 0x7f0bc10c9588 bp 0x7fff4676fef0 sp 0x7fff4676fee8
READ of size 1 at 0x60200000eff5 thread T0
    #0 0x7f0bc10c9587 in internal_fnmatch posix/./fnmatch_loop.c:951:8
    #1 0x7f0bc10b3014 in __GI_fnmatch posix/fnmatch.c:458:10
    #2 0x4c1e3f in main

0x60200000eff5 is located 0 bytes to the right of 5-byte region
[0x60200000eff0,0x60200000eff5)
allocated by thread T0 here:
    #0 0x48d23c in __interceptor_strdup 
    #1 0x4c1dfe in main


See also https://sourceware.org/bugzilla/show_bug.cgi?id=17062 -- a different
bug somewhere similar.

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