[Bug string/31509] New: Invalid read in __wcpncpy_avx2() via wcsxfrm()

[Bug string/31509] New: Invalid read in __wcpncpy_avx2() via wcsxfrm()

[slomo at coaxion dot net]

https://sourceware.org/bugzilla/show_bug.cgi?id=31509

            Bug ID: 31509
           Summary: Invalid read in __wcpncpy_avx2() via wcsxfrm()
           Product: glibc
           Version: 2.38
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: string
          Assignee: unassigned at sourceware dot org
          Reporter: slomo at coaxion dot net
  Target Milestone: ---

The following code reliably results in invalid reads when running under
valgrind. This is with glibc 2.38 from Fedora 39.

Whether an invalid read happens or not seems to depend on the alignment of the
input buffer or something else that I couldn't figure out completely yet. In
the code below it always happens on the last call, i.e. with the second
malloc'd buffer.

It seems like __wcpncpy_avx2() wrongly assumes under certain conditions that at
least 32 bytes (i.e. one AVX register) of input data is available.

--------

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <wchar.h>

int main() {

  const wchar_t in[] = {L'a', L'b', L'c', 0};
  wchar_t out[3+1] = {0, };

  printf("%p %p\n", in, out);
  size_t res = wcsxfrm(out, in, 3);
  printf("%lu\n", res);

  wchar_t *in2 = malloc(sizeof(wchar_t) * 4);
  memcpy(in2, in, sizeof(in));
  printf("%p %p\n", in2, out);
  res = wcsxfrm(out, in2, 3);
  printf("%lu\n", res);
  free(in2);

  wchar_t *in3 = malloc(sizeof(wchar_t) * 4);
  memcpy(in3, in, sizeof(in));
  printf("%p %p\n", in3, out);
  res = wcsxfrm(out, in3, 3);
  printf("%lu\n", res);
  free(in3);
}

--------

==139735== Memcheck, a memory error detector
==139735== Copyright (C) 2002-2022, and GNU GPL'd, by Julian Seward et al.
==139735== Using Valgrind-3.22.0 and LibVEX; rerun with -h for copyright info
==139735== Command: ./test
==139735== 
0x1fff000330 0x1fff000320
3
0x4a5c480 0x1fff000320
3
0x4a5c4d0 0x1fff000320
==139735== Invalid read of size 32
==139735==    at 0x49DAA2E: __wcpncpy_avx2 (strncpy-avx2.S:85)
==139735==    by 0x493C560: wcsxfrm_l (strxfrm_l.c:679)
==139735==    by 0x4012A1: main (in /home/slomo/tmp/test/test)
==139735==  Address 0x4a5c4d0 is 0 bytes inside a block of size 16 alloc'd
==139735==    at 0x484280F: malloc (vg_replace_malloc.c:442)
==139735==    by 0x401258: main (in /home/slomo/tmp/test/test)
==139735== 
3
==139735== 
==139735== HEAP SUMMARY:
==139735==     in use at exit: 0 bytes in 0 blocks
==139735==   total heap usage: 3 allocs, 3 frees, 1,056 bytes allocated
==139735== 
==139735== All heap blocks were freed -- no leaks are possible
==139735== 
==139735== For lists of detected and suppressed errors, rerun with: -s
==139735== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)

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[Bug string/31509] Invalid read in __wcpncpy_avx2() via wcsxfrm()

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=31509

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |fweimer at redhat dot com

--- Comment #1 from Florian Weimer <fweimer at redhat dot com> ---
(In reply to Sebastian Dröge from comment #0)
> ==139735== Invalid read of size 32
> ==139735==    at 0x49DAA2E: __wcpncpy_avx2 (strncpy-avx2.S:85)
> ==139735==    by 0x493C560: wcsxfrm_l (strxfrm_l.c:679)
> ==139735==    by 0x4012A1: main (in /home/slomo/tmp/test/test)
> ==139735==  Address 0x4a5c4d0 is 0 bytes inside a block of size 16 alloc'd
> ==139735==    at 0x484280F: malloc (vg_replace_malloc.c:442)
> ==139735==    by 0x401258: main (in /home/slomo/tmp/test/test)
> ==139735== 

This read does not case a page boundary, so overreading is harmless here. This
seems more like a valgrind limitation.

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[Bug string/31509] Invalid read in __wcpncpy_avx2() via wcsxfrm()

[schwab@linux-m68k.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=31509

--- Comment #2 from Andreas Schwab <schwab@linux-m68k.org> ---
Please report that to the valgrind project so that they can add that to their
default suppression list.

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[Bug string/31509] Invalid read in __wcpncpy_avx2() via wcsxfrm()

[slomo at coaxion dot net]

https://sourceware.org/bugzilla/show_bug.cgi?id=31509

--- Comment #3 from Sebastian Dröge <slomo at coaxion dot net> ---
Will do, thanks. OOC, how does the function make sure that it won't read across
a page boundary?

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[Bug string/31509] Invalid read in __wcpncpy_avx2() via wcsxfrm()

[slomo at coaxion dot net]

https://sourceware.org/bugzilla/show_bug.cgi?id=31509

--- Comment #4 from Sebastian Dröge <slomo at coaxion dot net> ---
Reported to valgrind at https://bugs.kde.org/show_bug.cgi?id=484002

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[Bug string/31509] Invalid read in __wcpncpy_avx2() via wcsxfrm()

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=31509

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |MOVED

--- Comment #5 from Florian Weimer <fweimer at redhat dot com> ---
Moved per comment 4.

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[Bug string/31509] Invalid read in __wcpncpy_avx2() via wcsxfrm()

[sam at gentoo dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=31509

Sam James <sam at gentoo dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |sam at gentoo dot org
           See Also|                            |https://bugs.kde.org/show_b
                   |                            |ug.cgi?id=484002

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