Re: [PATCH RESEND v2 2/2] migration/xbzrle: fix out-of-bounds write with axv512

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Matheus Tavares Bernardino (quic_mathbern@quicinc.com) wrote:
> xbzrle_encode_buffer_avx512() checks for overflows too scarcely in its
> outer loop, causing out-of-bounds writes:
> 
> $ ../configure --target-list=aarch64-softmmu --enable-sanitizers --enable-avx512bw
> $ make tests/unit/test-xbzrle && ./tests/unit/test-xbzrle
> 
> ==5518==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x62100000b100 at pc 0x561109a7714d bp 0x7ffed712a440 sp 0x7ffed712a430
> WRITE of size 1 at 0x62100000b100 thread T0
>     #0 0x561109a7714c in uleb128_encode_small ../util/cutils.c:831
>     #1 0x561109b67f6a in xbzrle_encode_buffer_avx512 ../migration/xbzrle.c:275
>     #2 0x5611099a7428 in test_encode_decode_overflow ../tests/unit/test-xbzrle.c:153
>     #3 0x7fb2fb65a58d  (/lib/x86_64-linux-gnu/libglib-2.0.so.0+0x7a58d)
>     #4 0x7fb2fb65a333  (/lib/x86_64-linux-gnu/libglib-2.0.so.0+0x7a333)
>     #5 0x7fb2fb65aa79 in g_test_run_suite (/lib/x86_64-linux-gnu/libglib-2.0.so.0+0x7aa79)
>     #6 0x7fb2fb65aa94 in g_test_run (/lib/x86_64-linux-gnu/libglib-2.0.so.0+0x7aa94)
>     #7 0x5611099a3a23 in main ../tests/unit/test-xbzrle.c:218
>     #8 0x7fb2fa78c082 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x24082)
>     #9 0x5611099a608d in _start (/qemu/build/tests/unit/test-xbzrle+0x28408d)
> 
> 0x62100000b100 is located 0 bytes to the right of 4096-byte region [0x62100000a100,0x62100000b100)
> allocated by thread T0 here:
>     #0 0x7fb2fb823a06 in __interceptor_calloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cc:153
>     #1 0x7fb2fb637ef0 in g_malloc0 (/lib/x86_64-linux-gnu/libglib-2.0.so.0+0x57ef0)
> 
> Fix that by performing the overflow check in the inner loop, instead.
> 
> Signed-off-by: Matheus Tavares Bernardino <quic_mathbern@quicinc.com>
> ---
>  migration/xbzrle.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/migration/xbzrle.c b/migration/xbzrle.c
> index 21b92d4eae..c6f8b20917 100644
> --- a/migration/xbzrle.c
> +++ b/migration/xbzrle.c
> @@ -197,10 +197,6 @@ int xbzrle_encode_buffer_avx512(uint8_t *old_buf, uint8_t *new_buf, int slen,
>      __m512i r = _mm512_set1_epi32(0);
>  
>      while (count512s) {
> -        if (d + 2 > dlen) {
> -            return -1;
> -        }
> -
>          int bytes_to_check = 64;
>          uint64_t mask = 0xffffffffffffffff;
>          if (count512s == 1) {
> @@ -216,6 +212,9 @@ int xbzrle_encode_buffer_avx512(uint8_t *old_buf, uint8_t *new_buf, int slen,
>  
>          bool is_same = (comp & 0x1);
>          while (bytes_to_check) {
> +            if (d + 2 > dlen) {
> +                return -1;
> +            }

I agree that's better, so:

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>


but is it sufficient?
In that bytes_to_check loop there are 4 calls to uleb128_encode_small
with another one just off the end of the loop.
I've not figured out all the legal combos, but I'm pretty sure at least
a few can trigger in one iteration - so don't we need those checks
before ecah call?

Dave

>              if (is_same) {
>                  if (nzrun_len) {
>                      d += uleb128_encode_small(dst + d, nzrun_len);
> -- 
> 2.39.1
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK