[FFmpeg-devel] [PATCH] avcodec/flac_parser: Do not loose header count in find_headers_search()

[Michael Niedermayer]

On Tue, Feb 04, 2020 at 06:34:47PM +0100, Michael Niedermayer wrote:
> On Tue, Feb 04, 2020 at 11:10:00AM +0000, Andreas Rheinhardt wrote:
> > Michael Niedermayer:
> > > Fixes: Timeout
> > > Fixes: out of array access
> > > Fixes: 20274/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FLAC_fuzzer-5649631988154368
> > > Fixes: 19275/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FLAC_fuzzer-5757535722405888
> > > 
> > > Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
> > > Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
> > > ---
> > >  libavcodec/flac_parser.c | 12 ++++++++----
> > >  1 file changed, 8 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/libavcodec/flac_parser.c b/libavcodec/flac_parser.c
> > > index 9ffa288548..3424583c49 100644
> > > --- a/libavcodec/flac_parser.c
> > > +++ b/libavcodec/flac_parser.c
> > > @@ -208,16 +208,20 @@ static int find_headers_search(FLACParseContext *fpc, uint8_t *buf,
> > >      uint32_t x;
> > >  
> > >      for (i = 0; i < mod_offset; i++) {
> > > -        if ((AV_RB16(buf + i) & 0xFFFE) == 0xFFF8)
> > > -            size = find_headers_search_validate(fpc, search_start + i);
> > > +        if ((AV_RB16(buf + i) & 0xFFFE) == 0xFFF8) {
> > > +            int ret = find_headers_search_validate(fpc, search_start + i);
> > > +            size = FFMAX(size, ret);
> > > +        }
> > >      }
> > >  
> > >      for (; i < buf_size - 1; i += 4) {
> > >          x = AV_RN32(buf + i);
> > >          if (((x & ~(x + 0x01010101)) & 0x80808080)) {
> > >              for (j = 0; j < 4; j++) {
> > > -                if ((AV_RB16(buf + i + j) & 0xFFFE) == 0xFFF8)
> > > -                    size = find_headers_search_validate(fpc, search_start + i + j);
> > > +                if ((AV_RB16(buf + i + j) & 0xFFFE) == 0xFFF8) {
> > > +                    int ret = find_headers_search_validate(fpc, search_start + i + j);
> > > +                    size = FFMAX(size, ret);
> > > +                }
> > >              }
> > >          }
> > >      }
> > > 
> > 1. Actually, find_headers_search_validate() can return an error and
> > these FFMAX (as well as the FFMAX already present in
> > find_new_headers()) will make sure that they are overwritten.
> 
> yes
> 
> 
> > 2. The linked list of buffered headers is currently traversed pretty
> > often: E.g. if it seems that no new headers have been found in a call
> > to find_new_headers(), the linked list will be traversed again in
> > order to count the number of buffered headers. Maybe one should
> > instead directly increment fpc->nb_headers_buffered after a new header
> > has been added to the list in find_headers_search_validate() and only
> > use the return value of find_headers_search_validate(),
> > find_headers_search(), find_new_headers() to forward the allocation
> > failure errors? (If one stores a pointer to the end of the linked
> > list, one can also remove the while-loop in
> > find_headers_search_validate(); don't know if this helps with the
> > timeout.)
> 
> These are good ideas. The problem iam facing a bit here is that i
> need to (or rather i want to) fix this in the releases too, so one
> goal is to keep the change simple as that makes it more likely to
> backport without too much issues.
> We can clean up the code afterwards and in fact there is probably
> more that can be cleaned up then what you mention (just by gut feeling
> from this code)

any objections ?
if not i will push this in a few days

thx


[...]

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Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

If you fake or manipulate statistics in a paper in physics you will never
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