Re: [FFmpeg-devel] [PATCH 3/3] avformat/mxfdec: Reduce overflows in essence_length computation

[Michael Niedermayer <michael@niedermayer.cc>]

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On Wed, Nov 16, 2022 at 12:49:10PM +0100, Tomas Härdin wrote:
> sön 2022-11-13 klockan 00:44 +0100 skrev Michael Niedermayer:
> > Fixes: signed integer overflow: -3741319169 - 9223372036823449370
> > cannot be represented in type 'long'
> > Fixes: 51896/clusterfuzz-testcase-minimized-ffmpeg_dem_MXF_fuzzer-
> > 513039428681728
> > 
> > Found-by: continuous fuzzing process 
> > https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
> > Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
> > ---
> >  libavformat/mxfdec.c | 10 +++++++---
> >  1 file changed, 7 insertions(+), 3 deletions(-)
> > 
> > diff --git a/libavformat/mxfdec.c b/libavformat/mxfdec.c
> > index e6118e141d..42109cb43a 100644
> > --- a/libavformat/mxfdec.c
> > +++ b/libavformat/mxfdec.c
> > @@ -100,7 +100,7 @@ typedef struct MXFPartition {
> >      uint64_t previous_partition;
> >      int index_sid;
> >      int body_sid;
> > -    int64_t this_partition;
> > +    uint64_t this_partition;
> >      int64_t essence_offset;         ///< absolute offset of essence
> >      int64_t essence_length;
> >      int32_t kag_size;
> > @@ -3519,8 +3519,12 @@ static void
> > mxf_compute_essence_containers(AVFormatContext *s)
> >              p->essence_offset = p->first_essence_klv.offset;
> >  
> >              /* essence container spans to the next partition */
> > -            if (x < mxf->partitions_count - 1)
> > -                p->essence_length = mxf-
> > >partitions[x+1].this_partition - p->essence_offset;
> > +            if (x < mxf->partitions_count - 1) {
> > +                if (mxf->partitions[x+1].this_partition < p-
> > >essence_offset) {
> > +                    p->essence_length = -1;
> > +                } else
> > +                    p->essence_length = mxf-
> > >partitions[x+1].this_partition - p->essence_offset;
> > +            }
> 
> A better solution might be to record the actual offset of the
> partitions rather than relying on ThisPartition. Then we can guarantee
> that they are strictly increasing.

do you mean that pack_ofs could be used here ?
(this is within 8 byte of this_partition) for the files i checked
or something else ?

but as pack_ofs is signed that either means dealing with potentially negative
or changing it to uint64_t or adding a new field and avio_tell() somewhere?

thx

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

It is a danger to trust the dream we wish for rather than
the science we have, -- Dr. Kenneth Brown

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