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

["Tomas Härdin" <git@haerdin.se>]

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.

/Tomas

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