Re: [FFmpeg-devel] [PATCH 1/5] avcodec/pngdec: Check last AVFrame before deref

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

Michael Niedermayer:
> Fixes: NULL pointer dereference
> Fixes: 68184/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_APNG_fuzzer-4926478069334016
> 
> Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
> Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
> ---
>  libavcodec/pngdec.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/libavcodec/pngdec.c b/libavcodec/pngdec.c
> index f7751223b81..b24bfa248dc 100644
> --- a/libavcodec/pngdec.c
> +++ b/libavcodec/pngdec.c
> @@ -1218,7 +1218,7 @@ static int decode_fctl_chunk(AVCodecContext *avctx, PNGDecContext *s,
>          return AVERROR_INVALIDDATA;
>      }
>  
> -    if ((sequence_number == 0 || !s->last_picture.f->data[0]) &&
> +    if ((sequence_number == 0 || !s->last_picture.f || !s->last_picture.f->data[0]) &&
>          dispose_op == APNG_DISPOSE_OP_PREVIOUS) {
>          // No previous frame to revert to for the first frame
>          // Spec says to just treat it as a APNG_DISPOSE_OP_BACKGROUND

Just checking for !s->last_picture.f is enough -- s->last_picture.f is
set if and only if s->last_picture.f->data[0] is set.

- Andreas

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