Re: [FFmpeg-devel] [PATCH] avcodec/vp8: Maintain consistency of frame pointers

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

Andreas Rheinhardt:
> Right now it is possible for the pointer for the current frame to
> be set in the context even when it could not be properly set up;
> this does not influence various the ordinary ref frames, but only
> VP8Context.prev_frame. And since this code has been ported to the
> ProgressFrame API in d48d7bc434f30dfbdf346f16715e4f2044b3e000,
> this leads to segfaults, because the ProgressFrame API is less
> forgiving than the ThreadFrame API (waiting on an uninitialized
> ProgressFrame segfaults, waiting on an uninitialized ThreadFrame
> is a no-op (the code behaves as if frame-threading is not in use)).
> 
> Fix this by maintaining the consistency of the frame pointers
> in the context (by setting them later).
> 
> Fixes: NULL pointer dereference
> Fixes: 68192/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_VP8_fuzzer-6180311026171904
> 
> Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
> ---
>  libavcodec/vp8.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/libavcodec/vp8.c b/libavcodec/vp8.c
> index d6df018655..8945447eb6 100644
> --- a/libavcodec/vp8.c
> +++ b/libavcodec/vp8.c
> @@ -541,9 +541,12 @@ static int vp7_fade_frame(VP8Context *s, int alpha, int beta)
>  
>          /* preserve the golden frame, write a new previous frame */
>          if (s->framep[VP8_FRAME_GOLDEN] == s->framep[VP8_FRAME_PREVIOUS]) {
> -            s->framep[VP8_FRAME_PREVIOUS] = vp8_find_free_buffer(s);
> -            if ((ret = vp8_alloc_frame(s, s->framep[VP8_FRAME_PREVIOUS], 1)) < 0)
> +            VP8Frame *prev_frame = vp8_find_free_buffer(s);
> +
> +            ret = vp8_alloc_frame(s, prev_frame, 1);
> +            if (ret < 0)
>                  return ret;
> +            s->framep[VP8_FRAME_PREVIOUS] = prev_frame;
>  
>              dst = s->framep[VP8_FRAME_PREVIOUS]->tf.f;
>  
> @@ -2699,7 +2702,7 @@ int vp78_decode_frame(AVCodecContext *avctx, AVFrame *rframe, int *got_frame,
>              &s->frames[i] != s->framep[VP8_FRAME_ALTREF])
>              vp8_release_frame(&s->frames[i]);
>  
> -    curframe = s->framep[VP8_FRAME_CURRENT] = vp8_find_free_buffer(s);
> +    curframe = vp8_find_free_buffer(s);
>  
>      if (!s->colorspace)
>          avctx->colorspace = AVCOL_SPC_BT470BG;
> @@ -2723,6 +2726,7 @@ int vp78_decode_frame(AVCodecContext *avctx, AVFrame *rframe, int *got_frame,
>  
>      if ((ret = vp8_alloc_frame(s, curframe, referenced)) < 0)
>          goto err;
> +    s->framep[VP8_FRAME_CURRENT] = curframe;
>      if (s->keyframe)
>          curframe->tf.f->flags |= AV_FRAME_FLAG_KEY;
>      else

Will apply. Sorry for the delay.

- Andreas

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