Re: [FFmpeg-devel] [PATCH v2] avcodec/hevc/hevcdec: Do not allow slices to depend on failed slices

[Anton Khirnov <anton@khirnov.net>]

Quoting Michael Niedermayer (2024-07-15 16:48:25)
> An alternative would be to leave the context unchanged on failure of hls_slice_header()
> 
> Fixes: out of array access
> Fixes: NULL pointer dereference
> Fixes: 69584/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_HEVC_fuzzer-5931086299856896
> Fixes: 69724/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_HEVC_fuzzer-5104066422702080
> 
> Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
> Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
> ---
>  libavcodec/hevc/hevcdec.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/libavcodec/hevc/hevcdec.c b/libavcodec/hevc/hevcdec.c
> index 0e4b26dad3b..80d59ab1916 100644
> --- a/libavcodec/hevc/hevcdec.c
> +++ b/libavcodec/hevc/hevcdec.c
> @@ -621,6 +621,10 @@ static int hls_slice_header(SliceHeader *sh, const HEVCContext *s, GetBitContext
>  
>          if (pps->dependent_slice_segments_enabled_flag)
>              sh->dependent_slice_segment_flag = get_bits1(gb);
> +        if (sh->dependent_slice_segment_flag && !s->slice_initialized) {
> +            av_log(s->avctx, AV_LOG_ERROR, "dependent slice failed\n");

The new error message seems worse than the old one. A slice is a passive
object, "slice failed" makes no sense.

> @@ -3155,8 +3156,11 @@ static int decode_slice(HEVCContext *s, const H2645NAL *nal, GetBitContext *gb)
>      int ret;
>  
>      ret = hls_slice_header(&s->sh, s, gb);
> -    if (ret < 0)
> +    if (ret < 0) {
> +        //The code is not capable to rewind from an error, the state now is inconsistant so we cannot use it on depandant slices
                                                                                        ^                                ^
                                                                                        e                                e
Also I'd drop everything before the comma, why mention rewinding that is
not implemented and may not be a good idea anyway.

Otherwise patch LGTM.

-- 
Anton Khirnov
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