Re: [FFmpeg-devel] [PATCH 2/3] avcodec/wmalosslessdec: Check channel mask against num channels

[James Almer <jamrial@gmail.com>]

On 3/17/2022 8:30 PM, Michael Niedermayer wrote:
> Fixes: Out of array write
> Fixes: 45613/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WMALOSSLESS_fuzzer-4539073606320128
> 
> Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
> Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
> ---
>   libavcodec/wmalosslessdec.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/libavcodec/wmalosslessdec.c b/libavcodec/wmalosslessdec.c
> index cd05b22689..1728920729 100644
> --- a/libavcodec/wmalosslessdec.c
> +++ b/libavcodec/wmalosslessdec.c
> @@ -281,6 +281,9 @@ static av_cold int decode_init(AVCodecContext *avctx)
>   
>       av_channel_layout_uninit(&avctx->ch_layout);
>       av_channel_layout_from_mask(&avctx->ch_layout, channel_mask);
> +    if (s->num_channels != avctx->ch_layout.nb_channels)
> +        return AVERROR_PATCHWELCOME; //are there non fuzzed files with this or is it an error ?

s->num_channels at this point is set to the channels count the user set 
before calling avcodec_open2() (Normally from lavf), but it could be 
anything.
If channel_mask is taken from extradata, maybe it should be used to set 
s->num_channels instead of aborting because the user set value and 
extradata disagreed.

Also, can you reproduce this crash before 3c933af493? s->num_channels 
was being set to the user set channel count too, same as now.

> +
>       return 0;
>   }
>   
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