Re: [FFmpeg-devel] [PATCH] avcodec/cbs_h266_syntax_template: Check num_output_layers_in_ols

[James Almer <jamrial@gmail.com>]

On 9/19/2023 12:25 PM, Nuo Mi wrote:
> from the specification:
> For each OLS, there shall be at least one layer that is an output layer. In other words, for any value of i in the range of 0
> to TotalNumOlss − 1, inclusive, the value of NumOutputLayersInOls[ i ] shall be greater than or equal to 1
> 
> Fixes: index 257 out of bounds for type 'uint8_t [257]'
> Fixes: 61160/clusterfuzz-testcase-minimized-ffmpeg_BSF_VVC_METADATA_fuzzer-6709397181825024
> 
> Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
> Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
> ---
>   libavcodec/cbs_h266_syntax_template.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/libavcodec/cbs_h266_syntax_template.c b/libavcodec/cbs_h266_syntax_template.c
> index f0d428a478..2077aa63bf 100644
> --- a/libavcodec/cbs_h266_syntax_template.c
> +++ b/libavcodec/cbs_h266_syntax_template.c
> @@ -891,6 +891,8 @@ static int FUNC(vps) (CodedBitstreamContext *ctx, RWContext *rw,
>                       }
>                   }
>               }
> +            if (!num_output_layers_in_ols[i])

The only way for num_output_layers_in_ols[i] to be less than 1 is if 
none of the checks above succeeded, and since the array is stored on 
stack uninitialized, this to me looks like a potential use of 
uninitialized value.

> +                return AVERROR_INVALIDDATA;
>           }
>           for (i = 1; i < total_num_olss; i++) {
>               int num_layers_in_ols = 0;
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