Re: [FFmpeg-devel] [PATCH] avcodec/jpegxl_parser: check remaining data buffer size

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

Kacper Michajłow:
> Fixes use of uninitialized value, reported by MSAN.
> 
> Found by OSS-Fuzz.
> 
> Signed-off-by: Kacper Michajłow <kasper93@gmail.com>
> ---
>  libavcodec/jpegxl_parser.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/libavcodec/jpegxl_parser.c b/libavcodec/jpegxl_parser.c
> index 8c45e1a1b7..8371d78a45 100644
> --- a/libavcodec/jpegxl_parser.c
> +++ b/libavcodec/jpegxl_parser.c
> @@ -504,9 +504,14 @@ static int read_dist_clustering(GetBitContext *gb, JXLEntropyDecoder *dec, JXLDi
>          return 0;
>      }
>  
> +    if (get_bits_left(gb) <= 0)
> +        return AVERROR_BUFFER_TOO_SMALL;
> +
>      if (get_bits1(gb)) {
>          /* simple clustering */
> -        uint32_t nbits = get_bits(gb, 2);
> +        int nbits = get_bits(gb, 2);
> +        if (get_bits_left(gb) < nbits * bundle->num_dist)
> +            return AVERROR_BUFFER_TOO_SMALL;
>          for (int i = 0; i < bundle->num_dist; i++)
>              bundle->cluster_map[i] = get_bitsz(gb, nbits);
>      } else {

Where is the uninitialized value that you are speaking of? When the
implicit checks of the GetBit-API are enabled, the values when
overreading come from reading the padding which is supposed to be
initialized. Is it here?

- Andreas

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