Re: [FFmpeg-devel] [PATCH 2/2] avcodec/vvc/refs: Check content_ref in set_pict_type()

[Frank Plowman <post@frankplowman.com>]

On 02/02/2025 21:17, Michael Niedermayer wrote:
> Fixes: 390565846/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_VVC_fuzzer-4990028521996288
> Fixes: Null pointer dereference
> 
> Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
> Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
> ---
>  libavcodec/vvc/refs.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/libavcodec/vvc/refs.c b/libavcodec/vvc/refs.c
> index 486515d06db..1cfca482047 100644
> --- a/libavcodec/vvc/refs.c
> +++ b/libavcodec/vvc/refs.c
> @@ -186,7 +186,7 @@ static void set_pict_type(AVFrame *frame, const VVCContext *s, const VVCFrameCon
>          const CodedBitstreamFragment *current = &s->current_frame;
>          for (int i = 0; i < current->nb_units && !has_b; i++) {
>              const CodedBitstreamUnit *unit = current->units + i;
> -            if (unit->type <= VVC_RSV_IRAP_11) {
> +            if (unit->content_ref && unit->type <= VVC_RSV_IRAP_11) {
>                  const H266RawSliceHeader *rsh = unit->content_ref;
>                  has_inter |= !IS_I(rsh);
>                  has_b     |= IS_B(rsh);

I did a little more sniffing around this.  unit->content and
unit->content_ref are NULL for NAL units with a type code corresponding
with a reserved or unspecified NAL unit type.  Due to the existing
condition on the NAL unit type being a VCL NAL unit type, this means
that unit->type will be in [4..6], which are all reserved.

Perhaps we might want to add a warning message or something similar
letting the user know some data is being skipped, particularly seeing as
we are talking about video data here?  On the other hand, if the
loglevel is set to verbose or above, cbs_read_fragment_content will
produce some log output which eludes to this, although it is a bit
obtuse as codec-specific information is not available there.  In any
case, I agree that adding the extra check on unit->content_ref is correct.

Thank you,
Frank

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