Re: [FFmpeg-devel] [PATCH] avcodec/cbs_h2645: Check NAL space

[Michael Niedermayer <michael@niedermayer.cc>]

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On Mon, Apr 22, 2024 at 05:46:10PM -0300, James Almer wrote:
> On 4/22/2024 5:40 PM, Mark Thompson wrote:
> > On 22/04/2024 02:31, Michael Niedermayer wrote:
> > > Found-by-reviewing: CID1419833 Untrusted loop bound
> > > 
> > > Sponsored-by: Sovereign Tech Fund
> > > Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
> > > ---
> > >   libavcodec/cbs_h2645.c | 4 ++++
> > >   1 file changed, 4 insertions(+)
> > > 
> > > diff --git a/libavcodec/cbs_h2645.c b/libavcodec/cbs_h2645.c
> > > index fe2e383ff33..1a45d424bae 100644
> > > --- a/libavcodec/cbs_h2645.c
> > > +++ b/libavcodec/cbs_h2645.c
> > > @@ -709,7 +709,11 @@ static int cbs_h2645_split_fragment(CodedBitstreamContext *ctx,
> > >               start = bytestream2_tell(&gbc);
> > >               for(i = 0; i < num_nalus; i++) {
> > > +                if (bytestream2_get_bytes_left(&gbc) < 2)
> > > +                    return AVERROR_INVALIDDATA;
> > >                   size = bytestream2_get_be16(&gbc);
> > > +                if (bytestream2_get_bytes_left(&gbc) < size)
> > > +                    return AVERROR_INVALIDDATA;
> > >                   bytestream2_skip(&gbc, size);
> > >               }
> > >               end = bytestream2_tell(&gbc);
> > 
> > Seems fair.
> > 
> > The problem looks more general with missing bounds checks in all the H.266 code around this, though?  Compare with H.26[45], which have checks on all the reads - seems like H.266 should be doing that.
> > 
> > Thanks,
> 
> Not against this approach, but since the bytestream2_get_* functions return
> 0, never overread the buffer or move the internal pointer, wouldn't it be
> enough to just ensure end > start?
> Particularly in ff_h2645_packet_split(), we can return an error if length
> (in this case being set to end - start) is < 4.

The patch adds the same kind of check as are used in the AV_CODEC_ID_HEVC
case earlier in the file already

I think what is done should approximately stay in sync

thx

[...]
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Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

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