Re: [FFmpeg-devel] [PATCH 1/4] avformat/iamf_parse: Check extradata size

[Michael Niedermayer <michael@niedermayer.cc>]

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On Fri, Jun 20, 2025 at 12:28:13AM +0200, Andreas Rheinhardt wrote:
> Michael Niedermayer:
> > Fixes: Assertion n>=0 && n<=32 failed at ./libavcodec/get_bits.h:406
> > Fixes: 398527871/clusterfuzz-testcase-minimized-ffmpeg_dem_IAMF_fuzzer-6602025714647040
> > 
> > Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
> > Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
> > ---
> >  libavformat/iamf_parse.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/libavformat/iamf_parse.c b/libavformat/iamf_parse.c
> > index 71497876ac3..330e01733dd 100644
> > --- a/libavformat/iamf_parse.c
> > +++ b/libavformat/iamf_parse.c
> > @@ -305,6 +305,8 @@ static int update_extradata(AVCodecParameters *codecpar)
> >          skip_bits(&gb, 4);
> >          put_bits(&pb, 4, codecpar->ch_layout.nb_channels); // set channel config
> >          ret = put_bits_left(&pb);
> > +        if (ret < 0)
> > +            return AVERROR_INVALIDDATA;
> >          while (ret >= 32) {
> >             put_bits32(&pb, get_bits_long(&gb, 32));
> >             ret -= 32;
> 
> There is only one way for put_bits_left() to return a negative value: If
> there is more data in the internal buffer than can be written out. And
> this scenario is already a violation of the PutBit API. Given that the
> size of the internal buffer depends upon the arch, it could be that one
> would have already hit an assert in case one is not using x64. In other
> words, your check is too late.

the patches puprose was mainly to show that
3f9420132441345b7ccd57001f230bb98f655696
was insufficient to fix 398527871

I do not expect my patch would be the correct solution even if the
check is done earlier. IAMF is cursed

thx

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

You can kill me, but you cannot change the truth.

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