[FFmpeg-devel] [PATCH] avcodec/mobiclip: Check quantizer before table setup

[Michael Niedermayer]

On Fri, Sep 11, 2020 at 11:12:33AM +0200, Paul B Mahol wrote:
> On Fri, Sep 11, 2020 at 01:19:59AM +0200, Michael Niedermayer wrote:
> > Fixes: index -1 out of bounds for type 'const uint8_t [6][16]'
> > Fixes: out of array read
> > Fixes: shift exponent -21 is negative
> > Fixes: 25422/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MOBICLIP_fuzzer-5748258226569216
> > Fixes: shift exponent 8039082 is too large for 32-bit type 'int'
> > Fixes: 25430/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MOBICLIP_fuzzer-5698567770210304
> > 
> > Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
> > Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
> > ---
> >  libavcodec/mobiclip.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/libavcodec/mobiclip.c b/libavcodec/mobiclip.c
> > index d147eddbae..13f0edc89d 100644
> > --- a/libavcodec/mobiclip.c
> > +++ b/libavcodec/mobiclip.c
> > @@ -1323,6 +1323,10 @@ static int mobiclip_decode(AVCodecContext *avctx, void *data,
> >          }
> >      } else {
> >          MotionXY *motion = s->motion;
> > +        int quantizer = s->quantizer + get_se_golomb(gb);
> > +
> > +        if (quantizer < 12 || quantizer > 161)
> 
> From where this numbers come?

It comes from the implementation
qtab is int and will overflow beyond 161

and below 12 the shift will be negative here:
s->qtab[1][i] = quant8x8_tab[qx][i] << (qy - 2);


> I think this is too aggressive.


[...]
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