[FFmpeg-devel] [PATCH] avcodec/mobiclip: Check that Motion vectors are within the input frame

[Michael Niedermayer]

On Sat, Oct 03, 2020 at 04:46:37PM +0200, Michael Niedermayer wrote:
> On Sat, Oct 03, 2020 at 10:43:04AM +0200, Paul B Mahol wrote:
> > On Fri, Oct 02, 2020 at 11:07:09PM +0200, Michael Niedermayer wrote:
> > > The MV checks did not consider the width and height of the block, also they
> > > had some off by 1 errors. This resulted in undefined behavior and crashes.
> > > This commit instead errors out on these
> > > 
> > > Fixes: out of array read
> > > Fixes: 26080/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MOBICLIP_fuzzer-5758146355920896
> > > 
> > > 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 | 30 ++++++------------------------
> > >  1 file changed, 6 insertions(+), 24 deletions(-)
> > > 
> > > diff --git a/libavcodec/mobiclip.c b/libavcodec/mobiclip.c
> > > index f890cb2599..a03ef8fb7d 100644
> > > --- a/libavcodec/mobiclip.c
> > > +++ b/libavcodec/mobiclip.c
> > > @@ -1189,14 +1189,14 @@ static int predict_motion(AVCodecContext *avctx,
> > >              dst_linesize = s->pic[s->current_pic]->linesize[i];
> > >              dst = s->pic[s->current_pic]->data[i] + offsetx + offsety * dst_linesize;
> > >  
> > > +            if (offsetx + (mv.x >> 1) < 0 ||
> > > +                offsety + (mv.y >> 1) < 0 ||
> > > +                offsetx + width  + (mv.x + 1 >> 1) > fwidth ||
> > > +                offsety + height + (mv.y + 1 >> 1) > fheight)
> > > +                return AVERROR_INVALIDDATA;
> [...]
> 
> > > @@ -1243,12 +1231,6 @@ static int predict_motion(AVCodecContext *avctx,
> > >                  }
> > >                  break;
> > >              case 3:
> > > -                if (offsety + (mv.y >> 1) < 0 ||
> > > -                    offsety + (mv.y >> 1) >= fheight - 1 ||
> > > -                    offsetx + (mv.x >> 1) < 0 ||
> > > -                    offsetx + (mv.x >> 1) >= fwidth)
> > > -                    return AVERROR_INVALIDDATA;
> > > -
> > 
> > You completely ignore this case, here it need one scanline extra.
> 
> case 3 implies method= 3 implies mv.y is odd which makes 
> (mv.y + 1 >> 1) 1 larger and that makes the test require 1 line more

will apply

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

If a bugfix only changes things apparently unrelated to the bug with no
further explanation, that is a good sign that the bugfix is wrong.
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