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

[Michael Niedermayer]

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

thx

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

If you think the mosad wants you dead since a long time then you are either
wrong or dead since a long time.
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