[FFmpeg-devel] [PATCH] avcodec/webp: Reinitilaize VP8 decoder on pixel format mismatch

[Michael Niedermayer]

On Wed, May 10, 2017 at 08:10:23AM -0400, Ronald S. Bultje wrote:
> Hi,
> 
> On Tue, May 9, 2017 at 9:24 PM, Michael Niedermayer <michael at niedermayer.cc>
> wrote:
> 
> > On Tue, May 09, 2017 at 09:08:08PM -0400, Ronald S. Bultje wrote:
> > > Hi,
> > >
> > > On Tue, May 9, 2017 at 8:37 PM, Michael Niedermayer
> > <michael at niedermayer.cc>
> > > wrote:
> > >
> > > > Fixes: out of array access
> > > > Fixes: 1434/clusterfuzz-testcase-minimized-6314998085189632
> > > > Fixes: 1435/clusterfuzz-testcase-minimized-6483783723253760
> > > >
> > > > Found-by: continuous fuzzing process https://github.com/google/oss-
> > > > fuzz/tree/master/targets/ffmpeg
> > > > Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
> > > > ---
> > > >  libavcodec/webp.c | 9 +++++++--
> > > >  1 file changed, 7 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/libavcodec/webp.c b/libavcodec/webp.c
> > > > index 16c3ae2662..23ed4bc26f 100644
> > > > --- a/libavcodec/webp.c
> > > > +++ b/libavcodec/webp.c
> > > > @@ -1330,12 +1330,17 @@ static int vp8_lossy_decode_frame(
> > AVCodecContext
> > > > *avctx, AVFrame *p,
> > > >      WebPContext *s = avctx->priv_data;
> > > >      AVPacket pkt;
> > > >      int ret;
> > > > +    enum AVPixelFormat wanted_pix_fmt = s->has_alpha ?
> > > > AV_PIX_FMT_YUVA420P : AV_PIX_FMT_YUV420P;
> > > > +
> > > > +    if (s->initialized && wanted_pix_fmt != avctx->pix_fmt) {
> > > > +        ff_vp8_decode_free(avctx);
> > > > +        s->initialized = 0;
> > > > +    }
> > > >
> > > >      if (!s->initialized) {
> > > >          ff_vp8_decode_init(avctx);
> > > >          s->initialized = 1;
> > > > -        if (s->has_alpha)
> > > > -            avctx->pix_fmt = AV_PIX_FMT_YUVA420P;
> > > > +        avctx->pix_fmt = wanted_pix_fmt;
> > > >      }
> > > >      s->lossless = 0;
> > >
> > >
> > > What is the out of array access? webp is intra only and the only thing
> > that
> > > is initialized with memory in that call is reference frames. What's going
> > > on here?
> >
> > webp uses the same context as VP8, and it changes the pixel format
> > as it needs. Vp8 doesnt work if its format is changed under its feet
> 
> 
> I think what you're trying to say is that it doesn't work with RGBA as
> pixel format (it shouldn't need a reset between yuv420p and yuv420pa). We
> indeed don't set pix_fmt if has_alpha = 0, which seems like the core of the
> issue. I'm not sure you need to call ff_vp8_decode_init() twice, in fact
> I'm pretty sure you don't have to. It may also help to assert that pix_fmt
> is yuv420p(a) when calling vp8 functions.

The reason why i replied privatly is because the details of security
issues should not be discussed in public before the fixes are available
in releases. As these details are usefull not only to us understanding
an issue but also to an attacker understanding and expoiting it

Ill reply privatly again to awnser your questions

thx

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

During times of universal deceit, telling the truth becomes a
revolutionary act. -- George Orwell
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