[FFmpeg-devel] [PATCH 2/2] avcodec/adpcm: Clip predictor for APC

[Michael Niedermayer]

On Fri, Nov 22, 2019 at 07:20:55PM +0800, Limin Wang wrote:
> On Thu, Nov 21, 2019 at 11:26:12PM +0100, Michael Niedermayer wrote:
> > Fixes: signed integer overflow: -2147483648 - 13 cannot be represented in type 'int'
> > Fixes: 18893/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ADPCM_IMA_APC_fuzzer-5630760442920960
> > 
> > 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/adpcm.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/libavcodec/adpcm.c b/libavcodec/adpcm.c
> > index edf7052636..d3919288b1 100644
> > --- a/libavcodec/adpcm.c
> > +++ b/libavcodec/adpcm.c
> > @@ -140,8 +140,8 @@ static av_cold int adpcm_decode_init(AVCodecContext * avctx)
> >          break;
> >      case AV_CODEC_ID_ADPCM_IMA_APC:
> >          if (avctx->extradata && avctx->extradata_size >= 8) {
> > -            c->status[0].predictor = AV_RL32(avctx->extradata);
> > -            c->status[1].predictor = AV_RL32(avctx->extradata + 4);
> > +            c->status[0].predictor = av_clip_intp2(AV_RL32(avctx->extradata    ), 18);
> > +            c->status[1].predictor = av_clip_intp2(AV_RL32(avctx->extradata + 4), 18);
> 
> 18 is 16?

18


> I understand c->status[i].predictor is 16bit.

ADPCMChannelStatus.predictor is an int so 32bit

the place where predictor is used is adpcm_ima_expand_nibble()
there its added to diff 

diff is maximally 32767*15 / 8 = 61438
if now predictor was -70000 then we would get -8562
if predictor was cliped to 16bit (-32768) would produce 28670
thats different. Same for 17bit which is why i choose 18.
Its the smallest that does not change the output in any case.

Also i dont see any cliping of the global header values in
https://wiki.multimedia.cx/index.php/CRYO_APC

thx

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

I have often repented speaking, but never of holding my tongue.
-- Xenocrates
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 181 bytes
Desc: not available
URL: <https://ffmpeg.org/pipermail/ffmpeg-devel/attachments/20191123/7fbb67af/attachment.sig>