On Fri, Jul 24, 2020 at 08:31:04PM +0200, Alexander Strasser wrote: > Hi Michael! > > On 2020-07-24 00:11 +0200, Michael Niedermayer wrote: > > Fixes: shift exponent 128 is too large for 32-bit type 'int' > > Fixes: > > 23860/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ALAC_fuzzer-5751138914402304 > > > > Found-by: continuous fuzzing process > > https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg > > Signed-off-by: Michael Niedermayer <mich...@niedermayer.cc> > > --- > > libavcodec/alac.c | 3 +++ > > 1 file changed, 3 insertions(+) > > > > diff --git a/libavcodec/alac.c b/libavcodec/alac.c > > index bf05406230..9040673528 100644 > > --- a/libavcodec/alac.c > > +++ b/libavcodec/alac.c > > @@ -302,6 +302,9 @@ static int decode_element(AVCodecContext *avctx, > > AVFrame *frame, int ch_index, > > decorr_shift = get_bits(&alac->gb, 8); > > decorr_left_weight = get_bits(&alac->gb, 8); > > > > + if (channels == 2 && decorr_left_weight && decorr_shift > 31) > > + return AVERROR_INVALIDDATA; > > IMHO this check looks a bit specific in this patch. If I read the > calling function and the conditions that guard the called function > where the problem was detected it makes sense. > > A little elaboration in the commit message body would have made this > change easier to agree on without checking the surroundings: >
> Later the decorrelate_stereo call is guarded by channels == 2 > and non-zero decorr_left_weight. Make sure decorr_shift is in > the expected shift range for that case. will add this > > I started thinking about the robustness of this fix because of > the channels == 2. Thinking if channel will end up somehow not > being 2, would that circumvent the check? As I found out no, > since decorrelate_stereo would not be called. > > Leaving the patch as is, seems fine to me. decode_element will > only ever be called with values 1 or 2 and the function is so > deep down in the decoder that it seems not justified to make it > more flexible. will apply thanks [...] -- Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB The misfortune of the wise is better than the prosperity of the fool. -- Epicurus
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