Re: [FFmpeg-devel] [PATCH 1/4] avcodec/jpeg2000htdec: Check M_b / magp before using it in a shift

From: Michael Niedermayer <michael@niedermayer.cc>
To: FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>
Subject: Re: [FFmpeg-devel] [PATCH 1/4] avcodec/jpeg2000htdec: Check M_b / magp before using it in a shift
Date: Wed, 20 Mar 2024 14:12:16 +0100
Message-ID: <20240320131216.GR6420@pb2> (raw)
In-Reply-To: <092a8cd44e3074e2d0462052e715281c08eb31b9.camel@haerdin.se>

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On Wed, Mar 20, 2024 at 12:20:11PM +0100, Tomas Härdin wrote:
> ons 2024-03-20 klockan 03:59 +0100 skrev Michael Niedermayer:
> > Fixes: shift exponent -1 is negative
> > Fixes: 65378/clusterfuzz-testcase-minimized-
> > ffmpeg_AV_CODEC_ID_JPEG2000_fuzzer-5457678193197056
> > 
> > Found-by: continuous fuzzing process
> > https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
> > Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
> > ---
> >  libavcodec/jpeg2000htdec.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/libavcodec/jpeg2000htdec.c b/libavcodec/jpeg2000htdec.c
> > index 6b9898d3ff..0b94bb5da2 100644
> > --- a/libavcodec/jpeg2000htdec.c
> > +++ b/libavcodec/jpeg2000htdec.c
> > @@ -1193,6 +1193,9 @@ ff_jpeg2000_decode_htj2k(const
> > Jpeg2000DecoderContext *s, Jpeg2000CodingStyle *c
> >  
> >      int32_t M_b = magp;
> >  
> > +    if (magp >= 31)
> > +        return AVERROR_INVALIDDATA;
> 
> This isn't where the error is, assuming it even is an error. It's
> either expn or nguardbits that are wrong, and they should be detected
> and reported as such in jpeg2000dec.c. Checking this in every call to
> ff_jpeg2000_decode_htj2k() is wasteful.
> 
> nguardbits can be 0..7 and expn can be 0..31. Table A.11 indicates that
> Ssize can be up to 38 bits, so M_b >= 31 is in fact perfectly valid.

> A
> more appropriate error might be AVERROR_PATCHWELCOME.

indeed, i will change it to AVERROR_PATCHWELCOME

thx

[...]
-- 
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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