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

["Tomas Härdin" <git@haerdin.se>]

mån 2024-03-25 klockan 21:04 +0100 skrev Michael Niedermayer:
> On Mon, Mar 25, 2024 at 08:13:13PM +0100, Michael Niedermayer wrote:
> > On Thu, Mar 21, 2024 at 04:07:14PM +0100, Tomas Härdin wrote:
> > > ons 2024-03-20 klockan 21:35 +0100 skrev Tomas Härdin:
> > > > ons 2024-03-20 klockan 14:12 +0100 skrev Michael Niedermayer:
> > > > > 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
> > > > 
> > > > Please also move it further up so as to not waste cycles
> > > > checking it
> > > > every time
> > > 
> > > To be more precise, get_qcx() looks like the proper place for it
> > 
> > will apply with teh check moved there
> 
> the values that are causing undefined behavior for htj2k are used in
> normal
> j2k knowing which type of j2k we have seems decided by COC/COD/COX
> 
> so i dont think we can check in QCX, because a later COX could
> make it both invalid or valid
> and we cannot check in COX as a later QCX can similarly change it

That all calls get_qcx(). If you git grep for nguardbits you'll see
it's only ever set there when decoding, and similarly with expn. Coding
style and quantization style are not the same thing.

/Tomas
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