[FFmpeg-devel] [PATCH 2/2] avcodec/h264_direct: Fix runtime error: signed integer overflow: 2147483647 - -14133 cannot be represented in type 'int'

[Michael Niedermayer]

On Mon, Mar 13, 2017 at 10:26:25PM +0100, wm4 wrote:
> On Mon, 13 Mar 2017 16:02:43 +0100
> Michael Niedermayer <michael at niedermayer.cc> wrote:
> 
> > On Mon, Mar 13, 2017 at 11:39:05AM +0100, wm4 wrote:
> > > On Sat, 11 Mar 2017 18:26:48 +0100
> > > Michael Niedermayer <michael at niedermayer.cc> wrote:
> > >   
> > > > On Sat, Mar 11, 2017 at 05:46:31PM +0100, wm4 wrote:  
> > > > > On Sat, 11 Mar 2017 14:50:42 +0100
> > > > > Michael Niedermayer <michael at niedermayer.cc> wrote:
> > > > >     
> > > > > > On Sat, Mar 11, 2017 at 02:04:25PM +0100, wm4 wrote:    
> > > > > > > On Sat, 11 Mar 2017 01:26:33 +0100
> > > > > > > Michael Niedermayer <michael at niedermayer.cc> wrote:
> > > > > > >       
> > > > > > > > On Fri, Mar 10, 2017 at 04:01:36PM +0100, wm4 wrote:      
> > > > > > > > > On Fri, 10 Mar 2017 15:24:52 +0100
> > > > > > > > > Michael Niedermayer <michael at niedermayer.cc> wrote:
> > > > > > > > >         
> > > > > > > > > > Fixes: 755/clusterfuzz-testcase-5369072516595712
> > > > > > > > > > 
> > > > > > > > > > See: [FFmpeg-devel] [PATCH 1/2] avcodec/h264_direct: Fix runtime error: signed integer overflow: 2147483647 - -14133 cannot be represented in type 'int'
> > > > > > > > > > 
> > > > > > > > > > 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/h264_direct.c | 7 ++++++-
> > > > > > > > > >  1 file changed, 6 insertions(+), 1 deletion(-)
> > > > > > > > > > 
> > > > > > > > > > diff --git a/libavcodec/h264_direct.c b/libavcodec/h264_direct.c
> > > > > > > > > > index cbb84665b3..66e54479d1 100644
> > > > > > > > > > --- a/libavcodec/h264_direct.c
> > > > > > > > > > +++ b/libavcodec/h264_direct.c
> > > > > > > > > > @@ -39,7 +39,12 @@ static int get_scale_factor(H264SliceContext *sl,
> > > > > > > > > >                              int poc, int poc1, int i)
> > > > > > > > > >  {
> > > > > > > > > >      int poc0 = sl->ref_list[0][i].poc;
> > > > > > > > > > -    int td = av_clip_int8(poc1 - poc0);
> > > > > > > > > > +    int64_t pocdiff = poc1 - (int64_t)poc0;
> > > > > > > > > > +    int td = av_clip_int8(pocdiff);
> > > > > > > > > > +
> > > > > > > > > > +    if (pocdiff != (int)pocdiff)
> > > > > > > > > > +        avpriv_request_sample(sl->h264->avctx, "pocdiff overflow\n");
> > > > > > > > > > +
> > > > > > > > > >      if (td == 0 || sl->ref_list[0][i].parent->long_ref) {
> > > > > > > > > >          return 256;
> > > > > > > > > >      } else {        
> > > > > > > > > 
> > > > > > > > > Hard to image that these poc values aren't bounded by something else,
> > > > > > > > > but I don't know.        
> > > > > > > >       
> > > > > > > > > 
> > > > > > > > > Also the previous patch didn't have this request_sample call, which
> > > > > > > > > inflates this whole thing by 5 lines of code.        
> > > > > > > > 
> > > > > > > > yes thats why i suggested it initially.
> > > > > > > > SUINT allows overflow detection simply by  #define CHECKED 1
> > > > > > > > and running under ubsan
> > > > > > > > 
> > > > > > > > otherwise an excplicit check is needed to detect such occurances      
> > > > > > > 
> > > > > > > You can either
> > > > > > > 1. ignore the error in some way that doesn't cause problems
> > > > > > > 2. ignore the error in some way that doesn't cause problems in debug
> > > > > > >    mode
> > > > > > > 3. make the error explicit and log it
> > > > > > > 
> > > > > > > Your first patch did 2 (which I find questionable, btw.), your current      
> > > > > > 
> > > > > > My first patch should have done 1, why do you think it does not?    
> > > > > 
> > > > > Well, it still allows the signed overflow, but only in release mode. Or    
> > > > 
> > > > i think you misread the code, the signed overflow is only possible
> > > > when CHECKED is enabled, its not enabled in release mode.
> > > > It is enabled in DEBUG mode so ubsan can be used to find such overflows
> > > > easily while there is no undefined behavior normally or in any default
> > > > build.  
> > > 
> > > Oh I see. Makes the while thing even stranger.  
> > 
> > can i apply one of the 2 patches or do you object?
> > if you dont object, which one do you prefer ?
> 
> I think the second patch is preferable, if it's between those 2. I
> think it could be improved, but don't want to hold you back further.

locally applied

thanks

[...]
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Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

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