[FFmpeg-devel] [PATCH 2/2] avformat/swfdec: Check outlen before allocation

[Michael Niedermayer]

On Sun, Jan 24, 2021 at 02:05:17PM +0100, Anton Khirnov wrote:
> Quoting Michael Niedermayer (2021-01-23 23:34:19)
> > On Sat, Jan 23, 2021 at 03:29:38PM +0100, Anton Khirnov wrote:
> > > Quoting Michael Niedermayer (2021-01-22 15:09:47)
> > > > Fixes: Timeout (too long -> 241ms)
> > > > Fixes: 29083/clusterfuzz-testcase-minimized-ffmpeg_dem_SWF_fuzzer-6273684478230528
> > > > 
> > > > Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
> > > > Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
> > > > ---
> > > >  libavformat/swfdec.c | 3 +++
> > > >  1 file changed, 3 insertions(+)
> > > > 
> > > > diff --git a/libavformat/swfdec.c b/libavformat/swfdec.c
> > > > index 1463f0ad4d..aa4be88f91 100644
> > > > --- a/libavformat/swfdec.c
> > > > +++ b/libavformat/swfdec.c
> > > > @@ -367,6 +367,9 @@ static int swf_read_packet(AVFormatContext *s, AVPacket *pkt)
> > > >              ff_dlog(s, "bitmap: ch=%d fmt=%d %dx%d (linesize=%d) len=%d->%ld pal=%d\n",
> > > >                      ch_id, bmp_fmt, width, height, linesize, len, out_len, colormapsize);
> > > >  
> > > > +            if (len * 17373LL < out_len)
> > > 
> > > Where does the magic number come from?
> > 
> > A very quick simulation of the best case compression for "compress"
> > below is not nice written code as i did not expect I or anyone else
> > would ever see it again
> > 
> > I would have preferred some nicer expression or course, but thats
> > what it seems to be asymptotically. For smaller amounts of data a
> > tighter bound is possible but i saw no nice way to consider that 
> > and it seems also overkill to try to do it more fine grained for
> > just this 
> > 
> > main(){
> >     int64_t bits = 0;
> >     int bank = 256;
> >     int bitbank = 8;
> >     for(unsigned i = 0; i<1024*1024*1024*4U-100000;) {
> >         int word_size = bank-255;
> >         i += word_size;
> >         bits += bitbank;
> > 
> >         if (!(bank & (bank-1)))
> >             bitbank ++;
> >         bank++;
> >         if (bitbank > 16) {
> >             printf("BEST %f \n", 8.0 * i / bits );
> >             bank = 256;
> >             bitbank = 8;
> >         }
> >     }
> > }
> > 
> > above assumes i remembered correctly how the algorithm works but the
> > value was close to what actual compession of zeros gave
> 
> People who read this code in the future will be interested in all this.
> So the content of your reply should be added to the commit message
> and/or the code itself.

will apply with the message in the commit message

thx

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

If you fake or manipulate statistics in a paper in physics you will never
get a job again.
If you fake or manipulate statistics in a paper in medicin you will get
a job for life at the pharma industry.
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