[FFmpeg-devel] [PATCH 6/9] avformat/vividas: Check for zero v_size

[Michael Niedermayer]

On Thu, Oct 22, 2020 at 07:55:31AM +0200, Andreas Rheinhardt wrote:
> Michael Niedermayer:
> > Fixes: SEGV on unknown address 0x000000000000
> > Fixes: 26482/clusterfuzz-testcase-minimized-ffmpeg_dem_VIVIDAS_fuzzer-4905102324006912
> > 
> > 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/vividas.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/libavformat/vividas.c b/libavformat/vividas.c
> > index 999c4a5d1c..6e93d96aef 100644
> > --- a/libavformat/vividas.c
> > +++ b/libavformat/vividas.c
> > @@ -682,7 +682,7 @@ static int viv_read_packet(AVFormatContext *s,
> >              return AVERROR_INVALIDDATA;
> >  
> >          ffio_read_varlen(pb);
> > -        if (v_size > INT_MAX)
> > +        if (v_size > INT_MAX || !v_size)
> >              return AVERROR_INVALIDDATA;
> >          ret = av_get_packet(pb, pkt, v_size);
> >          if (ret < 0)
> > @@ -711,7 +711,7 @@ static int viv_read_packet(AVFormatContext *s,
> >      } else {
> >          uint64_t v_size = ffio_read_varlen(pb);
> >  
> > -        if (v_size > INT_MAX)
> > +        if (v_size > INT_MAX || !v_size)
> >              return AVERROR_INVALIDDATA;
> >          ret = av_get_packet(pb, pkt, v_size);
> >          if (ret < 0)
> > 
> av_get_packet(pb, pkt, 0) should get a packet with pkt->data pointing to
> a zeroed buffer of size AV_INPUT_BUFFER_PADDING_SIZE. So accessing
> pkt->data[0] for the flags should not segfault (but it would set the
> wrong result: that it is a keyframe). So where is the segfault?

I tried to simulate this in a different demuxer:
(different to make sure its not the file or demuxer that somehow affects this)
--- a/libavformat/avidec.c
+++ b/libavformat/avidec.c
@@ -1446,9 +1446,10 @@ resync:
         if (size > ast->remaining)
             size = ast->remaining;
         avi->last_pkt_pos = avio_tell(pb);
-        err               = av_get_packet(pb, pkt, size);
+        err               = av_get_packet(pb, pkt, 0);
         if (err < 0)
             return err;
+        av_log(0,0, "P %p\n", pkt->data);
         size = err;
 
         if (ast->has_pal && pkt->size < (unsigned)INT_MAX / 2) {

what is printed is:
P (nil)

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

Asymptotically faster algorithms should always be preferred if you have
asymptotical amounts of data
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