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

[Michael Niedermayer]

On Thu, Oct 22, 2020 at 06:55:37PM +0200, Andreas Rheinhardt wrote:
> 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)
> > 
> My bad: append_packet_chunked() unrefs the packet if pkt->size is zero
> before returning (without returning an error). So your patch seems to be
> the way to go.

ok, will apply

thx

[...]
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