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

[Andreas Rheinhardt]

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.

- Andreas