[FFmpeg-devel] [PATCH 2/3] lavf/webm_chunk: Fix NULL dereference

[Andreas Rheinhardt]

Andreas Rheinhardt:
> Andreas Rheinhardt:
>> Andreas Rheinhardt:
>>> Andreas Rheinhardt:
>>>> The earlier version of the webm_chunk muxer had several bugs:
>>>>
>>>> 1. If the first packet of an audio stream didn't have a PTS of zero,
>>>> then no chunk will be started before a packet is delivered to the
>>>> underlying Matroska/WebM muxer, i.e. the AVFormatContext used to write
>>>> these packets had a NULL as AVIOContext for output. This is behind the
>>>> crash in ticket #5752.
>>>>
>>>> 2. If an error happens during writing a packet, the underlyimg
>>>> Matroska/WebM muxer context is freed. This leads to a use-after-free
>>>> coupled with a double-free in webm_chunk_write_trailer (which supposes
>>>> that the underlying AVFormatContext is still valid).
>>>>
>>>> 3. Even when no error occurs at all, webm_chunk_write_trailer is still
>>>> buggy: After the underlying Matroska/WebM muxer has written its trailer,
>>>> ending the chunk implicitly flushes it again which is illegal at this
>>>> point.
>>>>
>>>> These bugs have been fixed.
>>>>
>>>> Fixes #5752.
>>>>
>>>> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at gmail.com>
>>>> ---
>>>>  libavformat/webm_chunk.c | 44 +++++++++++++++++++++-------------------
>>>>  1 file changed, 23 insertions(+), 21 deletions(-)
>>>>
>>>> diff --git a/libavformat/webm_chunk.c b/libavformat/webm_chunk.c
>>>> index 2c99753b5b..391fee721a 100644
>>>> --- a/libavformat/webm_chunk.c
>>>> +++ b/libavformat/webm_chunk.c
>>>> @@ -168,7 +168,7 @@ static int chunk_start(AVFormatContext *s)
>>>>      return 0;
>>>>  }
>>>>  
>>>> -static int chunk_end(AVFormatContext *s)
>>>> +static int chunk_end(AVFormatContext *s, int flush)
>>>>  {
>>>>      WebMChunkContext *wc = s->priv_data;
>>>>      AVFormatContext *oc = wc->avf;
>>>> @@ -179,11 +179,14 @@ static int chunk_end(AVFormatContext *s)
>>>>      char filename[MAX_FILENAME_SIZE];
>>>>      AVDictionary *options = NULL;
>>>>  
>>>> -    if (wc->chunk_start_index == wc->chunk_index)
>>>> +    if (!oc->pb)
>>>>          return 0;
>>>> -    // Flush the cluster in WebM muxer.
>>>> -    oc->oformat->write_packet(oc, NULL);
>>>> +
>>>> +    if (flush)
>>>> +        // Flush the cluster in WebM muxer.
>>>> +        oc->oformat->write_packet(oc, NULL);
>>>>      buffer_size = avio_close_dyn_buf(oc->pb, &buffer);
>>>> +    oc->pb = NULL;
>>>>      ret = get_chunk_filename(s, 0, filename);
>>>>      if (ret < 0)
>>>>          goto fail;
>>>> @@ -194,7 +197,6 @@ static int chunk_end(AVFormatContext *s)
>>>>          goto fail;
>>>>      avio_write(pb, buffer, buffer_size);
>>>>      ff_format_io_close(s, &pb);
>>>> -    oc->pb = NULL;
>>>>  fail:
>>>>      av_dict_free(&options);
>>>>      av_free(buffer);
>>>> @@ -216,27 +218,19 @@ static int webm_chunk_write_packet(AVFormatContext *s, AVPacket *pkt)
>>>>      }
>>>>  
>>>>      // For video, a new chunk is started only on key frames. For audio, a new
>>>> -    // chunk is started based on chunk_duration.
>>>> -    if ((st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO &&
>>>> +    // chunk is started based on chunk_duration. Also, a new chunk is started
>>>> +    // unconditionally if there is no currently open chunk.
>>>> +    if (!oc->pb || (st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO &&
>>>>           (pkt->flags & AV_PKT_FLAG_KEY)) ||
>>>>          (st->codecpar->codec_type == AVMEDIA_TYPE_AUDIO &&
>>>> -         (pkt->pts == 0 || wc->duration_written >= wc->chunk_duration))) {
>>>> +         wc->duration_written >= wc->chunk_duration)) {
>>>>          wc->duration_written = 0;
>>>> -        if ((ret = chunk_end(s)) < 0 || (ret = chunk_start(s)) < 0) {
>>>> -            goto fail;
>>>> +        if ((ret = chunk_end(s, 1)) < 0 || (ret = chunk_start(s)) < 0) {
>>>> +            return ret;
>>>>          }
>>>>      }
>>>>  
>>>>      ret = oc->oformat->write_packet(oc, pkt);
>>>> -    if (ret < 0)
>>>> -        goto fail;
>>>> -
>>>> -fail:
>>>> -    if (ret < 0) {
>>>> -        oc->streams = NULL;
>>>> -        oc->nb_streams = 0;
>>>> -        avformat_free_context(oc);
>>>> -    }
>>>>  
>>>>      return ret;
>>>>  }
>>>> @@ -245,12 +239,20 @@ static int webm_chunk_write_trailer(AVFormatContext *s)
>>>>  {
>>>>      WebMChunkContext *wc = s->priv_data;
>>>>      AVFormatContext *oc = wc->avf;
>>>> +    int ret;
>>>> +
>>>> +    if (!oc->pb) {
>>>> +        ret = chunk_start(s);
>>>> +        if (ret < 0)
>>>> +            goto fail;
>>>> +    }
>>>>      oc->oformat->write_trailer(oc);
>>>> -    chunk_end(s);
>>>> +    ret = chunk_end(s, 0);
>>>> +fail:
>>>>      oc->streams = NULL;
>>>>      oc->nb_streams = 0;
>>>>      avformat_free_context(oc);
>>>> -    return 0;
>>>> +    return ret;
>>>>  }
>>>>  
>>>>  #define OFFSET(x) offsetof(WebMChunkContext, x)
>>>>
>>>>
>>> Ping for the last two patches of this patchset (i.e. this one and
>>> https://ffmpeg.org/pipermail/ffmpeg-devel/2019-April/242902.html)
>>>
>>> - Andreas
>>>
>> And another ping for these two patches.
>>
>> - Andreas
>>
> 
> Ping #3.
> 
> - Andreas
> 
Ping.

- Andreas