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

[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