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

[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