Re: [FFmpeg-devel] [PATCH] avformat/flvdec: use avio operation instead of pb->buf_ptr use

[Hendrik Leppkes <h.leppkes@gmail.com>]

On Thu, Jul 27, 2023 at 4:38 AM Steven Liu <lq@chinaffmpeg.org> wrote:
>
> fix segfaults:
> READ of size 1 at 0x6100000003b7 thread T0
>     #0 0x7f928d in flv_same_video_codec ffmpeg/libavformat/flvdec.c:317:29
>     #1 0x7f928d in flv_read_packet ffmpeg/libavformat/flvdec.c:1177
>     #2 0x6ff32f in ff_read_packet ffmpeg/libavformat/demux.c:575:15
>     #3 0x70a2fd in read_frame_internal ffmpeg/libavformat/demux.c:1263:15
>     #4 0x71d158 in avformat_find_stream_info ffmpeg/libavformat/demux.c:2634:15
>     #5 0x4c821b in LLVMFuzzerTestOneInput ffmpeg/tools/target_dem_fuzzer.c:206:11
>
> Signed-off-by: Steven Liu <lq@chinaffmpeg.org>
> ---
>  libavformat/flvdec.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/libavformat/flvdec.c b/libavformat/flvdec.c
> index 3fe21622f7..003e4d7691 100644
> --- a/libavformat/flvdec.c
> +++ b/libavformat/flvdec.c
> @@ -313,8 +313,9 @@ static int flv_same_video_codec(AVFormatContext *s, AVCodecParameters *vpar, int
>          return 1;
>
>      if (flv->exheader) {
> -        uint8_t *codec_id_str = (uint8_t *)s->pb->buf_ptr;
> -        uint32_t codec_id = codec_id_str[3] | codec_id_str[2] << 8 | codec_id_str[1] << 16 | codec_id_str[0] << 24;
> +        int64_t pos = avio_tell(s->pb);
> +        uint32_t codec_id = avio_rb32(s->pb);
> +        avio_seek(s->pb, pos, SEEK_SET);
>          switch(codec_id) {
>              case MKBETAG('h', 'v', 'c', '1'):
>                  return vpar->codec_id == AV_CODEC_ID_HEVC;

You don't need to store the position, just do a relative seek
backwards by 4 bytes.
I would also recommend to include a call to ffio_ensure_seekback so it
works with streams.

eg. something like this:
ffio_ensure_seekback(s->pb, 4);
avio_rb32(s->pb);
avio_seek(s->pb, -4, SEEK_CUR);

Add appropriate error checking, in particular for ffio_ensure_seekback, etc.

- Hendrik
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