[FFmpeg-devel] [PATCH 10/14] h264_sei: use a separate reader for the individual SEI messages

[Anton Khirnov]

Quoting James Almer (2020-03-27 16:08:11)
> On 3/27/2020 9:57 AM, Anton Khirnov wrote:
> > This tells the parsing functions the payload size and prevents them from
> > overreading.
> > ---
> >  libavcodec/h264_sei.c | 33 +++++++++++++++++++--------------
> >  1 file changed, 19 insertions(+), 14 deletions(-)
> > 
> > diff --git a/libavcodec/h264_sei.c b/libavcodec/h264_sei.c
> > index a565feabe2..32d13985f3 100644
> > --- a/libavcodec/h264_sei.c
> > +++ b/libavcodec/h264_sei.c
> > @@ -407,9 +407,9 @@ int ff_h264_sei_decode(H264SEIContext *h, GetBitContext *gb,
> >      int master_ret = 0;
> >  
> >      while (get_bits_left(gb) > 16 && show_bits(gb, 16)) {
> > +        GetBitContext gb_payload;
> >          int type = 0;
> >          unsigned size = 0;
> > -        unsigned next;
> >          int ret  = 0;
> >  
> >          do {
> > @@ -429,35 +429,38 @@ int ff_h264_sei_decode(H264SEIContext *h, GetBitContext *gb,
> >                     type, 8*size, get_bits_left(gb));
> >              return AVERROR_INVALIDDATA;
> >          }
> > -        next = get_bits_count(gb) + 8 * size;
> > +
> > +        ret = init_get_bits8(&gb_payload, gb->buffer + get_bits_count(gb) / 8, size);
> 
> Could use init_get_bits() instead, since size was already checked above.

ok, done locally

> 
> > +        if (ret < 0)
> > +            return ret;
> >  
> >          switch (type) {
> >          case H264_SEI_TYPE_PIC_TIMING: // Picture timing SEI
> > -            ret = decode_picture_timing(&h->picture_timing, gb, ps, logctx);
> > +            ret = decode_picture_timing(&h->picture_timing, &gb_payload, ps, logctx);
> >              break;
> >          case H264_SEI_TYPE_USER_DATA_REGISTERED:
> > -            ret = decode_registered_user_data(h, gb, logctx, size);
> > +            ret = decode_registered_user_data(h, &gb_payload, logctx, size);
> >              break;
> >          case H264_SEI_TYPE_USER_DATA_UNREGISTERED:
> > -            ret = decode_unregistered_user_data(&h->unregistered, gb, logctx, size);
> > +            ret = decode_unregistered_user_data(&h->unregistered, &gb_payload, logctx, size);
> >              break;
> >          case H264_SEI_TYPE_RECOVERY_POINT:
> > -            ret = decode_recovery_point(&h->recovery_point, gb, logctx);
> > +            ret = decode_recovery_point(&h->recovery_point, &gb_payload, logctx);
> >              break;
> >          case H264_SEI_TYPE_BUFFERING_PERIOD:
> > -            ret = decode_buffering_period(&h->buffering_period, gb, ps, logctx);
> > +            ret = decode_buffering_period(&h->buffering_period, &gb_payload, ps, logctx);
> >              break;
> >          case H264_SEI_TYPE_FRAME_PACKING:
> > -            ret = decode_frame_packing_arrangement(&h->frame_packing, gb);
> > +            ret = decode_frame_packing_arrangement(&h->frame_packing, &gb_payload);
> >              break;
> >          case H264_SEI_TYPE_DISPLAY_ORIENTATION:
> > -            ret = decode_display_orientation(&h->display_orientation, gb);
> > +            ret = decode_display_orientation(&h->display_orientation, &gb_payload);
> >              break;
> >          case H264_SEI_TYPE_GREEN_METADATA:
> > -            ret = decode_green_metadata(&h->green_metadata, gb);
> > +            ret = decode_green_metadata(&h->green_metadata, &gb_payload);
> >              break;
> >          case H264_SEI_TYPE_ALTERNATIVE_TRANSFER:
> > -            ret = decode_alternative_transfer(&h->alternative_transfer, gb);
> > +            ret = decode_alternative_transfer(&h->alternative_transfer, &gb_payload);
> >              break;
> >          default:
> >              av_log(logctx, AV_LOG_DEBUG, "unknown SEI type %d\n", type);
> > @@ -467,10 +470,12 @@ int ff_h264_sei_decode(H264SEIContext *h, GetBitContext *gb,
> >          if (ret < 0)
> >              master_ret = ret;
> >  
> > -        skip_bits_long(gb, next - get_bits_count(gb));
> > +        if (get_bits_left(&gb_payload) < 0) {
> > +            av_log(logctx, AV_LOG_WARNING, "SEI type %d overread by %d bits\n",
> > +                   type, -get_bits_left(&gb_payload));
> 
> Maybe check for explode and abort?

We don't have access to explode, so it'd have to be passed in
explicitly. That seemed like too much effort to be worth it.

-- 
Anton Khirnov