[FFmpeg-devel] [PATCH 2/2] avformat/subtitles: Check for NOPTS in ff_subtitles_queue_finalize()

[Nicolas George]

Michael Niedermayer (12020-06-24):
> On Tue, Jun 23, 2020 at 02:08:44PM +0200, Nicolas George wrote:
> > Michael Niedermayer (12020-06-23):
> > > Fixes; signed integer overflow: 1 - -9223372036854775808 cannot be represented in type 'long'
> > > Fixes: 23490/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5133490093031424
> > > 
> > > Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
> > > Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
> > > ---
> > >  libavformat/subtitles.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/libavformat/subtitles.c b/libavformat/subtitles.c
> > > index ad7f68938e..ccab80391f 100644
> > > --- a/libavformat/subtitles.c
> > > +++ b/libavformat/subtitles.c
> > > @@ -202,7 +202,7 @@ void ff_subtitles_queue_finalize(void *log_ctx, FFDemuxSubtitlesQueue *q)
> > >            q->sort == SUB_SORT_TS_POS ? cmp_pkt_sub_ts_pos
> > >                                       : cmp_pkt_sub_pos_ts);
> > >      for (i = 0; i < q->nb_subs; i++)
> > > -        if (q->subs[i].duration < 0 && i < q->nb_subs - 1)
> > > +        if (q->subs[i].duration < 0 && i < q->nb_subs - 1 && q->subs[i].pts != AV_NOPTS_VALUE)
> > >              q->subs[i].duration = q->subs[i + 1].pts - q->subs[i].pts;
> > >  
> > >      if (!q->keep_duplicates)
> > 
> > Having no PTS at this point makes no sense. We should examine why it
> > arrived there. 
> 
> it comes from microdvddec in this case, it can be fixed for example
> with the following: (i do not know if these are all cases which can
> generate this)

Thanks. It seems like a much more correct fix. We can see in
$FATE/sub/MicroDVD_capability_tester.sub that all lines have a number in
the first pair of braces except DEFAULT, which is parsed separately.

> commit 06eff32c6e4100b70a3a215a0756580a6fa124b5 (HEAD -> master)
> Author: Michael Niedermayer <michael at niedermayer.cc>
> Date:   Tue Jun 23 01:43:14 2020 +0200
> 

>     iavformat/microdvddec: Skip NOPTS values

skip malformed lines without frame number.

>     
>     Fixes; signed integer overflow: 1 - -9223372036854775808 cannot be represented in type 'long'

Colon / semicolon.

>     Fixes: 23490/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5133490093031424
>     
>     Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
>     Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
> 
> diff --git a/libavformat/microdvddec.c b/libavformat/microdvddec.c
> index 8759200f88..1f871b2518 100644
> --- a/libavformat/microdvddec.c
> +++ b/libavformat/microdvddec.c
> @@ -94,6 +94,7 @@ static int microdvd_read_header(AVFormatContext *s)
>          int64_t pos = avio_tell(s->pb);
>          int len = ff_get_line(s->pb, line_buf, sizeof(line_buf));
>          char *line = line_buf;
> +        int64_t pts;
>  
>          if (!strncmp(line, bom, 3))
>              line += 3;
> @@ -137,13 +138,16 @@ static int microdvd_read_header(AVFormatContext *s)
>          SKIP_FRAME_ID;
>          if (!*p)
>              continue;

> +        pts = get_pts(line);
> +        if (pts == AV_NOPTS_VALUE)
> +            continue;
>          sub = ff_subtitles_queue_insert(&microdvd->q, p, strlen(p), 0);
>          if (!sub) {
>              ret = AVERROR(ENOMEM);
>              goto fail;
>          }
>          sub->pos = pos;
> -        sub->pts = get_pts(line);
> +        sub->pts = pts;
>          sub->duration = get_duration(line);
>      }
>      ff_subtitles_queue_finalize(s, &microdvd->q);
> 
> 
> 
> 
> 
> > Can you share the test case?
> 
> Ill send you the file privatly

I really do not see the point. I only understood what I was looking at
after reading your public mail and the source code, which means anybody
could get the same information. And it is not as if it was a real bug,
even less a security issue, just a case of garbage in garbage out.

Regards,

-- 
  Nicolas George
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