[FFmpeg-devel] [PATCH] avcodec/gsm_parser: return -1 on parse error

[Hendrik Leppkes]

On Thu, Jan 31, 2019 at 11:29 PM Chris Cunningham
<chcunningham at chromium.org> wrote:
>
> On Wed, Jan 30, 2019 at 5:43 PM James Almer <jamrial at gmail.com> wrote:
>
> > On 1/30/2019 10:20 PM, Chris Cunningham wrote:
> > >>
> > >>>> And this definitely means there's a bug elsewhere. This parser should
> > >>>> have not been used for codecs ids other than GSM and GSM_MS. That's
> > >>>> precisely what this assert() is making sure of.
> > >>>>
> > >>>
> > >>> I'll take a closer look at how this parser was selected.
> > >>
> > >
> > > Ok, here are full details of how this happens:
> > >
> > >    1. AV_CODEC_ID_GSM_MS is assigned to on st->codecpar->codec_id
> > >    in ogm_header() (oggparseogm.c:75)
> > >    2. The (deprecated) st->codec->codec_id is not assigned at that time
> > and
> > >    remains 0 (UNKNOWN)
> > >    3. AV_CODEC_ID_GSM_MS from st->codecpar is passed to av_parser_init,
> > >    selecting the gsm parser (in read_frame_internal())
> > >    4. The fuzzer next (only) packet has a size of 0, so the first call to
> > >    parse_packet() (still in read_frame_internal()) does nothing
> > >    5. After this call we assign several members of st->internal->avctx to
> > >    st->codecpar. This leaves codecpar->codec_id = UNKNOWN.
> > Interestingly, we
> > >    do not reset the st->parser at this point (maybe we should?)
> >
> > Where does this happen? A call to avcodec_parameters_from_context()
> > should also copy codec_id.
> >
>
> Ignore #5 here - I'm not seeing that today - was likely confused.
>
>
> >
> > >    6. Next iteration of the read_frame_internal() loop we get EOF from
> > >    ff_read_packet. This triggers the "flush the parsers" call to
> > >    parse_packet() which finally reaches gsm_parse() to trigger the abort
> > >    (avctx->codec_id is still 0).
> > >
> > > Questions (guessing at the fix):
> > >
> > >    - At what point should st->codec->codec_id be synchronized with
> > >    st->codecpar->codec_id? It never is in this test.
> >
> > In avformat_find_stream_info(), i think. In any case, st->codec should
> > have no effect on parsers. What is used in parse_packet() however is
> > st->internal->avctx, so that context is the one that evidently contains
> > codec_id == UNKNOWN when it should be in sync with codecpar.
> >
>
> We do call avformat_find_stream_info, and avcodec_parameters_from_context
> is called during that process. Everything seems OK when
> avformat_find_stream_info is done. The codecpar->codec_id is in sync with
> internal->avctx->codec_id for all streams.
>
> But then we start calling av_read_frame as part of normal demuxing.
> avcodec_parameters_from_context() does not appear to be called during this
> process. Eventually we get this stack:
>
> ogm_header
> ogg_packet
> ogg_read_packet
> ff_read_packet
> read_frame_internal
> av_read_frame
>
> Inside ogm_header, st->codecpar->codec_id is assigned AV_CODEC_ID_GSM_MS
> (previous value was codec NONE). But *st->internal->avctx->codec_id is
> never updated. It remains NONE for the rest of this test. *
>
> When this unwinds back to read_frame_internal, st->parser is assigned using
> this new codec (GSM_MS)
>     st->parser = av_parser_init(st->codecpar->codec_id);
>
> Later on, still inside read_frame_internal's loop, we get EOF and call,
> parse_packet (/* flush the parsers */)
> parse_packet() calls av_parser_parse2(), passing st->internal->avctx
> (codec_id still NONE, while codecpar still says GSM_MS). This ultimately
> gets passed to gsm_parse, which triggers the assert0.
>
> The underlined bit above seems like the root cause. Where should we be
> updating st->internal->avctx->codec_id?

We have a flag to set that causes avformat to fix its internal state
if a demuxer changes it after the initial opening.
st->internal->need_context_update = 1

Try setting that at the position where the demuxer changes codecpar
(ie. in ogm_header?), and see if that resolves it.

- Hendrik