Re: [FFmpeg-devel] [PATCH 1/3] avformat/dashdec: fail on probing non mpd file extension

[Anton Khirnov <anton@khirnov.net>]

Quoting Michael Niedermayer (2023-05-09 00:35:08)
> On Mon, May 08, 2023 at 04:05:40PM +0200, Tobias Rapp wrote:
> > On 08/05/2023 14:00, James Almer wrote:
> > 
> > > On 5/6/2023 10:25 AM, Michael Niedermayer wrote:
> > > > Its unexpected that a .avi or other "standard" file turns into a
> > > > playlist.
> > > > The goal of this patch is to avoid this unexpected behavior and possible
> > > > privacy or security differences.
> > > > 
> > > > This is similar to the same change to hls
> > > > 
> > > > Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
> > > > ---
> > > >   libavformat/dashdec.c | 11 +++++++----
> > > >   1 file changed, 7 insertions(+), 4 deletions(-)
> > > > 
> > > > diff --git a/libavformat/dashdec.c b/libavformat/dashdec.c
> > > > index 29d4680c68..294e14150d 100644
> > > > --- a/libavformat/dashdec.c
> > > > +++ b/libavformat/dashdec.c
> > > > @@ -2336,10 +2336,13 @@ static int dash_probe(const AVProbeData *p)
> > > >           av_stristr(p->buf, "dash:profile:isoff-live:2011") ||
> > > >           av_stristr(p->buf, "dash:profile:isoff-live:2012") ||
> > > >           av_stristr(p->buf, "dash:profile:isoff-main:2011") ||
> > > > -        av_stristr(p->buf, "3GPP:PSS:profile:DASH1")) {
> > > > -        return AVPROBE_SCORE_MAX;
> > > > -    }
> > > > -    if (av_stristr(p->buf, "dash:profile")) {
> > > > +        av_stristr(p->buf, "3GPP:PSS:profile:DASH1") ||
> > > > +        av_stristr(p->buf, "dash:profile")) {
> > > > +        if (!av_match_ext(p->filename, "mpd")) {
> > > > +            av_log(NULL, AV_LOG_ERROR, "Not detecting dash with non
> > > > standard extension\n");
> > > > +            return 0;
> > > > +        }
> > > > +
> > > >           return AVPROBE_SCORE_MAX;
> > > >       }
> > > 
> > > Failing because it didn't match an extensions sort of goes against the
> > > point of probing, which even has a low score return value that's
> > > basically "it matched extension" as a sort of last resort.
> > > 
> > > I'd say wrap this in a FF_COMPLIANCE_STRICT check (since i assume the
> > > spec does state mpd must be the extension), but i think we have no
> > > access to the AVFormatContext here?
> > 
> > DASH is usually transferred over HTTP where file extensions are of minor
> > interest, the relevant type information is in the Mime-Type header.
> 
> would anyone be opposed to return 0 from dash_probe() when
> both the mime_type and the extension are wrong ?

I would.

probe() is for probing, not implementing security policies. IMO trying
to fix security issues at the wrong layer will only lead to more
confusion, more complexity, and LESS security.

-- 
Anton Khirnov
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