[FFmpeg-devel] [PATCH 1/8] avcodec/cinepak: Require 1 bit per 4x4 block as minimum input

[Michael Niedermayer]

On Sun, Aug 18, 2019 at 10:47:26AM +0200, Tomas Härdin wrote:
> sön 2019-08-18 klockan 02:35 +0200 skrev Tomas Härdin:
> > lör 2019-08-17 klockan 17:33 +0200 skrev Michael Niedermayer:
> > > On Sat, Aug 17, 2019 at 12:26:27AM +0200, Tomas Härdin wrote:
> > > > fre 2019-08-16 klockan 14:57 +0200 skrev Tomas Härdin:
> > > > 
> > > > I feel I should point out that being conservative here is at odds with
> > > > the general "best effort" approach taken in this project. These toy
> > > > codecs are useful as illustrative examples of this contradiction. I'm
> > > > sure there are many more examples of files that can cause ffmpeg to do
> > > > a lot more work than expected, for almost every codec. I know afl-fuzz
> > > > is likely to find out that it can make the ZMBV decoder do a lot of
> > > > work for a small input file, because I've seen it do that with gzip.
> > > > 
> > > > The user base for cinepak is of course miniscule, so I doubt anyone's
> > > > going to complain that their broken CVID files don't work any more. I
> > > > certainly don't care since cinepakenc only puts out valid files. 
> > > > But
> > > > again, for other formats we're just going to have to tell users to put
> > > > ffmpeg inside a sandbox and stick a CPU limit on it. Even ffprobe is
> > > > vulnerable to DoS-y things.
> > > 
> > > yes
> > > 
> > > the question ATM is just what to do here about this codec ?
> > > apply the patch ?
> > > change it ?
> > 
> > Well for a start, the file is 65535 x 209 pixels, 3166 frames. I
> > wouldn't call decoding that @ 263 fps particularly slow
> > 
> > Second, it's not the decoder which is slow. If I comment out the
> > "*got_frame = 1;" then the test also runs fast. I'm not sure what
> > happens elsewhere with the decoded buffer, but I suspect there's a
> > bunch of useless malloc()/memset()ing going on. Maybe the decoder is
> > using ff_reget_buffer() or av_frame_ref() incorrectly, I'm not sure.
> 
> I did some investigation, it is indeed ff_reget_buffer(). It copies the
> frame data for some reason. The fix is simple in this case: just call
> ff_get_buffer() once in cinepak_decode_init() and keep overwriting the
> same frame.
> 
> > As I said on IRC, this class of problems will exist for every codec.
> > Cinepak is easy to decode, even at these resolutions. Just imagine what
> > will happens when someone feeds in a 65535x209 av1 stream..
> 
> And related to this, ff_reget_buffer() is used for a lot of these
> codecs which only overwrite pixels in the old frame. flicvideo, gifdec,
> msrle, roqvideodec and others probably have the same flaw.

not calling any form of *get_buffer per frame breaks decoding into
user supplied buffers.

If you check the documentation of the get_buffer2 callback

" This callback is called at the beginning of each frame to get data buffer(s) for it."

That would not be possible if its just called once in init

and yes i too wish there was a magic fix but i think most things that
look like magic fixes have a fatal flaw. But maybe iam missing something
in fact i hope that iam missing something and that there is a magic fix

PS: if you think of changing the API, i dont think its the API.
I mean every user application will read the frames it receives, so
even if inside the decoder we just return the same frame with 2 pixels
different the user doesnt know this and has to read the whole frame.
The problem is moved around but its still there.

Thanks

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

During times of universal deceit, telling the truth becomes a
revolutionary act. -- George Orwell
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