Re: [FFmpeg-devel] [PATCH 1/3] avformat/cafdec: sanity check channels and bps

[Michael Niedermayer <michael@niedermayer.cc>]

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On Wed, Jun 26, 2024 at 09:52:44PM -0300, James Almer wrote:
> On 3/22/2024 8:08 PM, Michael Niedermayer wrote:
> > Fixes: Timeout
> > Fixes: 67044/clusterfuzz-testcase-minimized-ffmpeg_dem_CAF_fuzzer-5791144363491328
> > 
> > Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
> > Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
> > ---
> >   libavformat/cafdec.c | 5 +++++
> >   1 file changed, 5 insertions(+)
> > 
> > diff --git a/libavformat/cafdec.c b/libavformat/cafdec.c
> > index 426c56b9bd..334077efb5 100644
> > --- a/libavformat/cafdec.c
> > +++ b/libavformat/cafdec.c
> > @@ -33,6 +33,7 @@
> >   #include "isom.h"
> >   #include "mov_chan.h"
> >   #include "libavcodec/flac.h"
> > +#include "libavcodec/internal.h"
> >   #include "libavutil/intreadwrite.h"
> >   #include "libavutil/intfloat.h"
> >   #include "libavutil/dict.h"
> > @@ -87,6 +88,10 @@ static int read_desc_chunk(AVFormatContext *s)
> >       st->codecpar->ch_layout.nb_channels = avio_rb32(pb);
> >       st->codecpar->bits_per_coded_sample = avio_rb32(pb);
> > +    if (st->codecpar->ch_layout.nb_channels > FF_SANE_NB_CHANNELS ||
> > +        st->codecpar->bits_per_coded_sample > 64)
> 
> Where does the process take so long that oss-fuzz gets a timeout if these
> are unreasonably high? I don't see nb_channels used anywhere in here where
> that matters.
> Is it in the PCM decoder? Because that decoder is meant to handle any
> arbitrary amount of channels, so limiting it to whatever FF_SANE_NB_CHANNELS
> is set to is not ok.

libavutil/channel_layout.c:627:17
or it will OOM before depending on how much memory is available


> 
> And is the bits_per_coded_sample > 64 check to prevent codec_id being
> AV_CODEC_ID_NONE? if so, how does that affect demuxing time?
> AV_CODEC_ID_NONE for that matter could happen for valid files with a codec
> we don't currently support.

We generally check read values for validity at the earliest,
bits_per_coded_sample of more than 64 seem not valid to me.

thx

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

I have never wished to cater to the crowd; for what I know they do not
approve, and what they approve I do not know. -- Epicurus

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