[FFmpeg-devel] [PATCH] avcodec/argo: Check for end of input in decode_alcd()

[Michael Niedermayer]

On Fri, Jul 30, 2021 at 08:44:03AM +0200, Paul B Mahol wrote:
> On Fri, Jul 30, 2021 at 12:16 AM Andreas Rheinhardt <
> andreas.rheinhardt at outlook.com> wrote:
> 
> > Andreas Rheinhardt:
> > > Michael Niedermayer:
> > >> Fixes: reading over the end
> > >> Fixes:
> > 36346/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ARGO_fuzzer-5366943107383296
> > >>
> > >> Found-by: continuous fuzzing process
> > https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
> > >> Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
> > >> ---
> > >>  libavcodec/argo.c | 2 ++
> > >>  1 file changed, 2 insertions(+)
> > >>
> > >> diff --git a/libavcodec/argo.c b/libavcodec/argo.c
> > >> index bbdb6ae15f..79a44d2583 100644
> > >> --- a/libavcodec/argo.c
> > >> +++ b/libavcodec/argo.c
> > >> @@ -116,6 +116,8 @@ static int decode_alcd(AVCodecContext *avctx,
> > AVFrame *frame)
> > >>              int index;
> > >>
> > >>              if (count == 0) {
> > >> +                if (bytestream2_get_bytes_left(gb) < 1)
> > >> +                    return AVERROR_INVALIDDATA;
> > >>                  codes = bytestream2_get_byteu(&sb);
> > >>                  count = 8;
> > >>              }
> > >>
> > > Does the following also fix the issue?
> > >
> > > diff --git a/libavcodec/argo.c b/libavcodec/argo.c
> > >
> > > index bbdb6ae15f..602c042568 100644
> > >
> > > --- a/libavcodec/argo.c
> > >
> > > +++ b/libavcodec/argo.c
> > >
> > > @@ -102,13 +102,14 @@ static int decode_alcd(AVCodecContext *avctx,
> > > AVFrame *frame)
> > >
> > >      uint8_t *dst = frame->data[0];
> > >
> > >      uint8_t codes = 0;
> > >
> > >      int count = 0;
> > >
> > > +    int num_codes = ((frame->width + 1) / 2 * (frame->height + 1) / 2 +
> > > 7) >> 3;
> > >
> > >
> > >
> > > -    if (bytestream2_get_bytes_left(gb) < 1024 + (((frame->width / 2) *
> > > (frame->height / 2) + 7) >> 3))
> > >
> > > +    if (bytestream2_get_bytes_left(gb) < 1024 + num_codes)
> > >
> > >          return AVERROR_INVALIDDATA;
> > >
> > >
> > >
> > >      bytestream2_skipu(gb, 1024);
> > >
> > >      sb = *gb;
> > >
> > > -    bytestream2_skipu(gb, ((frame->width / 2) * (frame->height / 2) +
> > > 7) >> 3);
> > >
> > > +    bytestream2_skipu(gb, num_codes);
> > >
> > >
> > >
> > >      for (int y = 0; y < frame->height; y += 2) {
> > >
> > >          for (int x = 0; x < frame->width; x += 2) {
> > >
> > As can be seen from the above patch, my guess is that odd dimensions are
> > the cause (because num_codes as above is the number of codes that is
> > actually read); but my patch would not only change the criterion for
> > when to error out, but also how much to skip (i.e. where the real data
> > begins) and this makes me wonder whether we should not error out in this
> > case (and ask for a sample).
> >
> 
> I like Andreas solution much more, anway original video files are having
> even dimensions,
> so there is nothing to worry.

I did misread the code, andreas patch or a check for even dimensions is better
than what i posted

should i post a patch checking for even dimensions ?
if yes, which of the cases should be covered by a even check ?
i assume alcd and avcf at least 

Thanks

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

Frequently ignored answer#1 FFmpeg bugs should be sent to our bugtracker. User
questions about the command line tools should be sent to the ffmpeg-user ML.
And questions about how to use libav* should be sent to the libav-user ML.
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