[FFmpeg-devel] [PATCH 4/4] avcodec/faxcompr: Check for invalid VLC in decode_group3_1d_line()

[Michael Niedermayer]

On Wed, Apr 28, 2021 at 05:37:39PM +0200, Andreas Rheinhardt wrote:
> Michael Niedermayer:
> > Fixes: infinite loop
> > Fixes: 33674/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_TIFF_fuzzer-4816457818046464
> > 
> > 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/faxcompr.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/libavcodec/faxcompr.c b/libavcodec/faxcompr.c
> > index 7bf11d80ca..f1f7e67762 100644
> > --- a/libavcodec/faxcompr.c
> > +++ b/libavcodec/faxcompr.c
> > @@ -213,7 +213,7 @@ static int decode_group3_1d_line(AVCodecContext *avctx, GetBitContext *gb,
> >          run += t;
> >          if (t < 64) {
> >              *runs++ = run;
> > -            if (runs >= runend) {
> > +            if (runs >= runend || t < 0) {
> >                  av_log(avctx, AV_LOG_ERROR, "Run overrun\n");
> >                  return AVERROR_INVALIDDATA;
> >              }
> > 
> This t is unsigned, so your added check is void. There is btw an else
> part here that checks for errors: "} else if ((int)t == -1) {".
> The only way I can think of for an infinite loop is that the part after
> the end of the get_bits-reader needn't be zeroed and so it can be
> mistaken for a valid code and enter the codepath for valid codes; in
> particular, it can be a code corresponding to the symbol 0 in which case
> one is not saved by the "if (pix_left <= run) {" check. And given that
> this code does not use the unchecked bitstream reader, it will never
> advance.
> So it seems like the best way to fix this is to check for whether there
> are any bits left before the get_vlc2() call.

yes, ill do that.

thx

[...]
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Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

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