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

[Andreas Rheinhardt]

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.

- Andreas