[FFmpeg-devel] [PATCH] avcodec/msp2dec: Check available space in RLE decoder

[Michael Niedermayer]

On Wed, Apr 07, 2021 at 04:59:09PM +0200, Michael Niedermayer wrote:
> On Wed, Apr 07, 2021 at 12:42:50AM +0200, Andreas Rheinhardt wrote:
> > Michael Niedermayer:
> > > Fixes: out of array read
> > > Fixes: 32968/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MSP2_fuzzer-5315296027082752
> > > 
> > > 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/msp2dec.c | 3 ++-
> > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/libavcodec/msp2dec.c b/libavcodec/msp2dec.c
> > > index cc548d218a..87057fb5e2 100644
> > > --- a/libavcodec/msp2dec.c
> > > +++ b/libavcodec/msp2dec.c
> > > @@ -68,9 +68,10 @@ static int msp2_decode_frame(AVCodecContext *avctx,
> > >  
> > >          bytestream2_init(&gb, buf, pkt_size);
> > >          x = 0;
> > > -        while (bytestream2_get_bytes_left(&gb) && x < width) {
> > > +        while (bytestream2_get_bytes_left(&gb) > 0 && x < width) {
> > 
> > This decoder uses the checked bytestream2 API, so != 0 and > 0 should be
> > equivalent for bytestream2_get_bytes_left(&gb).
> 
> I changed it to "> 0" because it felt clearer&more robust
> i will drop that as its not needed for the bugfix
> 
> 
> > 
> > >              int size = bytestream2_get_byte(&gb);
> > >              if (size) {
> > > +                size = FFMIN(size, bytestream2_get_bytes_left(&gb));
> > >                  memcpy(p->data[0] + y * p->linesize[0] + x, gb.buffer, FFMIN(size, width - x));
> > 
> > width can include seven bytes of the packet's padding, but it stays
> > within the padding, so I wonder where the out of array read comes from.
> > The only fishy thing in this decoder I see is that 2 * avctx->height
> > might overflow.
> 
> size is a value read from the bytestream, theres no guarntee that
> the amount that byte calls for is not beyond the end of the bytestream
> and its not checked by memcpy

will apply and backport


[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

No great genius has ever existed without some touch of madness. -- Aristotle
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