Re: [FFmpeg-devel] [PATCH 1/2] avcodec/pictordec: Check that the image fits in the input

From: Anton Khirnov <anton@khirnov.net>
To: FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>
Subject: Re: [FFmpeg-devel] [PATCH 1/2] avcodec/pictordec: Check that the image fits in the input
Date: Fri, 25 Nov 2022 15:11:32 +0100
Message-ID: <166938549275.4503.16165224502546034668@lain.khirnov.net> (raw)
In-Reply-To: <20221125133846.GB3806951@pb2>

Quoting Michael Niedermayer (2022-11-25 14:38:46)
> t   On Fri, Nov 25, 2022 at 05:45:29PM +1100, Peter Ross wrote:
> > On Tue, Nov 22, 2022 at 11:56:51PM +0100, Michael Niedermayer wrote:
> > > Fixes: Timeout
> > > Fixes: 53438/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_PICTOR_fuzzer-5458939919859712
> > > 
> > > Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpe
> > > Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
> > > ---
> > >  libavcodec/pictordec.c | 3 +++
> > >  1 file changed, 3 insertions(+)
> > > 
> > > diff --git a/libavcodec/pictordec.c b/libavcodec/pictordec.c
> > > index 71bad40a0a..09229b94fd 100644
> > > --- a/libavcodec/pictordec.c
> > > +++ b/libavcodec/pictordec.c
> > > @@ -162,6 +162,9 @@ static int decode_frame(AVCodecContext *avctx, AVFrame *frame,
> > >  
> > >      if (av_image_check_size(s->width, s->height, 0, avctx) < 0)
> > >          return -1;
> > > +    if (bytestream2_get_bytes_left(&s->g) < s->width * s->height / 65536 * 5)
> > > +        return AVERROR_INVALIDDATA;
> > 
> > how did you arrive at this formula?
> 
> There are 2 coding modes, RLE and RAW
> I assume usable raw images will need around W*H and thus more than RLE
> RLE codes the most compressed runs by
> 1 byte for val (=marker)
> 1 byte run (=0)
> 2 bytes run 
> 1 byte val
> thats 5 bytes and the maximum run we can code is 65535
> 
> The RLE decoder loop exits before applying the last RLE run and then
> there is a seperate piece of code after it that fills the last color to
> the end. Iam not sure why its done like that way but if i remove that
> mid exit the seperate code piece becomes unused for all images i have
> so it seems all RLE images are always fully coded with no special case
> at the end.
> Based on this iam guesing that my formula is correct for undamaged images
> but of course i could find one tomorrow that exploits the special end
> handling and breaks this formula
> and of course its very possible that i missed some other thing that changes
> this limit

This should be written in a comment above the code then, otherwise all
future readers will be completely clueless.

-- 
Anton Khirnov
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