Re: [FFmpeg-devel] [PATCH 4/6] avformat/cafdec: saturate start + size into 64bit

[Michael Niedermayer <michael@niedermayer.cc>]

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On Sat, Sep 30, 2023 at 06:32:07PM +0200, Michael Niedermayer wrote:
> On Fri, Sep 29, 2023 at 08:41:23PM -0300, James Almer wrote:
> > On 9/29/2023 8:19 PM, Michael Niedermayer wrote:
> > > Fixes: signed integer overflow: 64 + 9223372036854775803 cannot be represented in type 'long long'
> > > Fixes: 51896/clusterfuzz-testcase-minimized-ffmpeg_dem_CAF_fuzzer-6536881135550464
> > > 
> > > Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
> > > Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
> > > ---
> > >   libavformat/cafdec.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/libavformat/cafdec.c b/libavformat/cafdec.c
> > > index e92e3279fc6..0e50a3cfe68 100644
> > > --- a/libavformat/cafdec.c
> > > +++ b/libavformat/cafdec.c
> > > @@ -435,7 +435,7 @@ static int read_packet(AVFormatContext *s, AVPacket *pkt)
> > >       /* don't read past end of data chunk */
> > >       if (caf->data_size > 0) {
> > > -        left = (caf->data_start + caf->data_size) - avio_tell(pb);
> > > +        left = av_sat_add64(caf->data_start, caf->data_size) - avio_tell(pb);
> > 
> > avio_tell(pb) here is guaranteed to be bigger than caf->data_start, which is
> > the offset where the DATA chunk starts, so the result of this calculation
> > will be <= INT64_MAX even if you don't saturate it and instead cast it to
> > uint64_t. Nonetheless, if the DATA chunk ends at an offset that would not
> > fit in an int64_t, then we can't parse it to begin with due to AVIOContext
> > API limitations.
> > 
> 
> > Maybe we should just abort in read_header() if data_start + data_size >
> > INT64_MAX, instead of pretending we can parse the file, invalid or not, by
> > saturating values.
> 
> yes ill try that

ossfuzz moved the testcases all to another unrelated issues (62276)

ill apply this as suggested with the check in read_header() below:

diff --git a/libavformat/cafdec.c b/libavformat/cafdec.c
index 72809fd1de2..07a2939a7a6 100644
--- a/libavformat/cafdec.c
+++ b/libavformat/cafdec.c
@@ -343,6 +343,9 @@ static int read_header(AVFormatContext *s)
             avio_skip(pb, 4); /* edit count */
             caf->data_start = avio_tell(pb);
             caf->data_size  = size < 0 ? -1 : size - 4;
+            if (caf->data_start < 0 || caf->data_size > INT64_MAX - caf->data_start)
+                return AVERROR_INVALIDDATA;
+
             if (caf->data_size > 0 && (pb->seekable & AVIO_SEEKABLE_NORMAL))
                 avio_skip(pb, caf->data_size);
             found_data = 1;

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Concerning the gods, I have no means of knowing whether they exist or not
or of what sort they may be, because of the obscurity of the subject, and
the brevity of human life -- Protagoras

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