[FFmpeg-devel] [PATCH 1/2] avformat/mxfdec: Fix integer overflow in next position in mxf_read_local_tags()

[Michael Niedermayer]

On Fri, Jan 22, 2021 at 11:23:09AM -0300, James Almer wrote:
> On 1/22/2021 11:09 AM, Michael Niedermayer wrote:
> > Fixes: signed integer overflow: 9223372036854775723 + 8192 cannot be represented in type 'long'
> > Fixes: 29072/clusterfuzz-testcase-minimized-ffmpeg_dem_MXF_fuzzer-4812604904177664
> > 
> > Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
> > Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
> > ---
> >   libavformat/mxfdec.c | 4 +++-
> >   1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/libavformat/mxfdec.c b/libavformat/mxfdec.c
> > index 4c932e954c..f1167761f9 100644
> > --- a/libavformat/mxfdec.c
> > +++ b/libavformat/mxfdec.c
> > @@ -2865,8 +2865,10 @@ static int mxf_read_local_tags(MXFContext *mxf, KLVPacket *klv, MXFMetadataReadF
> >           int ret;
> >           int tag = avio_rb16(pb);
> >           int size = avio_rb16(pb); /* KLV specified by 0x53 */
> > -        uint64_t next = avio_tell(pb) + size;
> > +        uint64_t next = avio_tell(pb);
> >           UID uid = {0};
> > +        if (next > INT64_MAX - size)
> > +            size = 0;
> 
> You're discarding the tag's size. The code below could misbehave as it will
> assume it's an empty tag when it's not. Not to mention the log message will
> print a bogus value.
> 
> Abort instead (Invalid data or erange) in this scenario since going beyond
> INT64_MAX is clearly not supported by avio, and then do next += size to
> ensure the avio_seek() at the end works as expected.

I dont think this strictly implies being beyond INT64_MAX.
but i wanted to redo this patch anyway as it contained another bug
so next suggestion:

@@ -2865,8 +2865,11 @@ static int mxf_read_local_tags(MXFContext *mxf, KLVPacket *klv, MXFMetadataReadF
         int ret;
         int tag = avio_rb16(pb);
         int size = avio_rb16(pb); /* KLV specified by 0x53 */
-        uint64_t next = avio_tell(pb) + size;
+        int64_t next = avio_tell(pb);
         UID uid = {0};
+        if (next < 0 || next > INT64_MAX - size)
+            return next < 0 ? next : AVERROR_INVALIDDATA;
+        next += size;
 
         av_log(mxf->fc, AV_LOG_TRACE, "local tag %#04x size %d\n", tag, size);
         if (!size) { /* ignore empty tag, needed for some files with empty UMID tag */


thx

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Breaking DRM is a little like attempting to break through a door even
though the window is wide open and the only thing in the house is a bunch
of things you dont want and which you would get tomorrow for free anyway
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