[FFmpeg-devel] [PATCH 04/11] avformat/utils: Move +1 to avoid overflow

[Michael Niedermayer]

On Sat, Nov 28, 2020 at 09:51:44PM +0100, Michael Niedermayer wrote:
> On Fri, Oct 30, 2020 at 09:24:15PM +0100, Michael Niedermayer wrote:
> > On Wed, Oct 21, 2020 at 08:47:22AM +0200, Andreas Rheinhardt wrote:
> > > Andreas Rheinhardt:
> > > > Michael Niedermayer:
> > > >> Fixes: signed integer overflow: 9223372036854775807 + 1 cannot be represented in type 'long'
> > > >> Fixes: Timeout
> > > >> Fixes: 26434/clusterfuzz-testcase-minimized-ffmpeg_dem_MV_fuzzer-5752845451919360
> > > >> Fixes: 26444/clusterfuzz-testcase-minimized-ffmpeg_dem_BINK_fuzzer-4697773380993024
> > > >>
> > > >> 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/utils.c | 2 +-
> > > >>  1 file changed, 1 insertion(+), 1 deletion(-)
> > > >>
> > > >> diff --git a/libavformat/utils.c b/libavformat/utils.c
> > > >> index e8335a601f..59d65a8092 100644
> > > >> --- a/libavformat/utils.c
> > > >> +++ b/libavformat/utils.c
> > > >> @@ -253,7 +253,7 @@ int ffio_limit(AVIOContext *s, int size)
> > > >>              remaining= FFMAX(remaining, 0);
> > > >>          }
> > > >>  
> > > >> -        if (s->maxsize>= 0 && remaining+1 < size) {
> > > >> +        if (s->maxsize>= 0 && remaining < size - (int64_t)1) {
> > > >>              av_log(NULL, remaining ? AV_LOG_ERROR : AV_LOG_DEBUG, "Truncating packet of size %d to %"PRId64"\n", size, remaining+1);
> > > >>              size = remaining+1;
> > > >>          }
> > > >>
> > > > The +1 here seems very weird. If remaining + 1 == size, the packet will
> > > > not be truncated, despite there not being enough data left.
> > > > 
> > > > The only reason for its existence I can think of is that it is designed
> > > > to counter the potential -1 from s->maxsize = newsize - !newsize a few
> > > > lines above. Yet this makes no sense: If newsize == 0, s->maxsize has
> > > > just been set to -1 (indicating that future calls to ffio_limit() should
> > > > not try again to get the filesize) and the second check above will not
> > > > be reached at all.
> > > > 
> > > > So how about just removing the +1 in all three lines above?
> > > > 
> > > > - Andreas
> > > > 
> > > > PS: And while just at it: You can also add a space to "s->maxsize>= 0".
> > > > 
> > > On second look (seeing "remaining ?") this seems to be done to make sure
> > >  not to truncate to zero in order to always read something at all. So
> > > how about this here (untested):
> > > 
> > > diff --git a/libavformat/utils.c b/libavformat/utils.c
> > > index e8335a601f..dd1b9e41c1 100644
> > > --- a/libavformat/utils.c
> > > +++ b/libavformat/utils.c
> > > @@ -253,9 +253,11 @@ int ffio_limit(AVIOContext *s, int size)
> > >              remaining= FFMAX(remaining, 0);
> > >          }
> > > 
> > > -        if (s->maxsize>= 0 && remaining+1 < size) {
> > > -            av_log(NULL, remaining ? AV_LOG_ERROR : AV_LOG_DEBUG,
> > > "Truncating packet of size %d to %"PRId64"\n", size, remaining+1);
> > > -            size = remaining+1;
> > > +        if (s->maxsize >= 0 && remaining < size && size > 1) {
> > > +            av_log(NULL, remaining ? AV_LOG_ERROR : AV_LOG_DEBUG,
> > > +                   "Truncating packet of size %d to %"PRId64"\n",
> > > +                   size, remaining + !remaining);
> > > +            size = remaining + !remaining;
> > >          }
> > >      }
> > >      return size;
> > 
> > LGTM, please apply
> 
> ping

This affects quite a few files, so i may apply this or something else myself 
if you dont apply anything and dont comment

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
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 195 bytes
Desc: not available
URL: <https://ffmpeg.org/pipermail/ffmpeg-devel/attachments/20201211/ef2884dd/attachment.sig>