[FFmpeg-devel] [PATCH] alsdec: validate time diff index

Michael Niedermayer michaelni at gmx.at
Sat Apr 18 20:47:08 CEST 2015


On Sat, Apr 18, 2015 at 08:42:58PM +0200, Michael Niedermayer wrote:
> On Sat, Apr 18, 2015 at 08:13:30PM +0200, Andreas Cadhalpun wrote:
> > On 18.04.2015 20:05, Michael Niedermayer wrote:
> > > On Sat, Apr 18, 2015 at 06:58:30PM +0200, Andreas Cadhalpun wrote:
> > >> If begin is smaller than t, the subtraction 'begin -= t' wraps around,
> > >> because begin is unsigned. The same applies for end < t.
> > >>
> > >> This causes segmentation faults.
> > >>
> > >> Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun at googlemail.com>
> > >> ---
> > >>  libavcodec/alsdec.c | 8 ++++++++
> > >>  1 file changed, 8 insertions(+)
> > >>
> > >> diff --git a/libavcodec/alsdec.c b/libavcodec/alsdec.c
> > >> index f85f1e8..ff6b6cf 100644
> > >> --- a/libavcodec/alsdec.c
> > >> +++ b/libavcodec/alsdec.c
> > >> @@ -1290,8 +1290,16 @@ static int revert_channel_correlation(ALSDecContext *ctx, ALSBlockData *bd,
> > >>  
> > >>              if (ch[dep].time_diff_sign) {
> > >>                  t      = -t;
> > >> +                if (begin < t) {
> > >> +                    av_log(ctx->avctx, AV_LOG_ERROR, "begin %u smaller than time diff index %d.\n", begin, t);
> > >> +                    return AVERROR_INVALIDDATA;
> > >> +                }
> > > 
> > > begin is 1, t < 0 also the comparission is unsigned so t will overflow
> > > in it
> > 
> > Sorry, I sent the wrong version of the patch. Fixed one attached.
> > The t < 0 case is not problematic for the subtraction.
> [...]
> > @@ -1290,8 +1290,16 @@ static int revert_channel_correlation(ALSDecContext *ctx, ALSBlockData *bd,
> >  
> >              if (ch[dep].time_diff_sign) {
> >                  t      = -t;
> > +                if (t > 0 && begin < t) {
> 
> time_diff_index is always positive, so t is always negative here
> so this cant be true unless the context got corrupted or iam missing
> something

or its uninitialized

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

The educated differ from the uneducated as much as the living from the
dead. -- Aristotle 
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