[PATCH 4.2.y-ckt 017/268] time: Avoid signed overflow in timekeeping_get_ns()

From: Kamal Mostafa
Date: Wed Jan 27 2016 - 17:01:51 EST

4.2.8-ckt3 -stable review patch. If anyone has any objections, please let me know.


From: David Gibson <david@xxxxxxxxxxxxxxxxxxxxx>

commit 35a4933a895927990772ae96fdcfd2f806929ee2 upstream.

1e75fa8 "time: Condense timekeeper.xtime into xtime_sec" replaced a call to
clocksource_cyc2ns() from timekeeping_get_ns() with an open-coded version
of the same logic to avoid keeping a semi-redundant struct timespec
in struct timekeeper.

However, the commit also introduced a subtle semantic change - where
clocksource_cyc2ns() uses purely unsigned math, the new version introduces
a signed temporary, meaning that if (delta * tk->mult) has a 63-bit
overflow the following shift will still give a negative result. The
choice of 'maxsec' in __clocksource_updatefreq_scale() means this will
generally happen if there's a ~10 minute pause in examining the

This can be triggered on a powerpc KVM guest by stopping it from qemu for
a bit over 10 minutes. After resuming time has jumped backwards several
minutes causing numerous problems (jiffies does not advance, msleep()s can
be extended by minutes..). It doesn't happen on x86 KVM guests, because
the guest TSC is effectively frozen while the guest is stopped, which is
not the case for the powerpc timebase.

Obviously an unsigned (64 bit) overflow will only take twice as long as a
signed, 63-bit overflow. I don't know the time code well enough to know
if that will still cause incorrect calculations, or if a 64-bit overflow
is avoided elsewhere.

Still, an incorrect forwards clock adjustment will cause less trouble than
time going backwards. So, this patch removes the potential for
intermediate signed overflow.

Suggested-by: Laurent Vivier <lvivier@xxxxxxxxxx>
Tested-by: Laurent Vivier <lvivier@xxxxxxxxxx>
Signed-off-by: David Gibson <david@xxxxxxxxxxxxxxxxxxxxx>
Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx>
Signed-off-by: Kamal Mostafa <kamal@xxxxxxxxxxxxx>
kernel/time/timekeeping.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 3688f1e..afb50be 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -305,8 +305,7 @@ static inline s64 timekeeping_get_ns(struct tk_read_base *tkr)

delta = timekeeping_get_delta(tkr);

- nsec = delta * tkr->mult + tkr->xtime_nsec;
- nsec >>= tkr->shift;
+ nsec = (delta * tkr->mult + tkr->xtime_nsec) >> tkr->shift;

/* If arch requires, add in get_arch_timeoffset() */
return nsec + arch_gettimeoffset();