From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757252AbcBCXx4 (ORCPT ); Wed, 3 Feb 2016 18:53:56 -0500 Received: from youngberry.canonical.com ([91.189.89.112]:46490 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756820AbcBCWej (ORCPT ); Wed, 3 Feb 2016 17:34:39 -0500 From: Luis Henriques To: linux-kernel@vger.kernel.org, stable@vger.kernel.org, kernel-team@lists.ubuntu.com Cc: David Gibson , John Stultz , Luis Henriques Subject: [PATCH 3.16.y-ckt 032/180] time: Avoid signed overflow in timekeeping_get_ns() Date: Wed, 3 Feb 2016 22:30:38 +0000 Message-Id: <1454538786-12215-33-git-send-email-luis.henriques@canonical.com> In-Reply-To: <1454538786-12215-1-git-send-email-luis.henriques@canonical.com> References: <1454538786-12215-1-git-send-email-luis.henriques@canonical.com> X-Extended-Stable: 3.16 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 3.16.7-ckt24 -stable review patch. If anyone has any objections, please let me know. ---8<------------------------------------------------------------ From: David Gibson 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 clocksource. 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 Tested-by: Laurent Vivier Signed-off-by: David Gibson Signed-off-by: John Stultz [ luis: backported to 3.16: adjusted context ] Signed-off-by: Luis Henriques --- 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 32d8d6aaedb8..268428930c99 100644 --- a/kernel/time/timekeeping.c +++ b/kernel/time/timekeeping.c @@ -178,8 +178,7 @@ static inline s64 timekeeping_get_ns(struct timekeeper *tk) /* calculate the delta since the last update_wall_time: */ cycle_delta = (cycle_now - clock->cycle_last) & clock->mask; - nsec = cycle_delta * tk->mult + tk->xtime_nsec; - nsec >>= tk->shift; + nsec = (cycle_delta * tk->mult + tk->xtime_nsec) >> tk->shift; /* If arch requires, add in get_arch_timeoffset() */ return nsec + get_arch_timeoffset();