[ 3.5.y.z extended stable ] Patch "hrtimer: Fix ktime_add_ns() overflow on 32bit architectures" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    hrtimer: Fix ktime_add_ns() overflow on 32bit architectures

to the linux-3.5.y-queue branch of the 3.5.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.5.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.5.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

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>From b454564b977ff855e38ec1bd5d76de3fb65bba0b Mon Sep 17 00:00:00 2001
From: David Engraf <david.engraf at sysgo.com>
Date: Tue, 19 Mar 2013 13:29:55 +0100
Subject: [PATCH] hrtimer: Fix ktime_add_ns() overflow on 32bit architectures

commit 51fd36f3fad8447c487137ae26b9d0b3ce77bb25 upstream.

One can trigger an overflow when using ktime_add_ns() on a 32bit
architecture not supporting CONFIG_KTIME_SCALAR.

When passing a very high value for u64 nsec, e.g. 7881299347898368000
the do_div() function converts this value to seconds (7881299347) which
is still to high to pass to the ktime_set() function as long. The result
in is a negative value.

The problem on my system occurs in the tick-sched.c,
tick_nohz_stop_sched_tick() when time_delta is set to
timekeeping_max_deferment(). The check for time_delta < KTIME_MAX is
valid, thus ktime_add_ns() is called with a too large value resulting in
a negative expire value. This leads to an endless loop in the ticker code:

time_delta: 7881299347898368000
expires = ktime_add_ns(last_update, time_delta)
expires: negative value

This fix caps the value to KTIME_MAX.

This error doesn't occurs on 64bit or architectures supporting
CONFIG_KTIME_SCALAR (e.g. ARM, x86-32).

Signed-off-by: David Engraf <david.engraf at sysgo.com>
[jstultz: Minor tweaks to commit message & header]
Signed-off-by: John Stultz <john.stultz at linaro.org>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 kernel/hrtimer.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
index 97d3742..60f7e32 100644
--- a/kernel/hrtimer.c
+++ b/kernel/hrtimer.c
@@ -298,6 +298,10 @@ ktime_t ktime_sub_ns(const ktime_t kt, u64 nsec)
 	} else {
 		unsigned long rem = do_div(nsec, NSEC_PER_SEC);

+		/* Make sure nsec fits into long */
+		if (unlikely(nsec > KTIME_SEC_MAX))
+			return (ktime_t){ .tv64 = KTIME_MAX };
+
 		tmp = ktime_set((long)nsec, rem);
 	}

--
1.8.1.2