clocksource: Prevent NULL pointer dereference

Writing a zero length string to sys/.../current_clocksource will cause
a NULL pointer dereference if the clock events system is in one shot
(highres or nohz) mode.

Pointed-out-by: Dan Carpenter <error27@gmail.com>
LKML-Reference: <alpine.DEB.2.00.0907191545580.12306@bicker>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 1 insertions, 1 deletions

diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index 592bf584d1d2..7466cb811251 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -513,7 +513,7 @@ static ssize_t sysfs_override_clocksource(struct sys_device *dev,
          * Check to make sure we don't switch to a non-highres capable
          * clocksource if the tick code is in oneshot mode (highres or nohz)
          */
-        if (tick_oneshot_mode_active() &&
+        if (tick_oneshot_mode_active() && ovr &&
             !(ovr->flags & CLOCK_SOURCE_VALID_FOR_HRES)) {
                 printk(KERN_WARNING "%s clocksource is not HRT compatible. "
                         "Cannot switch while in HRT/NOHZ mode\n", ovr->name);