Commit 8614ddef authored Nov 05, 2015 by Ulrich Obergfell Committed by Linus Torvalds Nov 05, 2015

kernel/watchdog.c: avoid races between /proc handlers and CPU hotplug



The handler functions for watchdog parameters in /proc/sys/kernel do not
protect themselves against races with CPU hotplug.  Hence, theoretically
it is possible that a new watchdog thread is started on a hotplugged CPU
while a parameter is being modified, and the thread could thus use a
parameter value that is 'in transition'.

For example, if 'watchdog_thresh' is being set to zero (note: this
disables the lockup detectors) the thread would erroneously use the value
zero as the sample period.

To avoid such races and to keep the /proc handler code consistent,
call
     {get|put}_online_cpus() in proc_watchdog_common()
     {get|put}_online_cpus() in proc_watchdog_thresh()
     {get|put}_online_cpus() in proc_watchdog_cpumask()

Signed-off-by: Ulrich Obergfell <uobergfe@redhat.com>
Acked-by: Don Zickus <dzickus@redhat.com>
Reviewed-by: Aaron Tomlin <atomlin@redhat.com>
Cc: Ulrich Obergfell <uobergfe@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent ee89e71e

kernel/watchdog.c

+6 −0

Original line number	Diff line number	Diff line
		@@ -857,6 +857,7 @@ static int proc_watchdog_common(int which, struct ctl_table *table, int write,
		int err, old, new;
		int watchdog_param = (int )table->data;

		get_online_cpus();
		mutex_lock(&watchdog_proc_mutex);

		if (watchdog_suspended) {
		@@ -908,6 +909,7 @@ static int proc_watchdog_common(int which, struct ctl_table *table, int write,
		}
		out:
		mutex_unlock(&watchdog_proc_mutex);
		put_online_cpus();
		return err;
		}

		@@ -949,6 +951,7 @@ int proc_watchdog_thresh(struct ctl_table *table, int write,
		{
		int err, old;

		get_online_cpus();
		mutex_lock(&watchdog_proc_mutex);

		if (watchdog_suspended) {
		@@ -974,6 +977,7 @@ int proc_watchdog_thresh(struct ctl_table *table, int write,
		}
		out:
		mutex_unlock(&watchdog_proc_mutex);
		put_online_cpus();
		return err;
		}

		@@ -988,6 +992,7 @@ int proc_watchdog_cpumask(struct ctl_table *table, int write,
		{
		int err;

		get_online_cpus();
		mutex_lock(&watchdog_proc_mutex);

		if (watchdog_suspended) {
		@@ -1015,6 +1020,7 @@ int proc_watchdog_cpumask(struct ctl_table *table, int write,
		}
		out:
		mutex_unlock(&watchdog_proc_mutex);
		put_online_cpus();
		return err;
		}

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