Patch "cpufreq: Fix governor module removal race" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

cpufreq: Fix governor module removal race

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
cpufreq-fix-governor-module-removal-race.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: "Rafael J. Wysocki" <[email protected]>
Date: Thu, 23 Nov 2017 14:27:07 +0100
Subject: cpufreq: Fix governor module removal race

From: "Rafael J. Wysocki" <[email protected]>

[ Upstream commit a8b149d32b663c1a4105273295184b78f53d33cf ]

It is possible to remove a cpufreq governor module after cpufreq_parse_governor() has returned success in store_scaling_governor() and before cpufreq_set_policy() acquires a reference to it, because the governor list is not protected during that period and nothing prevents the governor from being unregistered then.

Prevent that from happening by acquiring an extra reference to the governor module temporarily in cpufreq_parse_governor(), under cpufreq_governor_mutex, and dropping it in store_scaling_governor(), when cpufreq_set_policy() returns.

Note that the second cpufreq_parse_governor() call site is fine, because it only cares about the policy member of new_policy.

---
 drivers/cpufreq/cpufreq.c |    6 ++++++
 1 file changed, 6 insertions(+)

@@ -551,6 +551,8 @@ static int cpufreq_parse_governor(char *
 			*governor = t;
 			err = 0;
 		}
+		if (t && !try_module_get(t->owner))
+			t = NULL;
 
 		mutex_unlock(&cpufreq_governor_mutex);
 	}

@@ -669,6 +671,10 @@ static ssize_t store_scaling_governor(st
 		return -EINVAL;
 
 	ret = cpufreq_set_policy(policy, &new_policy);
+
+	if (new_policy.governor)
+		module_put(new_policy.governor->owner);
+
 	return ret ? ret : count;
 }
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/cpufreq-fix-governor-module-removal-race.patch