Patch "smp/hotplug: Handle removal correctly in cpuhp_store_callbacks()" has been added to the 4.13-stable tree

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

smp/hotplug: Handle removal correctly in cpuhp_store_callbacks()

to the 4.13-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:
smp-hotplug-handle-removal-correctly-in-cpuhp_store_callbacks.patch
and it can be found in the queue-4.13 subdirectory.

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

From: Ethan Barnes <[email protected]>
Date: Wed, 19 Jul 2017 22:36:00 +0000
Subject: smp/hotplug: Handle removal correctly in cpuhp_store_callbacks()

From: Ethan Barnes <[email protected]>

commit 0c96b27305faf06c068b45e07d28336c80dac286 upstream.

If cpuhp_store_callbacks() is called for CPUHP_AP_ONLINE_DYN or CPUHP_BP_PREPARE_DYN, which are the indicators for dynamically allocated states, then cpuhp_store_callbacks() allocates a new dynamic state. The first allocation in each range returns CPUHP_AP_ONLINE_DYN or CPUHP_BP_PREPARE_DYN.

If cpuhp_remove_state() is invoked for one of these states, then there is no protection against the allocation mechanism. So the removal, which should clear the callbacks and the name, gets a new state assigned and clears that one.

As a consequence the state which should be cleared stays initialized. A consecutive CPU hotplug operation dereferences the state callbacks and accesses either freed or reused memory, resulting in crashes.

Add a protection against this by checking the name argument for NULL. If it's NULL it's a removal. If not, it's an allocation.

[ tglx: Added a comment and massaged changelog ]

---
 kernel/cpu.c |   12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

@@ -1252,7 +1252,17 @@ static int cpuhp_store_callbacks(enum cp
 	struct cpuhp_step *sp;
 	int ret = 0;
 
-	if (state == CPUHP_AP_ONLINE_DYN || state == CPUHP_BP_PREPARE_DYN) {
+	/*
+	 * If name is NULL, then the state gets removed.
+	 *
+	 * CPUHP_AP_ONLINE_DYN and CPUHP_BP_PREPARE_DYN are handed out on
+	 * the first allocation from these dynamic ranges, so the removal
+	 * would trigger a new allocation and clear the wrong (already
+	 * empty) state, leaving the callbacks of the to be cleared state
+	 * dangling, which causes wreckage on the next hotplug operation.
+	 */
+	if (name && (state == CPUHP_AP_ONLINE_DYN ||
+		     state == CPUHP_BP_PREPARE_DYN)) {
 		ret = cpuhp_reserve_state(state);
 		if (ret < 0)
 			return ret;

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

queue-4.13/smp-hotplug-handle-removal-correctly-in-cpuhp_store_callbacks.patch