[PATCH 4.13 007/109] smp/hotplug: Handle removal correctly in cpuhp_store_callbacks()

[Greg Kroah-Hartman]

4.13-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Ethan Barnes <Ethan.Barnes@xxxxxxx>

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 ]

Fixes: 5b7aa87e0482 ("cpu/hotplug: Implement setup/removal interface")
Signed-off-by: Ethan Barnes <ethan.barnes@xxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxx>
Cc: "Srivatsa S. Bhat" <srivatsa@xxxxxxx>
Cc: Sebastian Siewior <bigeasy@xxxxxxxxxxxx>
Cc: Paul McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/DM2PR04MB398242FC7776D603D9F99C894A60@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

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

--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -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;