cpufreq: powernv: Fix use-after-free

commit d0a72ef upstream.

The cpufreq driver has a use-after-free that we can hit if:

a) There's an OCC message pending when the notifier is registered, and
b) The cpufreq driver fails to register with the core.

When a) occurs the notifier schedules a workqueue item to handle the
message. The backing work_struct is located on chips[].throttle and
when b) happens we clean up by freeing the array. Once we get to
the (now free) queued item and the kernel crashes.

Fixes: c5e29ea ("cpufreq: powernv: Fix bugs in powernv_cpufreq_{init/exit}")
Cc: [email protected] # v4.6+
Signed-off-by: Oliver O'Halloran <[email protected]>
Reviewed-by: Gautham R. Shenoy <[email protected]>
Signed-off-by: Michael Ellerman <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: oohal

drivers/cpufreq/powernv-cpufreq.c

@@ -1080,6 +1080,12 @@ static int init_chip_info(void)
 
 static inline void clean_chip_info(void)
 {
+	int i;
+
+	/* flush any pending work items */
+	if (chips)
+		for (i = 0; i < nr_chips; i++)
+			cancel_work_sync(&chips[i].throttle);
 	kfree(chips);
 }