[PATCH 2/2] PM: cpu: Restore synchronize_rcu() to cpu_pm_unregister_notifier()

[Bradley Morgan <include@grrlz.net>]

cpu_pm_notify() walks the notifier chain lockless under only
rcu_read_lock(). That only works if a removed notifier block is not
freed until every concurrent walker is done with it.

The chain used to be an atomic_notifier, whose unregister ends in
synchronize_rcu(). Commit b2f6662ac08d ("PM: cpu: Make notifier chain
use a raw_spinlock_t") switched it over to a raw_notifier, and in
doing so replaced that with raw_notifier_chain_unregister(), which
does not synchronize. The grace period quietly went away, so a driver
that frees the memory holding its notifier block right after
cpu_pm_unregister_notifier() returns can race with a concurrent
walker:

  cpu1 (idle exit)                  cpu2 (driver remove)
  ----------------                  --------------------
  cpu_pm_notify(CPU_PM_EXIT)
    rcu_read_lock()
    nb = rcu_dereference_raw(*nl)
                                    cpu_pm_unregister_notifier(&od->nb)
                                      unlink, no grace period
                                      remove() frees od (devm)
    nb->notifier_call(nb, ...)
      /* use after free */

The rcu_read_lock() on cpu1 does not stop cpu2 from freeing the
block, and nothing else does. The read side is the idle exit path,
so this can hit on any system where a driver with a cpu_pm notifier
gets unbound.

Add the grace period back. It is only needed when a notifier was
actually removed, so wait on success and return -ENOENT without
waiting otherwise. The kerneldoc gets back the "may sleep" note that
the same commit dropped.

Fixes: b2f6662ac08d ("PM: cpu: Make notifier chain use a raw_spinlock_t")
Cc: stable@vger.kernel.org
Signed-off-by: Bradley Morgan <include@grrlz.net>
---
 kernel/cpu_pm.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/cpu_pm.c b/kernel/cpu_pm.c
index 7481fbb947d3..a2a598ad6e6d 100644
--- a/kernel/cpu_pm.c
+++ b/kernel/cpu_pm.c
@@ -10,6 +10,7 @@
 #include <linux/cpu_pm.h>
 #include <linux/module.h>
 #include <linux/notifier.h>
+#include <linux/rcupdate.h>
 #include <linux/spinlock.h>
 #include <linux/syscore_ops.h>
 
@@ -76,7 +77,8 @@ EXPORT_SYMBOL_GPL(cpu_pm_register_notifier);
  *
  * Remove a driver from the CPU PM notifier list.
  *
- * This function has the same return conditions as raw_notifier_chain_unregister.
+ * This function may sleep, and has the same return conditions as
+ * raw_notifier_chain_unregister.
  */
 int cpu_pm_unregister_notifier(struct notifier_block *nb)
 {
@@ -86,6 +88,11 @@ int cpu_pm_unregister_notifier(struct notifier_block *nb)
 	raw_spin_lock_irqsave(&cpu_pm_notifier.lock, flags);
 	ret = raw_notifier_chain_unregister(&cpu_pm_notifier.chain, nb);
 	raw_spin_unlock_irqrestore(&cpu_pm_notifier.lock, flags);
+
+	/* Wait for the rcu_read_lock() walkers in cpu_pm_notify(). */
+	if (!ret)
+		synchronize_rcu();
+
 	return ret;
 }
 EXPORT_SYMBOL_GPL(cpu_pm_unregister_notifier);
-- 
2.47.3