md: Avoid waking up a thread after it has been freed.

Two related problems:

1/ some error paths call "md_unregister_thread(mddev->thread)"
   without subsequently clearing ->thread.  A subsequent call
   to mddev_unlock will try to wake the thread, and crash.

2/ Most calls to md_wakeup_thread are protected against the thread
   disappeared either by:
      - holding the ->mutex
      - having an active request, so something else must be keeping
        the array active.
   However mddev_unlock calls md_wakeup_thread after dropping the
   mutex and without any certainty of an active request, so the
   ->thread could theoretically disappear.
   So we need a spinlock to provide some protections.

So change md_unregister_thread to take a pointer to the thread
pointer, and ensure that it always does the required locking, and
clears the pointer properly.

Reported-by: "Moshe Melnikov" <moshe@zadarastorage.com>
Signed-off-by: NeilBrown <neilb@suse.de>
cc: stable@kernel.org

drivers/md/md.c

@@ -61,6 +61,11 @@
 static void autostart_arrays(int part);
 #endif
 
+/* pers_list is a list of registered personalities protected
+ * by pers_lock.
+ * pers_lock does extra service to protect accesses to
+ * mddev->thread when the mutex cannot be held.
+ */
 static LIST_HEAD(pers_list);
 static DEFINE_SPINLOCK(pers_lock);
 
@@ -739,7 +744,12 @@ static void mddev_unlock(mddev_t * mddev)
 	} else
 		mutex_unlock(&mddev->reconfig_mutex);
 
+	/* was we've dropped the mutex we need a spinlock to
+	 * make sur the thread doesn't disappear
+	 */
+	spin_lock(&pers_lock);
 	md_wakeup_thread(mddev->thread);
+	spin_unlock(&pers_lock);
 }
 
 static mdk_rdev_t * find_rdev_nr(mddev_t *mddev, int nr)
@@ -6429,11 +6439,18 @@ mdk_thread_t *md_register_thread(void (*run) (mddev_t *), mddev_t *mddev,
 	return thread;
 }
 
-void md_unregister_thread(mdk_thread_t *thread)
+void md_unregister_thread(mdk_thread_t **threadp)
 {
+	mdk_thread_t *thread = *threadp;
 	if (!thread)
 		return;
 	dprintk("interrupting MD-thread pid %d\n", task_pid_nr(thread->tsk));
+	/* Locking ensures that mddev_unlock does not wake_up a
+	 * non-existent thread
+	 */
+	spin_lock(&pers_lock);
+	*threadp = NULL;
+	spin_unlock(&pers_lock);
 
 	kthread_stop(thread->tsk);
 	kfree(thread);
@@ -7340,8 +7357,7 @@ static void reap_sync_thread(mddev_t *mddev)
 	mdk_rdev_t *rdev;
 
 	/* resync has finished, collect result */
-	md_unregister_thread(mddev->sync_thread);
-	mddev->sync_thread = NULL;
+	md_unregister_thread(&mddev->sync_thread);
 	if (!test_bit(MD_RECOVERY_INTR, &mddev->recovery) &&
 	    !test_bit(MD_RECOVERY_REQUESTED, &mddev->recovery)) {
 		/* success...*/

drivers/md/md.h

@@ -560,7 +560,7 @@ extern int register_md_personality(struct mdk_personality *p);
 extern int unregister_md_personality(struct mdk_personality *p);
 extern mdk_thread_t * md_register_thread(void (*run) (mddev_t *mddev),
 				mddev_t *mddev, const char *name);
-extern void md_unregister_thread(mdk_thread_t *thread);
+extern void md_unregister_thread(mdk_thread_t **threadp);
 extern void md_wakeup_thread(mdk_thread_t *thread);
 extern void md_check_recovery(mddev_t *mddev);
 extern void md_write_start(mddev_t *mddev, struct bio *bi);

drivers/md/multipath.c

@@ -514,8 +514,7 @@ static int multipath_stop (mddev_t *mddev)
 {
 	multipath_conf_t *conf = mddev->private;
 
-	md_unregister_thread(mddev->thread);
-	mddev->thread = NULL;
+	md_unregister_thread(&mddev->thread);
 	blk_sync_queue(mddev->queue); /* the unplug fn references 'conf'*/
 	mempool_destroy(conf->pool);
 	kfree(conf->multipaths);

drivers/md/raid1.c

@@ -2562,8 +2562,7 @@ static int stop(mddev_t *mddev)
 	raise_barrier(conf);
 	lower_barrier(conf);
 
-	md_unregister_thread(mddev->thread);
-	mddev->thread = NULL;
+	md_unregister_thread(&mddev->thread);
 	if (conf->r1bio_pool)
 		mempool_destroy(conf->r1bio_pool);
 	kfree(conf->mirrors);

drivers/md/raid10.c

@@ -2955,7 +2955,7 @@ static int run(mddev_t *mddev)
 	return 0;
 
 out_free_conf:
-	md_unregister_thread(mddev->thread);
+	md_unregister_thread(&mddev->thread);
 	if (conf->r10bio_pool)
 		mempool_destroy(conf->r10bio_pool);
 	safe_put_page(conf->tmppage);
@@ -2973,8 +2973,7 @@ static int stop(mddev_t *mddev)
 	raise_barrier(conf, 0);
 	lower_barrier(conf);
 
-	md_unregister_thread(mddev->thread);
-	mddev->thread = NULL;
+	md_unregister_thread(&mddev->thread);
 	blk_sync_queue(mddev->queue); /* the unplug fn references 'conf'*/
 	if (conf->r10bio_pool)
 		mempool_destroy(conf->r10bio_pool);

drivers/md/raid5.c

@@ -4941,8 +4941,7 @@ static int run(mddev_t *mddev)
 
 	return 0;
 abort:
-	md_unregister_thread(mddev->thread);
-	mddev->thread = NULL;
+	md_unregister_thread(&mddev->thread);
 	if (conf) {
 		print_raid5_conf(conf);
 		free_conf(conf);
@@ -4956,8 +4955,7 @@ static int stop(mddev_t *mddev)
 {
 	raid5_conf_t *conf = mddev->private;
 
-	md_unregister_thread(mddev->thread);
-	mddev->thread = NULL;
+	md_unregister_thread(&mddev->thread);
 	if (mddev->queue)
 		mddev->queue->backing_dev_info.congested_fn = NULL;
 	free_conf(conf);