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

1 files changed, 19 insertions, 3 deletions

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 5404b2295820..5c95ccb59500 100644
--- a/drivers/md/md.c
+++ b/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...*/