1 files changed, 69 insertions, 34 deletions

diff --git a/kernel/watch_queue.c b/kernel/watch_queue.c
index 869fea4fe26b..a6f9bdd956c3 100644
--- a/kernel/watch_queue.c
+++ b/kernel/watch_queue.c
@@ -34,6 +34,27 @@ MODULE_LICENSE("GPL");
 #define WATCH_QUEUE_NOTE_SIZE 128
 #define WATCH_QUEUE_NOTES_PER_PAGE (PAGE_SIZE / WATCH_QUEUE_NOTE_SIZE)
 
+/*
+ * This must be called under the RCU read-lock, which makes
+ * sure that the wqueue still exists. It can then take the lock,
+ * and check that the wqueue hasn't been destroyed, which in
+ * turn makes sure that the notification pipe still exists.
+ */
+static inline bool lock_wqueue(struct watch_queue *wqueue)
+{
+	spin_lock_bh(&wqueue->lock);
+	if (unlikely(wqueue->defunct)) {
+		spin_unlock_bh(&wqueue->lock);
+		return false;
+	}
+	return true;
+}
+
+static inline void unlock_wqueue(struct watch_queue *wqueue)
+{
+	spin_unlock_bh(&wqueue->lock);
+}
+
 static void watch_queue_pipe_buf_release(struct pipe_inode_info *pipe,
 					 struct pipe_buffer *buf)
 {
@@ -69,6 +90,10 @@ static const struct pipe_buf_operations watch_queue_pipe_buf_ops = {
 
 /*
  * Post a notification to a watch queue.
+ *
+ * Must be called with the RCU lock for reading, and the
+ * watch_queue lock held, which guarantees that the pipe
+ * hasn't been released.
  */
 static bool post_one_notification(struct watch_queue *wqueue,
 				  struct watch_notification *n)
@@ -85,9 +110,6 @@ static bool post_one_notification(struct watch_queue *wqueue,
 
 	spin_lock_irq(&pipe->rd_wait.lock);
 
-	if (wqueue->defunct)
-		goto out;
-
 	mask = pipe->ring_size - 1;
 	head = pipe->head;
 	tail = pipe->tail;
@@ -203,7 +225,10 @@ void __post_watch_notification(struct watch_list *wlist,
 		if (security_post_notification(watch->cred, cred, n) < 0)
 			continue;
 
-		post_one_notification(wqueue, n);
+		if (lock_wqueue(wqueue)) {
+			post_one_notification(wqueue, n);
+			unlock_wqueue(wqueue);
+		}
 	}
 
 	rcu_read_unlock();
@@ -429,6 +454,33 @@ void init_watch(struct watch *watch, struct watch_queue *wqueue)
 	rcu_assign_pointer(watch->queue, wqueue);
 }
 
+static int add_one_watch(struct watch *watch, struct watch_list *wlist, struct watch_queue *wqueue)
+{
+	const struct cred *cred;
+	struct watch *w;
+
+	hlist_for_each_entry(w, &wlist->watchers, list_node) {
+		struct watch_queue *wq = rcu_access_pointer(w->queue);
+		if (wqueue == wq && watch->id == w->id)
+			return -EBUSY;
+	}
+
+	cred = current_cred();
+	if (atomic_inc_return(&cred->user->nr_watches) > task_rlimit(current, RLIMIT_NOFILE)) {
+		atomic_dec(&cred->user->nr_watches);
+		return -EAGAIN;
+	}
+
+	watch->cred = get_cred(cred);
+	rcu_assign_pointer(watch->watch_list, wlist);
+
+	kref_get(&wqueue->usage);
+	kref_get(&watch->usage);
+	hlist_add_head(&watch->queue_node, &wqueue->watches);
+	hlist_add_head_rcu(&watch->list_node, &wlist->watchers);
+	return 0;
+}
+
 /**
  * add_watch_to_object - Add a watch on an object to a watch list
  * @watch: The watch to add
@@ -443,33 +495,21 @@ void init_watch(struct watch *watch, struct watch_queue *wqueue)
  */
 int add_watch_to_object(struct watch *watch, struct watch_list *wlist)
 {
-	struct watch_queue *wqueue = rcu_access_pointer(watch->queue);
-	struct watch *w;
-
-	hlist_for_each_entry(w, &wlist->watchers, list_node) {
-		struct watch_queue *wq = rcu_access_pointer(w->queue);
-		if (wqueue == wq && watch->id == w->id)
-			return -EBUSY;
-	}
+	struct watch_queue *wqueue;
+	int ret = -ENOENT;
 
-	watch->cred = get_current_cred();
-	rcu_assign_pointer(watch->watch_list, wlist);
+	rcu_read_lock();
 
-	if (atomic_inc_return(&watch->cred->user->nr_watches) >
-	    task_rlimit(current, RLIMIT_NOFILE)) {
-		atomic_dec(&watch->cred->user->nr_watches);
-		put_cred(watch->cred);
-		return -EAGAIN;
+	wqueue = rcu_access_pointer(watch->queue);
+	if (lock_wqueue(wqueue)) {
+		spin_lock(&wlist->lock);
+		ret = add_one_watch(watch, wlist, wqueue);
+		spin_unlock(&wlist->lock);
+		unlock_wqueue(wqueue);
 	}
 
-	spin_lock_bh(&wqueue->lock);
-	kref_get(&wqueue->usage);
-	kref_get(&watch->usage);
-	hlist_add_head(&watch->queue_node, &wqueue->watches);
-	spin_unlock_bh(&wqueue->lock);
-
-	hlist_add_head(&watch->list_node, &wlist->watchers);
-	return 0;
+	rcu_read_unlock();
+	return ret;
 }
 EXPORT_SYMBOL(add_watch_to_object);
 
@@ -520,20 +560,15 @@ found:
 
 	wqueue = rcu_dereference(watch->queue);
 
-	/* We don't need the watch list lock for the next bit as RCU is
-	 * protecting *wqueue from deallocation.
-	 */
-	if (wqueue) {
+	if (lock_wqueue(wqueue)) {
 		post_one_notification(wqueue, &n.watch);
 
-		spin_lock_bh(&wqueue->lock);
-
 		if (!hlist_unhashed(&watch->queue_node)) {
 			hlist_del_init_rcu(&watch->queue_node);
 			put_watch(watch);
 		}
 
-		spin_unlock_bh(&wqueue->lock);
+		unlock_wqueue(wqueue);
 	}
 
 	if (wlist->release_watch) {