[PATCH 5.10 07/65] watch_queue: Fix missing locking in add_watch_to_object()

[Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>]

From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>

commit e64ab2dbd882933b65cd82ff6235d705ad65dbb6 upstream.

If a watch is being added to a queue, it needs to guard against
interference from addition of a new watch, manual removal of a watch and
removal of a watch due to some other queue being destroyed.

KEYCTL_WATCH_KEY guards against this for the same {key,queue} pair by
holding the key->sem writelocked and by holding refs on both the key and
the queue - but that doesn't prevent interaction from other {key,queue}
pairs.

While add_watch_to_object() does take the spinlock on the event queue,
it doesn't take the lock on the source's watch list.  The assumption was
that the caller would prevent that (say by taking key->sem) - but that
doesn't prevent interference from the destruction of another queue.

Fix this by locking the watcher list in add_watch_to_object().

Fixes: c73be61cede5 ("pipe: Add general notification queue support")
Reported-by: syzbot+03d7b43290037d1f87ca@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
cc: keyrings@xxxxxxxxxxxxxxx
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 kernel/watch_queue.c |   58 +++++++++++++++++++++++++++++++--------------------
 1 file changed, 36 insertions(+), 22 deletions(-)

--- a/kernel/watch_queue.c
+++ b/kernel/watch_queue.c
@@ -457,6 +457,33 @@ void init_watch(struct watch *watch, str
 	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
@@ -471,34 +498,21 @@ void init_watch(struct watch *watch, str
  */
 int add_watch_to_object(struct watch *watch, struct watch_list *wlist)
 {
-	struct watch_queue *wqueue = rcu_access_pointer(watch->queue);
-	struct watch *w;
+	struct watch_queue *wqueue;
+	int ret = -ENOENT;
 
-	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;
-	}
-
-	watch->cred = get_current_cred();
-	rcu_assign_pointer(watch->watch_list, wlist);
-
-	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;
-	}
+	rcu_read_lock();
 
+	wqueue = rcu_access_pointer(watch->queue);
 	if (lock_wqueue(wqueue)) {
-		kref_get(&wqueue->usage);
-		kref_get(&watch->usage);
-		hlist_add_head(&watch->queue_node, &wqueue->watches);
+		spin_lock(&wlist->lock);
+		ret = add_one_watch(watch, wlist, wqueue);
+		spin_unlock(&wlist->lock);
 		unlock_wqueue(wqueue);
 	}
 
-	hlist_add_head_rcu(&watch->list_node, &wlist->watchers);
-	return 0;
+	rcu_read_unlock();
+	return ret;
 }
 EXPORT_SYMBOL(add_watch_to_object);