[PATCH 5.18 154/158] watchqueue: make sure to serialize wqueue->defunct properly

Greg Kroah-Hartman
2022-07-27T17:46:34Z

From: Linus Torvalds <torvalds@linux-foundation.org>

commit 353f7988dd8413c47718f7ca79c030b6fb62cfe5 upstream.

When the pipe is closed, we mark the associated watchqueue defunct by
calling watch_queue_clear(). However, while that is protected by the
watchqueue lock, new watchqueue entries aren't actually added under that
lock at all: they use the pipe->rd_wait.lock instead, and looking up
that pipe happens without any locking.

The watchqueue code uses the RCU read-side section to make sure that the
wqueue entry itself hasn't disappeared, but that does not protect the
pipe_info in any way.

So make sure to actually hold the wqueue lock when posting watch events,
properly serializing against the pipe being torn down.

Reported-by: Noam Rathaus <noamr@ssd-disclosure.com>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
kernel/watch_queue.c | 53 +++++++++++++++++++++++++++++++++++----------------
1 file changed, 37 insertions(+), 16 deletions(-)

--- 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

/*
* 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

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 wa
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();
@@ -462,11 +487,12 @@ int add_watch_to_object(struct watch *wa
return -EAGAIN;
}

- 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);
+ if (lock_wqueue(wqueue)) {
+ kref_get(&wqueue->usage);
+ kref_get(&watch->usage);
+ hlist_add_head(&watch->queue_node, &wqueue->watches);
+ unlock_wqueue(wqueue);
+ }

hlist_add_head(&watch->list_node, &wlist->watchers);
return 0;
@@ -520,20 +546,15 @@ 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) {