eventpoll: kill __ep_remove()

Remove the boolean conditional in __ep_remove() and restructure the code
so the check for racing with eventpoll_release_file() are only done in
the ep_remove_safe() path where they belong.

Link: https://patch.msgid.link/20260423-work-epoll-uaf-v1-3-2470f9eec0f5@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

Author: brauner

fs/eventpoll.c

@@ -826,49 +826,18 @@ static void ep_free(struct eventpoll *ep)
 	kfree_rcu(ep, rcu);
 }
 
-static void __ep_remove_file(struct eventpoll *ep, struct epitem *epi, struct file *file);
-static bool __ep_remove_epi(struct eventpoll *ep, struct epitem *epi);
-
-/*
- * Removes a "struct epitem" from the eventpoll RB tree and deallocates
- * all the associated resources. Must be called with "mtx" held.
- * If the dying flag is set, do the removal only if force is true.
- * This prevents ep_clear_and_put() from dropping all the ep references
- * while running concurrently with eventpoll_release_file().
- * Returns true if the eventpoll can be disposed.
- */
-static bool __ep_remove(struct eventpoll *ep, struct epitem *epi, bool force)
-{
-	struct file *file = epi->ffd.file;
-
-	lockdep_assert_irqs_enabled();
-
-	/*
-	 * Removes poll wait queue hooks.
-	 */
-	ep_unregister_pollwait(ep, epi);
-
-	/* Remove the current item from the list of epoll hooks */
-	spin_lock(&file->f_lock);
-	if (epi->dying && !force) {
-		spin_unlock(&file->f_lock);
-		return false;
-	}
-
-	__ep_remove_file(ep, epi, file);
-	return __ep_remove_epi(ep, epi);
-}
-
 /*
  * Called with &file->f_lock held,
  * returns with it released
  */
-static void __ep_remove_file(struct eventpoll *ep, struct epitem *epi, struct file *file)
+static void __ep_remove_file(struct eventpoll *ep, struct epitem *epi,
+			     struct file *file)
 {
 	struct epitems_head *to_free = NULL;
 	struct hlist_head *head = file->f_ep;
 
 	lockdep_assert_held(&ep->mtx);
+	lockdep_assert_held(&file->f_lock);
 
 	if (hlist_is_singular_node(&epi->fllink, head)) {
 		/* See eventpoll_release() for details. */
@@ -915,7 +884,25 @@ static bool __ep_remove_epi(struct eventpoll *ep, struct epitem *epi)
  */
 static void ep_remove_safe(struct eventpoll *ep, struct epitem *epi)
 {
-	if (__ep_remove(ep, epi, false))
+	struct file *file = epi->ffd.file;
+
+	lockdep_assert_irqs_enabled();
+	lockdep_assert_held(&ep->mtx);
+
+	ep_unregister_pollwait(ep, epi);
+
+	/* sync with eventpoll_release_file() */
+	if (unlikely(READ_ONCE(epi->dying)))
+		return;
+
+	spin_lock(&file->f_lock);
+	if (epi->dying) {
+		spin_unlock(&file->f_lock);
+		return;
+	}
+	__ep_remove_file(ep, epi, file);
+
+	if (__ep_remove_epi(ep, epi))
 		WARN_ON_ONCE(ep_refcount_dec_and_test(ep));
 }
 
@@ -1147,7 +1134,7 @@ void eventpoll_release_file(struct file *file)
 	spin_lock(&file->f_lock);
 	if (file->f_ep && file->f_ep->first) {
 		epi = hlist_entry(file->f_ep->first, struct epitem, fllink);
-		epi->dying = true;
+		WRITE_ONCE(epi->dying, true);
 		spin_unlock(&file->f_lock);
 
 		/*
@@ -1156,7 +1143,13 @@ void eventpoll_release_file(struct file *file)
 		 */
 		ep = epi->ep;
 		mutex_lock(&ep->mtx);
-		dispose = __ep_remove(ep, epi, true);
+
+		ep_unregister_pollwait(ep, epi);
+
+		spin_lock(&file->f_lock);
+		__ep_remove_file(ep, epi, file);
+		dispose = __ep_remove_epi(ep, epi);
+
 		mutex_unlock(&ep->mtx);
 
 		if (dispose && ep_refcount_dec_and_test(ep))