fanotify: fix fsnotify_prepare_user_wait() failure

If fsnotify_prepare_user_wait() fails, we leave the event on the
notification list.  Which will result in a warning in
fsnotify_destroy_event() and later use-after-free.

Instead of adding a new helper to remove the event from the list in this
case, I opted to move the prepare/finish up into fanotify_handle_event().

This will allow these to be moved further out into the generic code later,
and perhaps let us move to non-sleeping RCU.

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Fixes: 05f0e38724e8 ("fanotify: Release SRCU lock when waiting for userspace response")
Cc: <stable@vger.kernel.org> # v4.12
Signed-off-by: Jan Kara <jack@suse.cz>

fs/notify/fanotify/fanotify.c

@@ -64,19 +64,8 @@ static int fanotify_get_response(struct fsnotify_group *group,
 
 	pr_debug("%s: group=%p event=%p\n", __func__, group, event);
 
-	/*
-	 * fsnotify_prepare_user_wait() fails if we race with mark deletion.
-	 * Just let the operation pass in that case.
-	 */
-	if (!fsnotify_prepare_user_wait(iter_info)) {
-		event->response = FAN_ALLOW;
-		goto out;
-	}
-
 	wait_event(group->fanotify_data.access_waitq, event->response);
 
-	fsnotify_finish_user_wait(iter_info);
-out:
 	/* userspace responded, convert to something usable */
 	switch (event->response) {
 	case FAN_ALLOW:
@@ -211,9 +200,21 @@ static int fanotify_handle_event(struct fsnotify_group *group,
 	pr_debug("%s: group=%p inode=%p mask=%x\n", __func__, group, inode,
 		 mask);
 
+#ifdef CONFIG_FANOTIFY_ACCESS_PERMISSIONS
+	if (mask & FAN_ALL_PERM_EVENTS) {
+		/*
+		 * fsnotify_prepare_user_wait() fails if we race with mark
+		 * deletion.  Just let the operation pass in that case.
+		 */
+		if (!fsnotify_prepare_user_wait(iter_info))
+			return 0;
+	}
+#endif
+
 	event = fanotify_alloc_event(inode, mask, data);
+	ret = -ENOMEM;
 	if (unlikely(!event))
-		return -ENOMEM;
+		goto finish;
 
 	fsn_event = &event->fse;
 	ret = fsnotify_add_event(group, fsn_event, fanotify_merge);
@@ -223,7 +224,8 @@ static int fanotify_handle_event(struct fsnotify_group *group,
 		/* Our event wasn't used in the end. Free it. */
 		fsnotify_destroy_event(group, fsn_event);
 
-		return 0;
+		ret = 0;
+		goto finish;
 	}
 
 #ifdef CONFIG_FANOTIFY_ACCESS_PERMISSIONS
@@ -232,6 +234,11 @@ static int fanotify_handle_event(struct fsnotify_group *group,
 					    iter_info);
 		fsnotify_destroy_event(group, fsn_event);
 	}
+finish:
+	if (mask & FAN_ALL_PERM_EVENTS)
+		fsnotify_finish_user_wait(iter_info);
+#else
+finish:
 #endif
 	return ret;
 }