[PATCH 4.9 3/3] fanotify: Release SRCU lock when waiting for userspace response

[Matthew Ruffell <matthew.ruffell@canonical.com>]

From: Jan Kara <jack@suse.cz>

commit 05f0e38724e8449184acd8fbf0473ee5a07adc6c upstream.

When userspace task processing fanotify permission events screws up and
does not respond, fsnotify_mark_srcu SRCU is held indefinitely which
causes further hangs in the whole notification subsystem. Although we
cannot easily solve the problem of operations blocked waiting for
response from userspace, we can at least somewhat localize the damage by
dropping SRCU lock before waiting for userspace response and reacquiring
it when userspace responds.

Reviewed-by: Miklos Szeredi <mszeredi@redhat.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
[mruffell: cherry picked]
Signed-off-by: Matthew Ruffell <matthew.ruffell@canonical.com>
---
 fs/notify/fanotify/fanotify.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c
index 4944956cdbd9..eeb5cc1f6978 100644
--- a/fs/notify/fanotify/fanotify.c
+++ b/fs/notify/fanotify/fanotify.c
@@ -61,14 +61,26 @@ static int fanotify_merge(struct list_head *list, struct fsnotify_event *event)
 
 #ifdef CONFIG_FANOTIFY_ACCESS_PERMISSIONS
 static int fanotify_get_response(struct fsnotify_group *group,
-				 struct fanotify_perm_event_info *event)
+				 struct fanotify_perm_event_info *event,
+				 struct fsnotify_iter_info *iter_info)
 {
 	int ret;
 
 	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:
@@ -216,7 +228,8 @@ static int fanotify_handle_event(struct fsnotify_group *group,
 
 #ifdef CONFIG_FANOTIFY_ACCESS_PERMISSIONS
 	if (mask & FAN_ALL_PERM_EVENTS) {
-		ret = fanotify_get_response(group, FANOTIFY_PE(fsn_event));
+		ret = fanotify_get_response(group, FANOTIFY_PE(fsn_event),
+					    iter_info);
 		fsnotify_destroy_event(group, fsn_event);
 	}
 #endif
-- 
2.19.1