locks: close potential race in lease_get_mtime

lease_get_mtime is called without the i_lock held, so there's no
guarantee about the stability of the list. Between the time when we
assign "flock" and then dereference it to check whether it's a lease
and for write, the lease could be freed.

Ensure that that doesn't occur by taking the i_lock before trying
to check the lease.

Cc: J. Bruce Fields <bfields@fieldses.org>
Signed-off-by: Jeff Layton <jlayton@primarydata.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

1 files changed, 12 insertions, 2 deletions

diff --git a/fs/locks.c b/fs/locks.c
index f5f648e003dd..def1ac2e87bd 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -1456,8 +1456,18 @@ EXPORT_SYMBOL(__break_lease);
  */
 void lease_get_mtime(struct inode *inode, struct timespec *time)
 {
-        struct file_lock *flock = inode->i_flock;
-        if (flock && IS_LEASE(flock) && (flock->fl_type == F_WRLCK))
+        bool has_lease = false;
+        struct file_lock *flock;
+
+        if (inode->i_flock) {
+                spin_lock(&inode->i_lock);
+                flock = inode->i_flock;
+                if (flock && IS_LEASE(flock) && (flock->fl_type == F_WRLCK))
+                        has_lease = true;
+                spin_unlock(&inode->i_lock);
+        }
+
+        if (has_lease)
                 *time = current_fs_time(inode->i_sb);
         else
                 *time = inode->i_mtime;