fsnotify: pin both inode and vfsmount mark

We may fail to pin one of the marks in fsnotify_prepare_user_wait() when
dropping the srcu read lock, resulting in use after free at the next
iteration.

Solution is to store both marks in iter_info instead of just the one we'll
be sending the event for.
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Fixes: 9385a84d ("fsnotify: Pass fsnotify_iter_info into handle_event handler")
Cc: <stable@vger.kernel.org> # v4.12
Signed-off-by: Jan Kara <jack@suse.cz>

fs/notify/fsnotify.c

@@ -335,6 +335,13 @@ int fsnotify(struct inode *to_tell, __u32 mask, const void *data, int data_is,
 						    struct fsnotify_mark, obj_list);
 			vfsmount_group = vfsmount_mark->group;
 		}
+		/*
+		 * Need to protect both marks against freeing so that we can
+		 * continue iteration from this place, regardless of which mark
+		 * we actually happen to send an event for.
+		 */
+		iter_info.inode_mark = inode_mark;
+		iter_info.vfsmount_mark = vfsmount_mark;
 
 		if (inode_group && vfsmount_group) {
 			int cmp = fsnotify_compare_groups(inode_group,
@@ -348,9 +355,6 @@ int fsnotify(struct inode *to_tell, __u32 mask, const void *data, int data_is,
 			}
 		}
 
-		iter_info.inode_mark = inode_mark;
-		iter_info.vfsmount_mark = vfsmount_mark;
-
 		ret = send_to_group(to_tell, inode_mark, vfsmount_mark, mask,
 				    data, data_is, cookie, file_name,
 				    &iter_info);