fanotify: update connector fsid cache on add mark

When implementing connector fsid cache, we only initialized the cache
when the first mark added to object was added by FAN_REPORT_FID group.
We forgot to update conn->fsid when the second mark is added by
FAN_REPORT_FID group to an already attached connector without fsid
cache.

Reported-and-tested-by: syzbot+c277e8e2f46414645508@syzkaller.appspotmail.com
Fixes: 77115225 ("fanotify: cache fsid in fsnotify_mark_connector")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/notify/fanotify/fanotify.c

@@ -355,6 +355,10 @@ static __kernel_fsid_t fanotify_get_fsid(struct fsnotify_iter_info *iter_info)
 		/* Mark is just getting destroyed or created? */
 		if (!conn)
 			continue;
+		if (!(conn->flags & FSNOTIFY_CONN_FLAG_HAS_FSID))
+			continue;
+		/* Pairs with smp_wmb() in fsnotify_add_mark_list() */
+		smp_rmb();
 		fsid = conn->fsid;
 		if (WARN_ON_ONCE(!fsid.val[0] && !fsid.val[1]))
 			continue;

fs/notify/mark.c

@@ -482,10 +482,13 @@ static int fsnotify_attach_connector_to_object(fsnotify_connp_t *connp,
 	conn->type = type;
 	conn->obj = connp;
 	/* Cache fsid of filesystem containing the object */
-	if (fsid)
+	if (fsid) {
 		conn->fsid = *fsid;
-	else
+		conn->flags = FSNOTIFY_CONN_FLAG_HAS_FSID;
+	} else {
 		conn->fsid.val[0] = conn->fsid.val[1] = 0;
+		conn->flags = 0;
+	}
 	if (conn->type == FSNOTIFY_OBJ_TYPE_INODE)
 		inode = igrab(fsnotify_conn_inode(conn));
 	/*
@@ -560,7 +563,12 @@ static int fsnotify_add_mark_list(struct fsnotify_mark *mark,
 		if (err)
 			return err;
 		goto restart;
-	} else if (fsid && (conn->fsid.val[0] || conn->fsid.val[1]) &&
+	} else if (fsid && !(conn->flags & FSNOTIFY_CONN_FLAG_HAS_FSID)) {
+		conn->fsid = *fsid;
+		/* Pairs with smp_rmb() in fanotify_get_fsid() */
+		smp_wmb();
+		conn->flags |= FSNOTIFY_CONN_FLAG_HAS_FSID;
+	} else if (fsid && (conn->flags & FSNOTIFY_CONN_FLAG_HAS_FSID) &&
 		   (fsid->val[0] != conn->fsid.val[0] ||
 		    fsid->val[1] != conn->fsid.val[1])) {
 		/*

include/linux/fsnotify_backend.h

@@ -292,7 +292,9 @@ typedef struct fsnotify_mark_connector __rcu *fsnotify_connp_t;
  */
 struct fsnotify_mark_connector {
 	spinlock_t lock;
-	unsigned int type;	/* Type of object [lock] */
+	unsigned short type;	/* Type of object [lock] */
+#define FSNOTIFY_CONN_FLAG_HAS_FSID	0x01
+	unsigned short flags;	/* flags [lock] */
 	__kernel_fsid_t fsid;	/* fsid of filesystem containing object */
 	union {
 		/* Object pointer [lock] */