v2: merge the lower bound into the existing highest_inum check, as
    suggested by Zhihao Cheng.

diff --git a/fs/ubifs/master.c b/fs/ubifs/master.c
index a148760fa49e..e180091fab8c 100644
--- a/fs/ubifs/master.c
+++ b/fs/ubifs/master.c
@@ -186,7 +186,13 @@ static int validate_master(const struct ubifs_info *c)
 		goto out;
 	}
 
-	if (c->highest_inum >= INUM_WATERMARK) {
+	/*
+	 * Inode numbers below UBIFS_FIRST_INO are reserved.  If the master node
+	 * claims a smaller value, newly created inodes would reuse the root
+	 * inode number or another reserved one.
+	 */
+	if (c->highest_inum >= INUM_WATERMARK ||
+	    c->highest_inum < UBIFS_FIRST_INO) {
 		err = 3;
 		goto out;
 	}