ext4: avoid race conditions when remounting with options that change dax

[ Upstream commit 829b37b8 ]

Trying to change dax mount options when remounting could allow mount
options to be enabled for a small amount of time, and then the mount
option change would be reverted.

In the case of "mount -o remount,dax", this can cause a race where
files would temporarily treated as DAX --- and then not.

Cc: stable@kernel.org
Reported-by: syzbot+bca9799bf129256190da@syzkaller.appspotmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Sasha Levin <sashal@kernel.org>

fs/ext4/super.c

@@ -1989,6 +1989,16 @@ static int handle_mount_opt(struct super_block *sb, char *opt, int token,
 #endif
 	} else if (token == Opt_dax) {
 #ifdef CONFIG_FS_DAX
+		if (is_remount && test_opt(sb, DAX)) {
+			ext4_msg(sb, KERN_ERR, "can't mount with "
+				"both data=journal and dax");
+			return -1;
+		}
+		if (is_remount && !(sbi->s_mount_opt & EXT4_MOUNT_DAX)) {
+			ext4_msg(sb, KERN_ERR, "can't change "
+					"dax mount option while remounting");
+			return -1;
+		}
 		ext4_msg(sb, KERN_WARNING,
 		"DAX enabled. Warning: EXPERIMENTAL, use at your own risk");
 			sbi->s_mount_opt |= m->mount_opt;
@@ -5274,12 +5284,6 @@ static int ext4_remount(struct super_block *sb, int *flags, char *data)
 			err = -EINVAL;
 			goto restore_opts;
 		}
-		if (test_opt(sb, DAX)) {
-			ext4_msg(sb, KERN_ERR, "can't mount with "
-				 "both data=journal and dax");
-			err = -EINVAL;
-			goto restore_opts;
-		}
 	} else if (test_opt(sb, DATA_FLAGS) == EXT4_MOUNT_ORDERED_DATA) {
 		if (test_opt(sb, JOURNAL_ASYNC_COMMIT)) {
 			ext4_msg(sb, KERN_ERR, "can't mount with "
@@ -5295,12 +5299,6 @@ static int ext4_remount(struct super_block *sb, int *flags, char *data)
 		goto restore_opts;
 	}
 
-	if ((sbi->s_mount_opt ^ old_opts.s_mount_opt) & EXT4_MOUNT_DAX) {
-		ext4_msg(sb, KERN_WARNING, "warning: refusing change of "
-			"dax flag with busy inodes while remounting");
-		sbi->s_mount_opt ^= EXT4_MOUNT_DAX;
-	}
-
 	if (sbi->s_mount_flags & EXT4_MF_FS_ABORTED)
 		ext4_abort(sb, "Abort forced by user");