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

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>

fs/ext4/super.c

@@ -2104,16 +2104,40 @@ static int handle_mount_opt(struct super_block *sb, char *opt, int token,
 		switch (token) {
 		case Opt_dax:
 		case Opt_dax_always:
+			if (is_remount &&
+			    (!(sbi->s_mount_opt & EXT4_MOUNT_DAX_ALWAYS) ||
+			     (sbi->s_mount_opt2 & EXT4_MOUNT2_DAX_NEVER))) {
+			fail_dax_change_remount:
+				ext4_msg(sb, KERN_ERR, "can't change "
+					 "dax mount option while remounting");
+				return -1;
+			}
+			if (is_remount &&
+			    (test_opt(sb, DATA_FLAGS) ==
+			     EXT4_MOUNT_JOURNAL_DATA)) {
+				    ext4_msg(sb, KERN_ERR, "can't mount with "
+					     "both data=journal and dax");
+				    return -1;
+			}
 			ext4_msg(sb, KERN_WARNING,
 				"DAX enabled. Warning: EXPERIMENTAL, use at your own risk");
 			sbi->s_mount_opt |= EXT4_MOUNT_DAX_ALWAYS;
 			sbi->s_mount_opt2 &= ~EXT4_MOUNT2_DAX_NEVER;
 			break;
 		case Opt_dax_never:
+			if (is_remount &&
+			    (!(sbi->s_mount_opt2 & EXT4_MOUNT2_DAX_NEVER) ||
+			     (sbi->s_mount_opt & EXT4_MOUNT_DAX_ALWAYS)))
+				goto fail_dax_change_remount;
 			sbi->s_mount_opt2 |= EXT4_MOUNT2_DAX_NEVER;
 			sbi->s_mount_opt &= ~EXT4_MOUNT_DAX_ALWAYS;
 			break;
 		case Opt_dax_inode:
+			if (is_remount &&
+			    ((sbi->s_mount_opt & EXT4_MOUNT_DAX_ALWAYS) ||
+			     (sbi->s_mount_opt2 & EXT4_MOUNT2_DAX_NEVER) ||
+			     !(sbi->s_mount_opt2 & EXT4_MOUNT2_DAX_INODE)))
+				goto fail_dax_change_remount;
 			sbi->s_mount_opt &= ~EXT4_MOUNT_DAX_ALWAYS;
 			sbi->s_mount_opt2 &= ~EXT4_MOUNT2_DAX_NEVER;
 			/* Strictly for printing options */
@@ -5454,12 +5478,6 @@ static int ext4_remount(struct super_block *sb, int *flags, char *data)
 			err = -EINVAL;
 			goto restore_opts;
 		}
-		if (test_opt(sb, DAX_ALWAYS)) {
-			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 "
@@ -5475,18 +5493,6 @@ 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_ALWAYS ||
-	    (sbi->s_mount_opt2 ^ old_opts.s_mount_opt2) & EXT4_MOUNT2_DAX_NEVER ||
-	    (sbi->s_mount_opt2 ^ old_opts.s_mount_opt2) & EXT4_MOUNT2_DAX_INODE) {
-		ext4_msg(sb, KERN_WARNING, "warning: refusing change of "
-			"dax mount option with busy inodes while remounting");
-		sbi->s_mount_opt &= ~EXT4_MOUNT_DAX_ALWAYS;
-		sbi->s_mount_opt |= old_opts.s_mount_opt & EXT4_MOUNT_DAX_ALWAYS;
-		sbi->s_mount_opt2 &= ~(EXT4_MOUNT2_DAX_NEVER | EXT4_MOUNT2_DAX_INODE);
-		sbi->s_mount_opt2 |= old_opts.s_mount_opt2 &
-				     (EXT4_MOUNT2_DAX_NEVER | EXT4_MOUNT2_DAX_INODE);
-	}
-
 	if (sbi->s_mount_flags & EXT4_MF_FS_ABORTED)
 		ext4_abort(sb, EXT4_ERR_ESHUTDOWN, "Abort forced by user");