ntfs3: enforce read-only when used as legacy ntfs driver

Ensure that ntfs3 is mounted read-only when it is used to provide the
legacy ntfs driver.
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/ntfs3/ntfs_fs.h

@@ -1154,4 +1154,6 @@ static inline void le64_sub_cpu(__le64 *var, u64 val)
 	*var = cpu_to_le64(le64_to_cpu(*var) - val);
 }
 
+bool is_legacy_ntfs(struct super_block *sb);
+
 #endif /* _LINUX_NTFS3_NTFS_FS_H */

fs/ntfs3/super.c

@@ -408,6 +408,12 @@ static int ntfs_fs_reconfigure(struct fs_context *fc)
 	struct ntfs_mount_options *new_opts = fc->fs_private;
 	int ro_rw;
 
+	/* If ntfs3 is used as legacy ntfs enforce read-only mode. */
+	if (is_legacy_ntfs(sb)) {
+		fc->sb_flags |= SB_RDONLY;
+		goto out;
+	}
+
 	ro_rw = sb_rdonly(sb) && !(fc->sb_flags & SB_RDONLY);
 	if (ro_rw && (sbi->flags & NTFS_FLAGS_NEED_REPLAY)) {
 		errorf(fc,
@@ -427,8 +433,6 @@ static int ntfs_fs_reconfigure(struct fs_context *fc)
 			fc,
 			"ntfs3: Cannot use different iocharset when remounting!");
 
-	sync_filesystem(sb);
-
 	if (ro_rw && (sbi->volume.flags & VOLUME_FLAG_DIRTY) &&
 	    !new_opts->force) {
 		errorf(fc,
@@ -436,6 +440,8 @@ static int ntfs_fs_reconfigure(struct fs_context *fc)
 		return -EINVAL;
 	}
 
+out:
+	sync_filesystem(sb);
 	swap(sbi->options, fc->fs_private);
 
 	return 0;
@@ -1613,6 +1619,8 @@ static int ntfs_fill_super(struct super_block *sb, struct fs_context *fc)
 	}
 #endif
 
+	if (is_legacy_ntfs(sb))
+		sb->s_flags |= SB_RDONLY;
 	return 0;
 
 put_inode_out:
@@ -1730,7 +1738,7 @@ static const struct fs_context_operations ntfs_context_ops = {
  * This will called when mount/remount. We will first initialize
  * options so that if remount we can use just that.
  */
-static int ntfs_init_fs_context(struct fs_context *fc)
+static int __ntfs_init_fs_context(struct fs_context *fc)
 {
 	struct ntfs_mount_options *opts;
 	struct ntfs_sb_info *sbi;
@@ -1778,6 +1786,11 @@ static int ntfs_init_fs_context(struct fs_context *fc)
 	return -ENOMEM;
 }
 
+static int ntfs_init_fs_context(struct fs_context *fc)
+{
+	return __ntfs_init_fs_context(fc);
+}
+
 static void ntfs3_kill_sb(struct super_block *sb)
 {
 	struct ntfs_sb_info *sbi = sb->s_fs_info;
@@ -1800,10 +1813,20 @@ static struct file_system_type ntfs_fs_type = {
 };
 
 #if IS_ENABLED(CONFIG_NTFS_FS)
+static int ntfs_legacy_init_fs_context(struct fs_context *fc)
+{
+	int ret;
+
+	ret = __ntfs_init_fs_context(fc);
+	/* If ntfs3 is used as legacy ntfs enforce read-only mode. */
+	fc->sb_flags |= SB_RDONLY;
+	return ret;
+}
+
 static struct file_system_type ntfs_legacy_fs_type = {
 	.owner			= THIS_MODULE,
 	.name			= "ntfs",
-	.init_fs_context	= ntfs_init_fs_context,
+	.init_fs_context	= ntfs_legacy_init_fs_context,
 	.parameters		= ntfs_fs_parameters,
 	.kill_sb		= ntfs3_kill_sb,
 	.fs_flags		= FS_REQUIRES_DEV | FS_ALLOW_IDMAP,
@@ -1821,9 +1844,14 @@ static inline void unregister_as_ntfs_legacy(void)
 {
 	unregister_filesystem(&ntfs_legacy_fs_type);
 }
+bool is_legacy_ntfs(struct super_block *sb)
+{
+	return sb->s_type == &ntfs_legacy_fs_type;
+}
 #else
 static inline void register_as_ntfs_legacy(void) {}
 static inline void unregister_as_ntfs_legacy(void) {}
+bool is_legacy_ntfs(struct super_block *sb) { return false; }
 #endif