btrfs: sanitize security_mnt_opts use

1) keeping a copy in btrfs_fs_info is completely pointless - we never
use it for anything. Getting rid of that allows for simpler calling
conventions for setup_security_options() (caller is responsible for
freeing mnt_opts in all cases).

2) on remount we want to use ->sb_remount(), not ->sb_set_mnt_opts(),
same as we would if not for FS_BINARY_MOUNTDATA. Behaviours *are*
close (in fact, selinux sb_set_mnt_opts() ought to punt to
sb_remount() in "already initialized" case), but let's handle
that uniformly. And the only reason why the original btrfs changes
didn't go for security_sb_remount() in btrfs_remount() case is that
it hadn't been exported. Let's export it for a while - it'll be
going away soon anyway.

Reviewed-by: David Howells <[e-mail]>
Signed-off-by: Al Viro <[e-mail]>

fs/btrfs/ctree.h

···
 	struct mutex unused_bg_unpin_mutex;
 	struct mutex delete_unused_bgs_mutex;
 
-	/* For btrfs to record security options */
-	struct security_mnt_opts security_opts;
-
 	/*
 	 * Chunks that can't be freed yet (under a trim/discard operation)
 	 * and will be latter freed. Protected by fs_info->chunk_mutex.
···
 	kfree(fs_info->free_space_root);
 	kfree(fs_info->super_copy);
 	kfree(fs_info->super_for_commit);
-	security_free_mnt_opts(&fs_info->security_opts);
 	kvfree(fs_info);
 }
 

fs/btrfs/super.c

···
 	return root;
 }
 
-static int parse_security_options(char *orig_opts,
-				  struct security_mnt_opts *sec_opts)
-{
-	return security_sb_eat_lsm_opts(orig_opts, sec_opts);
-}
-
-static int setup_security_options(struct btrfs_fs_info *fs_info,
-				  struct super_block *sb,
-				  struct security_mnt_opts *sec_opts)
-{
-	int ret = 0;
-
-	/*
-	 * Call security_sb_set_mnt_opts() to check whether new sec_opts
-	 * is valid.
-	 */
-	ret = security_sb_set_mnt_opts(sb, sec_opts, 0, NULL);
-	if (ret)
-		return ret;
-
-#ifdef CONFIG_SECURITY
-	if (!fs_info->security_opts.num_mnt_opts) {
-		/* first time security setup, copy sec_opts to fs_info */
-		memcpy(&fs_info->security_opts, sec_opts, sizeof(*sec_opts));
-	} else {
-		/*
-		 * Since SELinux (the only one supporting security_mnt_opts)
-		 * does NOT support changing context during remount/mount of
-		 * the same sb, this must be the same or part of the same
-		 * security options, just free it.
-		 */
-		security_free_mnt_opts(sec_opts);
-	}
-#endif
-	return ret;
-}
-
 /*
  * Find a superblock for the given device / mount point.
  *
···
 
 	security_init_mnt_opts(&new_sec_opts);
 	if (data) {
-		error = parse_security_options(data, &new_sec_opts);
+		error = security_sb_eat_lsm_opts(data, &new_sec_opts);
 		if (error)
 			return ERR_PTR(error);
 	}
···
 
 	fs_info->super_copy = kzalloc(BTRFS_SUPER_INFO_SIZE, GFP_KERNEL);
 	fs_info->super_for_commit = kzalloc(BTRFS_SUPER_INFO_SIZE, GFP_KERNEL);
-	security_init_mnt_opts(&fs_info->security_opts);
 	if (!fs_info->super_copy || !fs_info->super_for_commit) {
 		error = -ENOMEM;
 		goto error_fs_info;
···
 		btrfs_sb(s)->bdev_holder = fs_type;
 		error = btrfs_fill_super(s, fs_devices, data);
 	}
+	if (!error)
+		error = security_sb_set_mnt_opts(s, &new_sec_opts, 0, NULL);
+	security_free_mnt_opts(&new_sec_opts);
 	if (error) {
 		deactivate_locked_super(s);
-		goto error_sec_opts;
-	}
-
-	fs_info = btrfs_sb(s);
-	error = setup_security_options(fs_info, s, &new_sec_opts);
-	if (error) {
-		deactivate_locked_super(s);
-		goto error_sec_opts;
+		return ERR_PTR(error);
 	}
 
 	return dget(s->s_root);
···
 		struct security_mnt_opts new_sec_opts;
 
 		security_init_mnt_opts(&new_sec_opts);
-		ret = parse_security_options(data, &new_sec_opts);
+		ret = security_sb_eat_lsm_opts(data, &new_sec_opts);
+		if (!ret)
+			ret = security_sb_remount(sb, &new_sec_opts);
+		security_free_mnt_opts(&new_sec_opts);
 		if (ret)
 			goto restore;
-		ret = setup_security_options(fs_info, sb,
-					     &new_sec_opts);
-		if (ret) {
-			security_free_mnt_opts(&new_sec_opts);
-			goto restore;
-		}
 	}
 
 	ret = btrfs_parse_options(fs_info, data, *flags);

security/security.c

···
 {
 	return call_int_hook(sb_remount, 0, sb, opts);
 }
+EXPORT_SYMBOL(security_sb_remount);
 
 int security_sb_kern_mount(struct super_block *sb)
 {