[RFC ONLY 15/23] btrfs: add a subvolume flag for whole-volume encryption

From: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
To: Chris Mason <clm@fb.com>, Josef Bacik <josef@toxicpanda.com>,
	David Sterba <dsterba@suse.com>,
	linux-btrfs@vger.kernel.org
Cc: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
Subject: [RFC ONLY 15/23] btrfs: add a subvolume flag for whole-volume encryption
Date: Wed, 13 Jul 2022 06:29:48 -0400	[thread overview]
Message-ID: <9b7039604b577a36ff2e49e0470b21c3d2aa2ed0.1657707687.git.sweettea-kernel@dorminy.me> (raw)
In-Reply-To: <cover.1657707686.git.sweettea-kernel@dorminy.me>

In many cases, we will want to encrypt all of a subvolume; adding a
subvolume flag allows this.  However, since an unencrypted subvolume
would be unable to read encrypted data, encrypted subvolumes should only
be snapshottable to other encrypted subvolumes.

Signed-off-by: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
---
 fs/btrfs/ctree.h                |   3 +
 fs/btrfs/fscrypt.c              | 158 ++++++++++++++++++++++++++++++++
 fs/btrfs/inode.c                |  38 ++++++++
 fs/btrfs/ioctl.c                |  10 +-
 fs/btrfs/tree-checker.c         |   1 +
 include/uapi/linux/btrfs_tree.h |  10 ++
 6 files changed, 219 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index b01030cbe79b..a1fea2833bff 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -33,6 +33,7 @@
 #include "extent-io-tree.h"
 #include "extent_io.h"
 #include "extent_map.h"
+#include "fscrypt.h"
 #include "async-thread.h"
 #include "block-rsv.h"
 #include "locking.h"
@@ -1589,6 +1590,7 @@ do {                                                                   \
 #define BTRFS_INODE_NOATIME		(1U << 9)
 #define BTRFS_INODE_DIRSYNC		(1U << 10)
 #define BTRFS_INODE_COMPRESS		(1U << 11)
+#define BTRFS_INODE_FSCRYPT_CONTEXT	(1U << 12)
 
 #define BTRFS_INODE_ROOT_ITEM_INIT	(1U << 31)
 
@@ -1605,6 +1607,7 @@ do {                                                                   \
 	 BTRFS_INODE_NOATIME |						\
 	 BTRFS_INODE_DIRSYNC |						\
 	 BTRFS_INODE_COMPRESS |						\
+	 BTRFS_INODE_FSCRYPT_CONTEXT |					\
 	 BTRFS_INODE_ROOT_ITEM_INIT)
 
 #define BTRFS_INODE_RO_VERITY		(1U << 0)
diff --git a/fs/btrfs/fscrypt.c b/fs/btrfs/fscrypt.c
index 9829d280a6bc..cffe63e18ef4 100644
--- a/fs/btrfs/fscrypt.c
+++ b/fs/btrfs/fscrypt.c
@@ -3,8 +3,13 @@
  * Copyright (C) 2020 Facebook
  */
 
+#include <linux/iversion.h>
 #include "ctree.h"
+#include "btrfs_inode.h"
+#include "disk-io.h"
 #include "fscrypt.h"
+#include "transaction.h"
+#include "xattr.h"
 
 /* fscrypt_match_name() but for an extent_buffer. */
 bool btrfs_fscrypt_match_name(const struct fscrypt_name *fname,
@@ -31,5 +36,158 @@ bool btrfs_fscrypt_match_name(const struct fscrypt_name *fname,
 	return !memcmp(digest, nokey_name->sha256, sizeof(digest));
 }
 
+static int btrfs_fscrypt_get_context(struct inode *inode, void *ctx, size_t len)
+{
+	struct btrfs_root *root = BTRFS_I(inode)->root;
+	struct inode *put_inode = NULL;
+	struct btrfs_key key;
+	struct btrfs_path *path;
+	int ret;
+
+	if (S_ISREG(inode->i_mode) &&
+	    (btrfs_root_flags(&root->root_item) & BTRFS_ROOT_SUBVOL_FSCRYPT)) {
+		/* TODO: maybe cache the item */
+		inode = btrfs_iget(inode->i_sb, BTRFS_FIRST_FREE_OBJECTID,
+				   root);
+		if (IS_ERR(inode))
+			return PTR_ERR(inode);
+		put_inode = inode;
+	}
+
+	path = btrfs_alloc_path();
+	if (!path)
+		return -ENOMEM;
+
+	key = (struct btrfs_key) {
+		.objectid = btrfs_ino(BTRFS_I(inode)),
+		.type = BTRFS_FSCRYPT_CTXT_ITEM_KEY,
+		.offset = 0,
+	};
+
+	ret = btrfs_search_slot(NULL, BTRFS_I(inode)->root, &key, path, 0, 0);
+	if (ret) {
+		len = -EINVAL;
+		goto out;
+	}
+
+	struct extent_buffer *leaf = path->nodes[0];
+	unsigned long ptr = btrfs_item_ptr_offset(leaf, path->slots[0]);
+	/* fscrypt provides max context length, but it could be less */
+	len = min_t(size_t, len, btrfs_item_size(leaf, path->slots[0]));
+	read_extent_buffer(leaf, ctx, ptr, len);
+
+out:
+	btrfs_free_path(path);
+	iput(put_inode);
+	return len;
+}
+
+static int btrfs_fscrypt_set_context(struct inode *inode, const void *ctx,
+				     size_t len, void *fs_data)
+{
+	struct btrfs_root *root = BTRFS_I(inode)->root;
+	struct btrfs_trans_handle *trans;
+	int is_subvolume = inode->i_ino == BTRFS_FIRST_FREE_OBJECTID;
+	int ret;
+	struct btrfs_path *path;
+	struct btrfs_key key = {
+		.objectid = btrfs_ino(BTRFS_I(inode)),
+		.type = BTRFS_FSCRYPT_CTXT_ITEM_KEY,
+		.offset = 0,
+	};
+
+	/*
+	 * If the whole subvolume is encrypted, we can get the policy for
+	 * regular files from the root inode.
+	 */
+	if (S_ISREG(inode->i_mode) &&
+	    (btrfs_root_flags(&root->root_item) & BTRFS_ROOT_SUBVOL_FSCRYPT))
+		return 0;
+
+	if (fs_data) {
+		/*
+		 * We are setting the context as part of an existing
+		 * transaction. This happens when we are inheriting the context
+		 * for a new inode.
+		 */
+		trans = fs_data;
+	} else {
+		/*
+		 * 1 for the inode item
+		 * 1 for the root item if the inode is a subvolume
+		 */
+		trans = btrfs_start_transaction(root, 1 + is_subvolume);
+		if (IS_ERR(trans))
+			return PTR_ERR(trans);
+	}
+
+	path = btrfs_alloc_path();
+	if (!path)
+		return -ENOMEM;
+	ret = btrfs_search_slot(trans, BTRFS_I(inode)->root, &key, path, 0, 1);
+	if (ret == 0) {
+		struct extent_buffer *leaf = path->nodes[0];
+		unsigned long ptr = btrfs_item_ptr_offset(leaf, path->slots[0]);
+		len = min_t(size_t, len, btrfs_item_size(leaf, path->slots[0]));
+		write_extent_buffer(leaf, ctx, ptr, len);
+		btrfs_mark_buffer_dirty(leaf);
+		btrfs_free_path(path);
+		goto out;
+	} else if (ret < 0) {
+		goto out;
+	}
+	btrfs_free_path(path);
+
+	ret = btrfs_insert_item(trans, BTRFS_I(inode)->root, &key, ctx, len);
+	if (ret)
+		goto out;
+
+	BTRFS_I(inode)->flags |= BTRFS_INODE_FSCRYPT_CONTEXT;
+	btrfs_sync_inode_flags_to_i_flags(inode);
+	if (!fs_data) {
+		inode_inc_iversion(inode);
+		inode->i_ctime = current_time(inode);
+		ret = btrfs_update_inode(trans, root, BTRFS_I(inode));
+		if (ret)
+			goto out;
+		/*
+		 * For new subvolumes, the root item is already initialized with
+		 * the BTRFS_ROOT_SUBVOL_FSCRYPT flag.
+		 */
+		if (is_subvolume) {
+			u64 root_flags = btrfs_root_flags(&root->root_item);
+
+			btrfs_set_root_flags(&root->root_item,
+					     root_flags |
+					     BTRFS_ROOT_SUBVOL_FSCRYPT);
+			ret = btrfs_update_root(trans, root->fs_info->tree_root,
+						&root->root_key,
+						&root->root_item);
+		}
+	}
+out:
+	if (fs_data)
+		return ret;
+
+	if (ret)
+		btrfs_abort_transaction(trans, ret);
+	else
+		btrfs_end_transaction(trans);
+	return ret;
+}
+
+static bool btrfs_fscrypt_empty_dir(struct inode *inode)
+{
+	/*
+	 * We don't care about turning on encryption on a non-empty directory
+	 * so we always return true.
+	 */
+	return true;
+}
+
 const struct fscrypt_operations btrfs_fscrypt_ops = {
+	.key_prefix = "btrfs:",
+	.get_context = btrfs_fscrypt_get_context,
+	.set_context = btrfs_fscrypt_set_context,
+	.empty_dir = btrfs_fscrypt_empty_dir,
 };
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index b043342f2820..a74b0d326baa 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -6286,6 +6286,34 @@ int btrfs_new_inode_prepare(struct btrfs_new_inode_args *args,
 	struct inode *inode = args->inode;
 	int ret;
 
+	if (fscrypt_is_nokey_name(args->dentry))
+		return -ENOKEY;
+
+	if (IS_ENCRYPTED(dir) &&
+	    !(BTRFS_I(dir)->flags & BTRFS_INODE_FSCRYPT_CONTEXT)) {
+		struct inode *root_inode;
+		bool encrypt;
+
+		root_inode = btrfs_iget(inode->i_sb, BTRFS_FIRST_FREE_OBJECTID,
+					BTRFS_I(dir)->root);
+		if (IS_ERR(root_inode))
+			return PTR_ERR(root_inode);
+		/*
+		 * TODO: perhaps instead of faking making a new dir to get a
+		 * new context, it would be better to expose
+		 * fscrypt_setup_encryption_info() for our use.
+		 */
+		ret = fscrypt_prepare_new_inode(root_inode, dir, &encrypt);
+		if (!ret) {
+			ret = fscrypt_set_context(dir, NULL);
+			if (ret)
+				fscrypt_put_encryption_info(dir);
+		}
+		iput(root_inode);
+		if (ret)
+			return ret;
+	}
+
 	if (!args->orphan) {
 		ret = fscrypt_setup_filename(dir, &args->dentry->d_name, 0,
 					     &args->fname);
@@ -6319,6 +6347,8 @@ int btrfs_new_inode_prepare(struct btrfs_new_inode_args *args,
 	if (dir->i_security)
 		(*trans_num_items)++;
 #endif
+	if (args->encrypt)
+		(*trans_num_items)++; /* 1 to add fscrypt item */
 	if (args->orphan) {
 		/* 1 to add orphan item */
 		(*trans_num_items)++;
@@ -6572,6 +6602,14 @@ int btrfs_create_new_inode(struct btrfs_trans_handle *trans,
 		}
 	}
 
+	if (args->encrypt) {
+		ret = fscrypt_set_context(inode, trans);
+		if (ret) {
+			btrfs_abort_transaction(trans, ret);
+			goto discard;
+		}
+	}
+
 	inode_tree_add(inode);
 
 	trace_btrfs_inode_new(inode);
diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 352f890c53ec..9d3d447f58ad 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -658,7 +658,8 @@ static noinline int create_subvol(struct user_namespace *mnt_userns,
 				     fs_info->nodesize);
 	btrfs_set_stack_inode_mode(inode_item, S_IFDIR | 0755);
 
-	btrfs_set_root_flags(root_item, 0);
+	btrfs_set_root_flags(root_item, new_inode_args.encrypt ?
+			     BTRFS_ROOT_SUBVOL_FSCRYPT : 0);
 	btrfs_set_root_limit(root_item, 0);
 	btrfs_set_stack_inode_flags(inode_item, BTRFS_INODE_ROOT_ITEM_INIT);
 
@@ -787,6 +788,13 @@ static int create_snapshot(struct btrfs_root *root, struct inode *dir,
 		return -ETXTBSY;
 	}
 
+	if ((btrfs_root_flags(&root->root_item) & BTRFS_ROOT_SUBVOL_FSCRYPT) &&
+	    !IS_ENCRYPTED(dir)) {
+		btrfs_warn(fs_info,
+			   "cannot snapshot encrypted volume to unencrypted destination");
+		return -EXDEV;
+	}
+
 	pending_snapshot = kzalloc(sizeof(*pending_snapshot), GFP_KERNEL);
 	if (!pending_snapshot)
 		return -ENOMEM;
diff --git a/fs/btrfs/tree-checker.c b/fs/btrfs/tree-checker.c
index f861cc52be41..6f8e53d0cd6e 100644
--- a/fs/btrfs/tree-checker.c
+++ b/fs/btrfs/tree-checker.c
@@ -1124,6 +1124,7 @@ static int check_root_item(struct extent_buffer *leaf, struct btrfs_key *key,
 	struct btrfs_fs_info *fs_info = leaf->fs_info;
 	struct btrfs_root_item ri = { 0 };
 	const u64 valid_root_flags = BTRFS_ROOT_SUBVOL_RDONLY |
+				     BTRFS_ROOT_SUBVOL_FSCRYPT |
 				     BTRFS_ROOT_SUBVOL_DEAD;
 	int ret;
 
diff --git a/include/uapi/linux/btrfs_tree.h b/include/uapi/linux/btrfs_tree.h
index 428ae75b9f73..da44d3355385 100644
--- a/include/uapi/linux/btrfs_tree.h
+++ b/include/uapi/linux/btrfs_tree.h
@@ -144,6 +144,8 @@
 #define BTRFS_VERITY_DESC_ITEM_KEY	36
 #define BTRFS_VERITY_MERKLE_ITEM_KEY	37
 
+#define BTRFS_FSCRYPT_CTXT_ITEM_KEY	41
+
 #define BTRFS_ORPHAN_ITEM_KEY		48
 /* reserve 2-15 close to the inode for later flexibility */
 
@@ -633,6 +635,8 @@ struct btrfs_dir_item {
 } __attribute__ ((__packed__));
 
 #define BTRFS_ROOT_SUBVOL_RDONLY	(1ULL << 0)
+/* Top-level subvolume directory is encrypted with fscrypt. */
+#define BTRFS_ROOT_SUBVOL_FSCRYPT	(1ULL << 1)
 
 /*
  * Internal in-memory flag that a subvolume has been marked for deletion but
@@ -788,6 +792,12 @@ enum {
 	BTRFS_NR_FILE_EXTENT_TYPES = 3,
 };
 
+enum {
+	BTRFS_ENCRYPTION_NONE,
+	BTRFS_ENCRYPTION_FSCRYPT,
+	BTRFS_NR_ENCRYPTION_TYPES,
+};
+
 struct btrfs_file_extent_item {
 	/*
 	 * transaction id that created this extent
-- 
2.35.1

