[PATCH v2 13/20] btrfs: add fscrypt_context items.

[Sweet Tea Dorminy <sweettea-kernel@dorminy.me>]

From: Omar Sandoval <osandov@osandov.com>

In order to store per-inode information such as the inode nonce and the
key identifier, fscrypt stores a context item with each encrypted inode.
This can be implemented as a new item type, as fscrypt provides an
arbitrary blob for the filesystem to store.

This also provides a good place to implement full-subvolume encryption:
a subvolume flag permits setting one context for the whole subvolume.
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: Omar Sandoval <osandov@osandov.com>
Signed-off-by: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
---
 fs/btrfs/ctree.h                |   3 +
 fs/btrfs/fscrypt.c              | 167 ++++++++++++++++++++++++++++++++
 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, 228 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 2b9ba8d77861..f0a16c32110d 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"
@@ -1667,6 +1668,7 @@ 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)
 
@@ -1683,6 +1685,7 @@ 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..b824bbd964bc 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 +36,167 @@ 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;
+	struct extent_buffer *leaf;
+	unsigned long ptr;
+	int ret;
+
+	if (btrfs_root_flags(&root->root_item) & BTRFS_ROOT_SUBVOL_FSCRYPT) {
+		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;
+	}
+
+	leaf = path->nodes[0];
+	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 expect that all children
+	 * have the same policy.
+	 */
+	if (btrfs_root_flags(&root->root_item) & BTRFS_ROOT_SUBVOL_FSCRYPT) {
+		bool same_policy;
+		struct inode *root_inode = NULL;
+		root_inode = btrfs_iget(inode->i_sb, BTRFS_FIRST_FREE_OBJECTID,
+				   root);
+		if (IS_ERR(inode))
+			return PTR_ERR(inode);
+		same_policy = fscrypt_have_same_policy(inode, root_inode);
+		iput(root_inode);
+		if (same_policy)
+			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 fscrypt item
+		 * 1 for the root item if the inode is a subvolume
+		 */
+		trans = btrfs_start_transaction(root, 2 + 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, (void *) ctx, len);
+	if (ret)
+		goto out;
+
+	BTRFS_I(inode)->flags |= BTRFS_INODE_FSCRYPT_CONTEXT;
+	btrfs_sync_inode_flags_to_i_flags(inode);
+	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 (!fs_data && 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 eb42e4bf55b9..007abdf6de93 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -6278,6 +6278,34 @@ 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);
@@ -6311,6 +6339,8 @@ 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)++;
@@ -6564,6 +6594,14 @@ 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 10b8db56edda..8f5b65c43c8d 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 2c6d48af1d2a..05f404c7498e 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 e6031d18d521..061aabca56f3 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