Adding upstream version 6.6.15.upstream/6.6.15

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 811 insertions, 0 deletions

diff --git a/fs/btrfs/verity.c b/fs/btrfs/verity.c
new file mode 100644
index 0000000000..744f4f4d4c
--- /dev/null
+++ b/fs/btrfs/verity.c
@@ -0,0 +1,811 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/init.h>
+#include <linux/fs.h>
+#include <linux/slab.h>
+#include <linux/rwsem.h>
+#include <linux/xattr.h>
+#include <linux/security.h>
+#include <linux/posix_acl_xattr.h>
+#include <linux/iversion.h>
+#include <linux/fsverity.h>
+#include <linux/sched/mm.h>
+#include "messages.h"
+#include "ctree.h"
+#include "btrfs_inode.h"
+#include "transaction.h"
+#include "disk-io.h"
+#include "locking.h"
+#include "fs.h"
+#include "accessors.h"
+#include "ioctl.h"
+#include "verity.h"
+#include "orphan.h"
+
+/*
+ * Implementation of the interface defined in struct fsverity_operations.
+ *
+ * The main question is how and where to store the verity descriptor and the
+ * Merkle tree. We store both in dedicated btree items in the filesystem tree,
+ * together with the rest of the inode metadata. This means we'll need to do
+ * extra work to encrypt them once encryption is supported in btrfs, but btrfs
+ * has a lot of careful code around i_size and it seems better to make a new key
+ * type than try and adjust all of our expectations for i_size.
+ *
+ * Note that this differs from the implementation in ext4 and f2fs, where
+ * this data is stored as if it were in the file, but past EOF. However, btrfs
+ * does not have a widespread mechanism for caching opaque metadata pages, so we
+ * do pretend that the Merkle tree pages themselves are past EOF for the
+ * purposes of caching them (as opposed to creating a virtual inode).
+ *
+ * fs verity items are stored under two different key types on disk.
+ * The descriptor items:
+ * [ inode objectid, BTRFS_VERITY_DESC_ITEM_KEY, offset ]
+ *
+ * At offset 0, we store a btrfs_verity_descriptor_item which tracks the
+ * size of the descriptor item and some extra data for encryption.
+ * Starting at offset 1, these hold the generic fs verity descriptor.
+ * The latter are opaque to btrfs, we just read and write them as a blob for
+ * the higher level verity code.  The most common descriptor size is 256 bytes.
+ *
+ * The merkle tree items:
+ * [ inode objectid, BTRFS_VERITY_MERKLE_ITEM_KEY, offset ]
+ *
+ * These also start at offset 0, and correspond to the merkle tree bytes.
+ * So when fsverity asks for page 0 of the merkle tree, we pull up one page
+ * starting at offset 0 for this key type.  These are also opaque to btrfs,
+ * we're blindly storing whatever fsverity sends down.
+ *
+ * Another important consideration is the fact that the Merkle tree data scales
+ * linearly with the size of the file (with 4K pages/blocks and SHA-256, it's
+ * ~1/127th the size) so for large files, writing the tree can be a lengthy
+ * operation. For that reason, we guard the whole enable verity operation
+ * (between begin_enable_verity and end_enable_verity) with an orphan item.
+ * Again, because the data can be pretty large, it's quite possible that we
+ * could run out of space writing it, so we try our best to handle errors by
+ * stopping and rolling back rather than aborting the victim transaction.
+ */
+
+#define MERKLE_START_ALIGN			65536
+
+/*
+ * Compute the logical file offset where we cache the Merkle tree.
+ *
+ * @inode:  inode of the verity file
+ *
+ * For the purposes of caching the Merkle tree pages, as required by
+ * fs-verity, it is convenient to do size computations in terms of a file
+ * offset, rather than in terms of page indices.
+ *
+ * Use 64K to be sure it's past the last page in the file, even with 64K pages.
+ * That rounding operation itself can overflow loff_t, so we do it in u64 and
+ * check.
+ *
+ * Returns the file offset on success, negative error code on failure.
+ */
+static loff_t merkle_file_pos(const struct inode *inode)
+{
+	u64 sz = inode->i_size;
+	u64 rounded = round_up(sz, MERKLE_START_ALIGN);
+
+	if (rounded > inode->i_sb->s_maxbytes)
+		return -EFBIG;
+
+	return rounded;
+}
+
+/*
+ * Drop all the items for this inode with this key_type.
+ *
+ * @inode:     inode to drop items for
+ * @key_type:  type of items to drop (BTRFS_VERITY_DESC_ITEM or
+ *             BTRFS_VERITY_MERKLE_ITEM)
+ *
+ * Before doing a verity enable we cleanup any existing verity items.
+ * This is also used to clean up if a verity enable failed half way through.
+ *
+ * Returns number of dropped items on success, negative error code on failure.
+ */
+static int drop_verity_items(struct btrfs_inode *inode, u8 key_type)
+{
+	struct btrfs_trans_handle *trans;
+	struct btrfs_root *root = inode->root;
+	struct btrfs_path *path;
+	struct btrfs_key key;
+	int count = 0;
+	int ret;
+
+	path = btrfs_alloc_path();
+	if (!path)
+		return -ENOMEM;
+
+	while (1) {
+		/* 1 for the item being dropped */
+		trans = btrfs_start_transaction(root, 1);
+		if (IS_ERR(trans)) {
+			ret = PTR_ERR(trans);
+			goto out;
+		}
+
+		/*
+		 * Walk backwards through all the items until we find one that
+		 * isn't from our key type or objectid
+		 */
+		key.objectid = btrfs_ino(inode);
+		key.type = key_type;
+		key.offset = (u64)-1;
+
+		ret = btrfs_search_slot(trans, root, &key, path, -1, 1);
+		if (ret > 0) {
+			ret = 0;
+			/* No more keys of this type, we're done */
+			if (path->slots[0] == 0)
+				break;
+			path->slots[0]--;
+		} else if (ret < 0) {
+			btrfs_end_transaction(trans);
+			goto out;
+		}
+
+		btrfs_item_key_to_cpu(path->nodes[0], &key, path->slots[0]);
+
+		/* No more keys of this type, we're done */
+		if (key.objectid != btrfs_ino(inode) || key.type != key_type)
+			break;
+
+		/*
+		 * This shouldn't be a performance sensitive function because
+		 * it's not used as part of truncate.  If it ever becomes
+		 * perf sensitive, change this to walk forward and bulk delete
+		 * items
+		 */
+		ret = btrfs_del_items(trans, root, path, path->slots[0], 1);
+		if (ret) {
+			btrfs_end_transaction(trans);
+			goto out;
+		}
+		count++;
+		btrfs_release_path(path);
+		btrfs_end_transaction(trans);
+	}
+	ret = count;
+	btrfs_end_transaction(trans);
+out:
+	btrfs_free_path(path);
+	return ret;
+}
+
+/*
+ * Drop all verity items
+ *
+ * @inode:  inode to drop verity items for
+ *
+ * In most contexts where we are dropping verity items, we want to do it for all
+ * the types of verity items, not a particular one.
+ *
+ * Returns: 0 on success, negative error code on failure.
+ */
+int btrfs_drop_verity_items(struct btrfs_inode *inode)
+{
+	int ret;
+
+	ret = drop_verity_items(inode, BTRFS_VERITY_DESC_ITEM_KEY);
+	if (ret < 0)
+		return ret;
+	ret = drop_verity_items(inode, BTRFS_VERITY_MERKLE_ITEM_KEY);
+	if (ret < 0)
+		return ret;
+
+	return 0;
+}
+
+/*
+ * Insert and write inode items with a given key type and offset.
+ *
+ * @inode:     inode to insert for
+ * @key_type:  key type to insert
+ * @offset:    item offset to insert at
+ * @src:       source data to write
+ * @len:       length of source data to write
+ *
+ * Write len bytes from src into items of up to 2K length.
+ * The inserted items will have key (ino, key_type, offset + off) where off is
+ * consecutively increasing from 0 up to the last item ending at offset + len.
+ *
+ * Returns 0 on success and a negative error code on failure.
+ */
+static int write_key_bytes(struct btrfs_inode *inode, u8 key_type, u64 offset,
+			   const char *src, u64 len)
+{
+	struct btrfs_trans_handle *trans;
+	struct btrfs_path *path;
+	struct btrfs_root *root = inode->root;
+	struct extent_buffer *leaf;
+	struct btrfs_key key;
+	unsigned long copy_bytes;
+	unsigned long src_offset = 0;
+	void *data;
+	int ret = 0;
+
+	path = btrfs_alloc_path();
+	if (!path)
+		return -ENOMEM;
+
+	while (len > 0) {
+		/* 1 for the new item being inserted */
+		trans = btrfs_start_transaction(root, 1);
+		if (IS_ERR(trans)) {
+			ret = PTR_ERR(trans);
+			break;
+		}
+
+		key.objectid = btrfs_ino(inode);
+		key.type = key_type;
+		key.offset = offset;
+
+		/*
+		 * Insert 2K at a time mostly to be friendly for smaller leaf
+		 * size filesystems
+		 */
+		copy_bytes = min_t(u64, len, 2048);
+
+		ret = btrfs_insert_empty_item(trans, root, path, &key, copy_bytes);
+		if (ret) {
+			btrfs_end_transaction(trans);
+			break;
+		}
+
+		leaf = path->nodes[0];
+
+		data = btrfs_item_ptr(leaf, path->slots[0], void);
+		write_extent_buffer(leaf, src + src_offset,
+				    (unsigned long)data, copy_bytes);
+		offset += copy_bytes;
+		src_offset += copy_bytes;
+		len -= copy_bytes;
+
+		btrfs_release_path(path);
+		btrfs_end_transaction(trans);
+	}
+
+	btrfs_free_path(path);
+	return ret;
+}
+
+/*
+ * Read inode items of the given key type and offset from the btree.
+ *
+ * @inode:      inode to read items of
+ * @key_type:   key type to read
+ * @offset:     item offset to read from
+ * @dest:       Buffer to read into. This parameter has slightly tricky
+ *              semantics.  If it is NULL, the function will not do any copying
+ *              and will just return the size of all the items up to len bytes.
+ *              If dest_page is passed, then the function will kmap_local the
+ *              page and ignore dest, but it must still be non-NULL to avoid the
+ *              counting-only behavior.
+ * @len:        length in bytes to read
+ * @dest_page:  copy into this page instead of the dest buffer
+ *
+ * Helper function to read items from the btree.  This returns the number of
+ * bytes read or < 0 for errors.  We can return short reads if the items don't
+ * exist on disk or aren't big enough to fill the desired length.  Supports
+ * reading into a provided buffer (dest) or into the page cache
+ *
+ * Returns number of bytes read or a negative error code on failure.
+ */
+static int read_key_bytes(struct btrfs_inode *inode, u8 key_type, u64 offset,
+			  char *dest, u64 len, struct page *dest_page)
+{
+	struct btrfs_path *path;
+	struct btrfs_root *root = inode->root;
+	struct extent_buffer *leaf;
+	struct btrfs_key key;
+	u64 item_end;
+	u64 copy_end;
+	int copied = 0;
+	u32 copy_offset;
+	unsigned long copy_bytes;
+	unsigned long dest_offset = 0;
+	void *data;
+	char *kaddr = dest;
+	int ret;
+
+	path = btrfs_alloc_path();
+	if (!path)
+		return -ENOMEM;
+
+	if (dest_page)
+		path->reada = READA_FORWARD;
+
+	key.objectid = btrfs_ino(inode);
+	key.type = key_type;
+	key.offset = offset;
+
+	ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
+	if (ret < 0) {
+		goto out;
+	} else if (ret > 0) {
+		ret = 0;
+		if (path->slots[0] == 0)
+			goto out;
+		path->slots[0]--;
+	}
+
+	while (len > 0) {
+		leaf = path->nodes[0];
+		btrfs_item_key_to_cpu(leaf, &key, path->slots[0]);
+
+		if (key.objectid != btrfs_ino(inode) || key.type != key_type)
+			break;
+
+		item_end = btrfs_item_size(leaf, path->slots[0]) + key.offset;
+
+		if (copied > 0) {
+			/*
+			 * Once we've copied something, we want all of the items
+			 * to be sequential
+			 */
+			if (key.offset != offset)
+				break;
+		} else {
+			/*
+			 * Our initial offset might be in the middle of an
+			 * item.  Make sure it all makes sense.
+			 */
+			if (key.offset > offset)
+				break;
+			if (item_end <= offset)
+				break;
+		}
+
+		/* desc = NULL to just sum all the item lengths */
+		if (!dest)
+			copy_end = item_end;
+		else
+			copy_end = min(offset + len, item_end);
+
+		/* Number of bytes in this item we want to copy */
+		copy_bytes = copy_end - offset;
+
+		/* Offset from the start of item for copying */
+		copy_offset = offset - key.offset;
+
+		if (dest) {
+			if (dest_page)
+				kaddr = kmap_local_page(dest_page);
+
+			data = btrfs_item_ptr(leaf, path->slots[0], void);
+			read_extent_buffer(leaf, kaddr + dest_offset,
+					   (unsigned long)data + copy_offset,
+					   copy_bytes);
+
+			if (dest_page)
+				kunmap_local(kaddr);
+		}
+
+		offset += copy_bytes;
+		dest_offset += copy_bytes;
+		len -= copy_bytes;
+		copied += copy_bytes;
+
+		path->slots[0]++;
+		if (path->slots[0] >= btrfs_header_nritems(path->nodes[0])) {
+			/*
+			 * We've reached the last slot in this leaf and we need
+			 * to go to the next leaf.
+			 */
+			ret = btrfs_next_leaf(root, path);
+			if (ret < 0) {
+				break;
+			} else if (ret > 0) {
+				ret = 0;
+				break;
+			}
+		}
+	}
+out:
+	btrfs_free_path(path);
+	if (!ret)
+		ret = copied;
+	return ret;
+}
+
+/*
+ * Delete an fsverity orphan
+ *
+ * @trans:  transaction to do the delete in
+ * @inode:  inode to orphan
+ *
+ * Capture verity orphan specific logic that is repeated in the couple places
+ * we delete verity orphans. Specifically, handling ENOENT and ignoring inodes
+ * with 0 links.
+ *
+ * Returns zero on success or a negative error code on failure.
+ */
+static int del_orphan(struct btrfs_trans_handle *trans, struct btrfs_inode *inode)
+{
+	struct btrfs_root *root = inode->root;
+	int ret;
+
+	/*
+	 * If the inode has no links, it is either already unlinked, or was
+	 * created with O_TMPFILE. In either case, it should have an orphan from
+	 * that other operation. Rather than reference count the orphans, we
+	 * simply ignore them here, because we only invoke the verity path in
+	 * the orphan logic when i_nlink is 1.
+	 */
+	if (!inode->vfs_inode.i_nlink)
+		return 0;
+
+	ret = btrfs_del_orphan_item(trans, root, btrfs_ino(inode));
+	if (ret == -ENOENT)
+		ret = 0;
+	return ret;
+}
+
+/*
+ * Rollback in-progress verity if we encounter an error.
+ *
+ * @inode:  inode verity had an error for
+ *
+ * We try to handle recoverable errors while enabling verity by rolling it back
+ * and just failing the operation, rather than having an fs level error no
+ * matter what. However, any error in rollback is unrecoverable.
+ *
+ * Returns 0 on success, negative error code on failure.
+ */
+static int rollback_verity(struct btrfs_inode *inode)
+{
+	struct btrfs_trans_handle *trans = NULL;
+	struct btrfs_root *root = inode->root;
+	int ret;
+
+	ASSERT(inode_is_locked(&inode->vfs_inode));
+	truncate_inode_pages(inode->vfs_inode.i_mapping, inode->vfs_inode.i_size);
+	clear_bit(BTRFS_INODE_VERITY_IN_PROGRESS, &inode->runtime_flags);
+	ret = btrfs_drop_verity_items(inode);
+	if (ret) {
+		btrfs_handle_fs_error(root->fs_info, ret,
+				"failed to drop verity items in rollback %llu",
+				(u64)inode->vfs_inode.i_ino);
+		goto out;
+	}
+
+	/*
+	 * 1 for updating the inode flag
+	 * 1 for deleting the orphan
+	 */
+	trans = btrfs_start_transaction(root, 2);
+	if (IS_ERR(trans)) {
+		ret = PTR_ERR(trans);
+		trans = NULL;
+		btrfs_handle_fs_error(root->fs_info, ret,
+			"failed to start transaction in verity rollback %llu",
+			(u64)inode->vfs_inode.i_ino);
+		goto out;
+	}
+	inode->ro_flags &= ~BTRFS_INODE_RO_VERITY;
+	btrfs_sync_inode_flags_to_i_flags(&inode->vfs_inode);
+	ret = btrfs_update_inode(trans, root, inode);
+	if (ret) {
+		btrfs_abort_transaction(trans, ret);
+		goto out;
+	}
+	ret = del_orphan(trans, inode);
+	if (ret) {
+		btrfs_abort_transaction(trans, ret);
+		goto out;
+	}
+out:
+	if (trans)
+		btrfs_end_transaction(trans);
+	return ret;
+}
+
+/*
+ * Finalize making the file a valid verity file
+ *
+ * @inode:      inode to be marked as verity
+ * @desc:       contents of the verity descriptor to write (not NULL)
+ * @desc_size:  size of the verity descriptor
+ *
+ * Do the actual work of finalizing verity after successfully writing the Merkle
+ * tree:
+ *
+ * - write out the descriptor items
+ * - mark the inode with the verity flag
+ * - delete the orphan item
+ * - mark the ro compat bit
+ * - clear the in progress bit
+ *
+ * Returns 0 on success, negative error code on failure.
+ */
+static int finish_verity(struct btrfs_inode *inode, const void *desc,
+			 size_t desc_size)
+{
+	struct btrfs_trans_handle *trans = NULL;
+	struct btrfs_root *root = inode->root;
+	struct btrfs_verity_descriptor_item item;
+	int ret;
+
+	/* Write out the descriptor item */
+	memset(&item, 0, sizeof(item));
+	btrfs_set_stack_verity_descriptor_size(&item, desc_size);
+	ret = write_key_bytes(inode, BTRFS_VERITY_DESC_ITEM_KEY, 0,
+			      (const char *)&item, sizeof(item));
+	if (ret)
+		goto out;
+
+	/* Write out the descriptor itself */
+	ret = write_key_bytes(inode, BTRFS_VERITY_DESC_ITEM_KEY, 1,
+			      desc, desc_size);
+	if (ret)
+		goto out;
+
+	/*
+	 * 1 for updating the inode flag
+	 * 1 for deleting the orphan
+	 */
+	trans = btrfs_start_transaction(root, 2);
+	if (IS_ERR(trans)) {
+		ret = PTR_ERR(trans);
+		goto out;
+	}
+	inode->ro_flags |= BTRFS_INODE_RO_VERITY;
+	btrfs_sync_inode_flags_to_i_flags(&inode->vfs_inode);
+	ret = btrfs_update_inode(trans, root, inode);
+	if (ret)
+		goto end_trans;
+	ret = del_orphan(trans, inode);
+	if (ret)
+		goto end_trans;
+	clear_bit(BTRFS_INODE_VERITY_IN_PROGRESS, &inode->runtime_flags);
+	btrfs_set_fs_compat_ro(root->fs_info, VERITY);
+end_trans:
+	btrfs_end_transaction(trans);
+out:
+	return ret;
+
+}
+
+/*
+ * fsverity op that begins enabling verity.
+ *
+ * @filp:  file to enable verity on
+ *
+ * Begin enabling fsverity for the file. We drop any existing verity items, add
+ * an orphan and set the in progress bit.
+ *
+ * Returns 0 on success, negative error code on failure.
+ */
+static int btrfs_begin_enable_verity(struct file *filp)
+{
+	struct btrfs_inode *inode = BTRFS_I(file_inode(filp));
+	struct btrfs_root *root = inode->root;
+	struct btrfs_trans_handle *trans;
+	int ret;
+
+	ASSERT(inode_is_locked(file_inode(filp)));
+
+	if (test_bit(BTRFS_INODE_VERITY_IN_PROGRESS, &inode->runtime_flags))
+		return -EBUSY;
+
+	/*
+	 * This should almost never do anything, but theoretically, it's
+	 * possible that we failed to enable verity on a file, then were
+	 * interrupted or failed while rolling back, failed to cleanup the
+	 * orphan, and finally attempt to enable verity again.
+	 */
+	ret = btrfs_drop_verity_items(inode);
+	if (ret)
+		return ret;
+
+	/* 1 for the orphan item */
+	trans = btrfs_start_transaction(root, 1);
+	if (IS_ERR(trans))
+		return PTR_ERR(trans);
+
+	ret = btrfs_orphan_add(trans, inode);
+	if (!ret)
+		set_bit(BTRFS_INODE_VERITY_IN_PROGRESS, &inode->runtime_flags);
+	btrfs_end_transaction(trans);
+
+	return 0;
+}
+
+/*
+ * fsverity op that ends enabling verity.
+ *
+ * @filp:              file we are finishing enabling verity on
+ * @desc:              verity descriptor to write out (NULL in error conditions)
+ * @desc_size:         size of the verity descriptor (variable with signatures)
+ * @merkle_tree_size:  size of the merkle tree in bytes
+ *
+ * If desc is null, then VFS is signaling an error occurred during verity
+ * enable, and we should try to rollback. Otherwise, attempt to finish verity.
+ *
+ * Returns 0 on success, negative error code on error.
+ */
+static int btrfs_end_enable_verity(struct file *filp, const void *desc,
+				   size_t desc_size, u64 merkle_tree_size)
+{
+	struct btrfs_inode *inode = BTRFS_I(file_inode(filp));
+	int ret = 0;
+	int rollback_ret;
+
+	ASSERT(inode_is_locked(file_inode(filp)));
+
+	if (desc == NULL)
+		goto rollback;
+
+	ret = finish_verity(inode, desc, desc_size);
+	if (ret)
+		goto rollback;
+	return ret;
+
+rollback:
+	rollback_ret = rollback_verity(inode);
+	if (rollback_ret)
+		btrfs_err(inode->root->fs_info,
+			  "failed to rollback verity items: %d", rollback_ret);
+	return ret;
+}
+
+/*
+ * fsverity op that gets the struct fsverity_descriptor.
+ *
+ * @inode:     inode to get the descriptor of
+ * @buf:       output buffer for the descriptor contents
+ * @buf_size:  size of the output buffer. 0 to query the size
+ *
+ * fsverity does a two pass setup for reading the descriptor, in the first pass
+ * it calls with buf_size = 0 to query the size of the descriptor, and then in
+ * the second pass it actually reads the descriptor off disk.
+ *
+ * Returns the size on success or a negative error code on failure.
+ */
+int btrfs_get_verity_descriptor(struct inode *inode, void *buf, size_t buf_size)
+{
+	u64 true_size;
+	int ret = 0;
+	struct btrfs_verity_descriptor_item item;
+
+	memset(&item, 0, sizeof(item));
+	ret = read_key_bytes(BTRFS_I(inode), BTRFS_VERITY_DESC_ITEM_KEY, 0,
+			     (char *)&item, sizeof(item), NULL);
+	if (ret < 0)
+		return ret;
+
+	if (item.reserved[0] != 0 || item.reserved[1] != 0)
+		return -EUCLEAN;
+
+	true_size = btrfs_stack_verity_descriptor_size(&item);
+	if (true_size > INT_MAX)
+		return -EUCLEAN;
+
+	if (buf_size == 0)
+		return true_size;
+	if (buf_size < true_size)
+		return -ERANGE;
+
+	ret = read_key_bytes(BTRFS_I(inode), BTRFS_VERITY_DESC_ITEM_KEY, 1,
+			     buf, buf_size, NULL);
+	if (ret < 0)
+		return ret;
+	if (ret != true_size)
+		return -EIO;
+
+	return true_size;
+}
+
+/*
+ * fsverity op that reads and caches a merkle tree page.
+ *
+ * @inode:         inode to read a merkle tree page for
+ * @index:         page index relative to the start of the merkle tree
+ * @num_ra_pages:  number of pages to readahead. Optional, we ignore it
+ *
+ * The Merkle tree is stored in the filesystem btree, but its pages are cached
+ * with a logical position past EOF in the inode's mapping.
+ *
+ * Returns the page we read, or an ERR_PTR on error.
+ */
+static struct page *btrfs_read_merkle_tree_page(struct inode *inode,
+						pgoff_t index,
+						unsigned long num_ra_pages)
+{
+	struct folio *folio;
+	u64 off = (u64)index << PAGE_SHIFT;
+	loff_t merkle_pos = merkle_file_pos(inode);
+	int ret;
+
+	if (merkle_pos < 0)
+		return ERR_PTR(merkle_pos);
+	if (merkle_pos > inode->i_sb->s_maxbytes - off - PAGE_SIZE)
+		return ERR_PTR(-EFBIG);
+	index += merkle_pos >> PAGE_SHIFT;
+again:
+	folio = __filemap_get_folio(inode->i_mapping, index, FGP_ACCESSED, 0);
+	if (!IS_ERR(folio)) {
+		if (folio_test_uptodate(folio))
+			goto out;
+
+		folio_lock(folio);
+		/* If it's not uptodate after we have the lock, we got a read error. */
+		if (!folio_test_uptodate(folio)) {
+			folio_unlock(folio);
+			folio_put(folio);
+			return ERR_PTR(-EIO);
+		}
+		folio_unlock(folio);
+		goto out;
+	}
+
+	folio = filemap_alloc_folio(mapping_gfp_constraint(inode->i_mapping, ~__GFP_FS),
+				    0);
+	if (!folio)
+		return ERR_PTR(-ENOMEM);
+
+	ret = filemap_add_folio(inode->i_mapping, folio, index, GFP_NOFS);
+	if (ret) {
+		folio_put(folio);
+		/* Did someone else insert a folio here? */
+		if (ret == -EEXIST)
+			goto again;
+		return ERR_PTR(ret);
+	}
+
+	/*
+	 * Merkle item keys are indexed from byte 0 in the merkle tree.
+	 * They have the form:
+	 *
+	 * [ inode objectid, BTRFS_MERKLE_ITEM_KEY, offset in bytes ]
+	 */
+	ret = read_key_bytes(BTRFS_I(inode), BTRFS_VERITY_MERKLE_ITEM_KEY, off,
+			     folio_address(folio), PAGE_SIZE, &folio->page);
+	if (ret < 0) {
+		folio_put(folio);
+		return ERR_PTR(ret);
+	}
+	if (ret < PAGE_SIZE)
+		folio_zero_segment(folio, ret, PAGE_SIZE);
+
+	folio_mark_uptodate(folio);
+	folio_unlock(folio);
+
+out:
+	return folio_file_page(folio, index);
+}
+
+/*
+ * fsverity op that writes a Merkle tree block into the btree.
+ *
+ * @inode:	inode to write a Merkle tree block for
+ * @buf:	Merkle tree block to write
+ * @pos:	the position of the block in the Merkle tree (in bytes)
+ * @size:	the Merkle tree block size (in bytes)
+ *
+ * Returns 0 on success or negative error code on failure
+ */
+static int btrfs_write_merkle_tree_block(struct inode *inode, const void *buf,
+					 u64 pos, unsigned int size)
+{
+	loff_t merkle_pos = merkle_file_pos(inode);
+
+	if (merkle_pos < 0)
+		return merkle_pos;
+	if (merkle_pos > inode->i_sb->s_maxbytes - pos - size)
+		return -EFBIG;
+
+	return write_key_bytes(BTRFS_I(inode), BTRFS_VERITY_MERKLE_ITEM_KEY,
+			       pos, buf, size);
+}
+
+const struct fsverity_operations btrfs_verityops = {
+	.begin_enable_verity     = btrfs_begin_enable_verity,
+	.end_enable_verity       = btrfs_end_enable_verity,
+	.get_verity_descriptor   = btrfs_get_verity_descriptor,
+	.read_merkle_tree_page   = btrfs_read_merkle_tree_page,
+	.write_merkle_tree_block = btrfs_write_merkle_tree_block,
+};