diff --git a/fs/verity/Makefile b/fs/verity/Makefile
index 7fa628cd5eba24..04b37475fd280a 100644
--- a/fs/verity/Makefile
+++ b/fs/verity/Makefile
@@ -1,6 +1,7 @@
 # SPDX-License-Identifier: GPL-2.0
 
-obj-$(CONFIG_FS_VERITY) += hash_algs.o \
+obj-$(CONFIG_FS_VERITY) += enable.o \
+			   hash_algs.o \
 			   init.o \
 			   open.o \
 			   verify.o
diff --git a/fs/verity/enable.c b/fs/verity/enable.c
new file mode 100644
index 00000000000000..144721bbe4aab9
--- /dev/null
+++ b/fs/verity/enable.c
@@ -0,0 +1,341 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * fs/verity/enable.c: ioctl to enable verity on a file
+ *
+ * Copyright 2019 Google LLC
+ */
+
+#include "fsverity_private.h"
+
+#include <crypto/hash.h>
+#include <linux/mount.h>
+#include <linux/pagemap.h>
+#include <linux/sched/signal.h>
+#include <linux/uaccess.h>
+
+static int build_merkle_tree_level(struct inode *inode, unsigned int level,
+				   u64 num_blocks_to_hash,
+				   const struct merkle_tree_params *params,
+				   u8 *pending_hashes,
+				   struct ahash_request *req)
+{
+	const struct fsverity_operations *vops = inode->i_sb->s_vop;
+	unsigned int pending_size = 0;
+	u64 dst_block_num;
+	u64 i;
+	int err;
+
+	if (WARN_ON(params->block_size != PAGE_SIZE)) /* checked earlier too */
+		return -EINVAL;
+
+	if (level < params->num_levels) {
+		dst_block_num = params->level_start[level];
+	} else {
+		if (WARN_ON(num_blocks_to_hash != 1))
+			return -EINVAL;
+		dst_block_num = 0; /* unused */
+	}
+
+	for (i = 0; i < num_blocks_to_hash; i++) {
+		struct page *src_page;
+
+		if ((pgoff_t)i % 10000 == 0 || i + 1 == num_blocks_to_hash)
+			pr_debug("Hashing block %llu of %llu for level %u\n",
+				 i + 1, num_blocks_to_hash, level);
+
+		if (level == 0)
+			/* Leaf: hashing a data block */
+			src_page = read_mapping_page(inode->i_mapping, i, NULL);
+		else
+			/* Non-leaf: hashing hash block from level below */
+			src_page = vops->read_merkle_tree_page(inode,
+					params->level_start[level - 1] + i);
+		if (IS_ERR(src_page)) {
+			err = PTR_ERR(src_page);
+			fsverity_err(inode,
+				     "Error %d reading Merkle tree page %llu",
+				     err, params->level_start[level - 1] + i);
+			return err;
+		}
+
+		err = fsverity_hash_page(params, inode, req, src_page,
+					 &pending_hashes[pending_size]);
+		put_page(src_page);
+		if (err)
+			return err;
+		pending_size += params->digest_size;
+
+		if (level == params->num_levels) /* Root hash? */
+			return 0;
+
+		if (pending_size + params->digest_size > params->block_size ||
+		    i + 1 == num_blocks_to_hash) {
+			/* Flush the pending hash block */
+			memset(&pending_hashes[pending_size], 0,
+			       params->block_size - pending_size);
+			err = vops->write_merkle_tree_block(inode,
+					pending_hashes,
+					dst_block_num,
+					params->log_blocksize);
+			if (err) {
+				fsverity_err(inode,
+					     "Error %d writing Merkle tree block %llu",
+					     err, dst_block_num);
+				return err;
+			}
+			dst_block_num++;
+			pending_size = 0;
+		}
+
+		if (fatal_signal_pending(current))
+			return -EINTR;
+		cond_resched();
+	}
+	return 0;
+}
+
+/*
+ * Build the Merkle tree for the given inode using the given parameters, and
+ * return the root hash in @root_hash.
+ *
+ * The tree is written to a filesystem-specific location as determined by the
+ * ->write_merkle_tree_block() method.  However, the blocks that comprise the
+ * tree are the same for all filesystems.
+ */
+static int build_merkle_tree(struct inode *inode,
+			     const struct merkle_tree_params *params,
+			     u8 *root_hash)
+{
+	u8 *pending_hashes;
+	struct ahash_request *req;
+	u64 blocks;
+	unsigned int level;
+	int err = -ENOMEM;
+
+	if (inode->i_size == 0) {
+		/* Empty file is a special case; root hash is all 0's */
+		memset(root_hash, 0, params->digest_size);
+		return 0;
+	}
+
+	pending_hashes = kmalloc(params->block_size, GFP_KERNEL);
+	req = ahash_request_alloc(params->hash_alg->tfm, GFP_KERNEL);
+	if (!pending_hashes || !req)
+		goto out;
+
+	/*
+	 * Build each level of the Merkle tree, starting at the leaf level
+	 * (level 0) and ascending to the root node (level 'num_levels - 1').
+	 * Then at the end (level 'num_levels'), calculate the root hash.
+	 */
+	blocks = (inode->i_size + params->block_size - 1) >>
+		 params->log_blocksize;
+	for (level = 0; level <= params->num_levels; level++) {
+		err = build_merkle_tree_level(inode, level, blocks, params,
+					      pending_hashes, req);
+		if (err)
+			goto out;
+		blocks = (blocks + params->hashes_per_block - 1) >>
+			 params->log_arity;
+	}
+	memcpy(root_hash, pending_hashes, params->digest_size);
+	err = 0;
+out:
+	kfree(pending_hashes);
+	ahash_request_free(req);
+	return err;
+}
+
+static int enable_verity(struct file *filp,
+			 const struct fsverity_enable_arg *arg)
+{
+	struct inode *inode = file_inode(filp);
+	const struct fsverity_operations *vops = inode->i_sb->s_vop;
+	struct merkle_tree_params params = { };
+	struct fsverity_descriptor *desc;
+	size_t desc_size = sizeof(*desc);
+	struct fsverity_info *vi;
+	int err;
+
+	/* Start initializing the fsverity_descriptor */
+	desc = kzalloc(desc_size, GFP_KERNEL);
+	if (!desc)
+		return -ENOMEM;
+	desc->version = 1;
+	desc->hash_algorithm = arg->hash_algorithm;
+	desc->log_blocksize = ilog2(arg->block_size);
+
+	/* Get the salt if the user provided one */
+	if (arg->salt_size &&
+	    copy_from_user(desc->salt,
+			   (const u8 __user *)(uintptr_t)arg->salt_ptr,
+			   arg->salt_size)) {
+		err = -EFAULT;
+		goto out;
+	}
+	desc->salt_size = arg->salt_size;
+
+	desc->data_size = cpu_to_le64(inode->i_size);
+
+	pr_debug("Building Merkle tree...\n");
+
+	/* Prepare the Merkle tree parameters */
+	err = fsverity_init_merkle_tree_params(&params, inode,
+					       arg->hash_algorithm,
+					       desc->log_blocksize,
+					       desc->salt, desc->salt_size);
+	if (err)
+		goto out;
+
+	/* Tell the filesystem that verity is being enabled on the file */
+	err = vops->begin_enable_verity(filp);
+	if (err)
+		goto out;
+
+	/* Build the Merkle tree */
+	BUILD_BUG_ON(sizeof(desc->root_hash) < FS_VERITY_MAX_DIGEST_SIZE);
+	err = build_merkle_tree(inode, &params, desc->root_hash);
+	if (err) {
+		fsverity_err(inode, "Error %d building Merkle tree", err);
+		goto rollback;
+	}
+	pr_debug("Done building Merkle tree.  Root hash is %s:%*phN\n",
+		 params.hash_alg->name, params.digest_size, desc->root_hash);
+
+	/*
+	 * Create the fsverity_info.  Don't bother trying to save work by
+	 * reusing the merkle_tree_params from above.  Instead, just create the
+	 * fsverity_info from the fsverity_descriptor as if it were just loaded
+	 * from disk.  This is simpler, and it serves as an extra check that the
+	 * metadata we're writing is valid before actually enabling verity.
+	 */
+	vi = fsverity_create_info(inode, desc, desc_size);
+	if (IS_ERR(vi)) {
+		err = PTR_ERR(vi);
+		goto rollback;
+	}
+
+	/* Tell the filesystem to finish enabling verity on the file */
+	err = vops->end_enable_verity(filp, desc, desc_size, params.tree_size);
+	if (err) {
+		fsverity_err(inode, "%ps() failed with err %d",
+			     vops->end_enable_verity, err);
+		fsverity_free_info(vi);
+	} else if (WARN_ON(!IS_VERITY(inode))) {
+		err = -EINVAL;
+		fsverity_free_info(vi);
+	} else {
+		/* Successfully enabled verity */
+
+		/*
+		 * Readers can start using ->i_verity_info immediately, so it
+		 * can't be rolled back once set.  So don't set it until just
+		 * after the filesystem has successfully enabled verity.
+		 */
+		fsverity_set_info(inode, vi);
+	}
+out:
+	kfree(params.hashstate);
+	kfree(desc);
+	return err;
+
+rollback:
+	(void)vops->end_enable_verity(filp, NULL, 0, params.tree_size);
+	goto out;
+}
+
+/**
+ * fsverity_ioctl_enable() - enable verity on a file
+ *
+ * Enable fs-verity on a file.  See the "FS_IOC_ENABLE_VERITY" section of
+ * Documentation/filesystems/fsverity.rst for the documentation.
+ *
+ * Return: 0 on success, -errno on failure
+ */
+int fsverity_ioctl_enable(struct file *filp, const void __user *uarg)
+{
+	struct inode *inode = file_inode(filp);
+	struct fsverity_enable_arg arg;
+	int err;
+
+	if (copy_from_user(&arg, uarg, sizeof(arg)))
+		return -EFAULT;
+
+	if (arg.version != 1)
+		return -EINVAL;
+
+	if (arg.__reserved1 ||
+	    memchr_inv(arg.__reserved2, 0, sizeof(arg.__reserved2)))
+		return -EINVAL;
+
+	if (arg.block_size != PAGE_SIZE)
+		return -EINVAL;
+
+	if (arg.salt_size > FIELD_SIZEOF(struct fsverity_descriptor, salt))
+		return -EMSGSIZE;
+
+	if (arg.sig_size)
+		return -EINVAL;
+
+	/*
+	 * Require a regular file with write access.  But the actual fd must
+	 * still be readonly so that we can lock out all writers.  This is
+	 * needed to guarantee that no writable fds exist to the file once it
+	 * has verity enabled, and to stabilize the data being hashed.
+	 */
+
+	err = inode_permission(inode, MAY_WRITE);
+	if (err)
+		return err;
+
+	if (IS_APPEND(inode))
+		return -EPERM;
+
+	if (S_ISDIR(inode->i_mode))
+		return -EISDIR;
+
+	if (!S_ISREG(inode->i_mode))
+		return -EINVAL;
+
+	err = mnt_want_write_file(filp);
+	if (err) /* -EROFS */
+		return err;
+
+	err = deny_write_access(filp);
+	if (err) /* -ETXTBSY */
+		goto out_drop_write;
+
+	inode_lock(inode);
+
+	if (IS_VERITY(inode)) {
+		err = -EEXIST;
+		goto out_unlock;
+	}
+
+	err = enable_verity(filp, &arg);
+	if (err)
+		goto out_unlock;
+
+	/*
+	 * Some pages of the file may have been evicted from pagecache after
+	 * being used in the Merkle tree construction, then read into pagecache
+	 * again by another process reading from the file concurrently.  Since
+	 * these pages didn't undergo verification against the file measurement
+	 * which fs-verity now claims to be enforcing, we have to wipe the
+	 * pagecache to ensure that all future reads are verified.
+	 */
+	filemap_write_and_wait(inode->i_mapping);
+	truncate_inode_pages(inode->i_mapping, 0);
+
+	/*
+	 * allow_write_access() is needed to pair with deny_write_access().
+	 * Regardless, the filesystem won't allow writing to verity files.
+	 */
+out_unlock:
+	inode_unlock(inode);
+	allow_write_access(filp);
+out_drop_write:
+	mnt_drop_write_file(filp);
+	return err;
+}
+EXPORT_SYMBOL_GPL(fsverity_ioctl_enable);
diff --git a/include/linux/fsverity.h b/include/linux/fsverity.h
index ecd47e748c7f64..7ef2ef82653409 100644
--- a/include/linux/fsverity.h
+++ b/include/linux/fsverity.h
@@ -17,6 +17,42 @@
 /* Verity operations for filesystems */
 struct fsverity_operations {
 
+	/**
+	 * Begin enabling verity on the given file.
+	 *
+	 * @filp: a readonly file descriptor for the file
+	 *
+	 * The filesystem must do any needed filesystem-specific preparations
+	 * for enabling verity, e.g. evicting inline data.
+	 *
+	 * i_rwsem is held for write.
+	 *
+	 * Return: 0 on success, -errno on failure
+	 */
+	int (*begin_enable_verity)(struct file *filp);
+
+	/**
+	 * End enabling verity on the given file.
+	 *
+	 * @filp: a readonly file descriptor for the file
+	 * @desc: the verity descriptor to write, or NULL on failure
+	 * @desc_size: size of verity descriptor, or 0 on failure
+	 * @merkle_tree_size: total bytes the Merkle tree took up
+	 *
+	 * If desc == NULL, then enabling verity failed and the filesystem only
+	 * must do any necessary cleanups.  Else, it must also store the given
+	 * verity descriptor to a fs-specific location associated with the inode
+	 * and do any fs-specific actions needed to mark the inode as a verity
+	 * inode, e.g. setting a bit in the on-disk inode.  The filesystem is
+	 * also responsible for setting the S_VERITY flag in the VFS inode.
+	 *
+	 * i_rwsem is held for write.
+	 *
+	 * Return: 0 on success, -errno on failure
+	 */
+	int (*end_enable_verity)(struct file *filp, const void *desc,
+				 size_t desc_size, u64 merkle_tree_size);
+
 	/**
 	 * Get the verity descriptor of the given inode.
 	 *
@@ -50,6 +86,22 @@ struct fsverity_operations {
 	 */
 	struct page *(*read_merkle_tree_page)(struct inode *inode,
 					      pgoff_t index);
+
+	/**
+	 * Write a Merkle tree block to the given inode.
+	 *
+	 * @inode: the inode for which the Merkle tree is being built
+	 * @buf: block to write
+	 * @index: 0-based index of the block within the Merkle tree
+	 * @log_blocksize: log base 2 of the Merkle tree block size
+	 *
+	 * This is only called between ->begin_enable_verity() and
+	 * ->end_enable_verity().  i_rwsem is held for write.
+	 *
+	 * Return: 0 on success, -errno on failure
+	 */
+	int (*write_merkle_tree_block)(struct inode *inode, const void *buf,
+				       u64 index, int log_blocksize);
 };
 
 #ifdef CONFIG_FS_VERITY
@@ -60,6 +112,10 @@ static inline struct fsverity_info *fsverity_get_info(const struct inode *inode)
 	return READ_ONCE(inode->i_verity_info);
 }
 
+/* enable.c */
+
+extern int fsverity_ioctl_enable(struct file *filp, const void __user *arg);
+
 /* open.c */
 
 extern int fsverity_file_open(struct inode *inode, struct file *filp);
@@ -79,6 +135,14 @@ static inline struct fsverity_info *fsverity_get_info(const struct inode *inode)
 	return NULL;
 }
 
+/* enable.c */
+
+static inline int fsverity_ioctl_enable(struct file *filp,
+					const void __user *arg)
+{
+	return -EOPNOTSUPP;
+}
+
 /* open.c */
 
 static inline int fsverity_file_open(struct inode *inode, struct file *filp)
