fs/verity/enable.c - linux - Git at Google

 // SPDX-License-Identifier: GPL-2.0
 /*
  * 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/sched/signal.h>
 #include <linux/uaccess.h>

 struct block_buffer {
 	u32 filled;
 	bool is_root_hash;
 	u8 *data;
 };

 /* Hash a block, writing the result to the next level's pending block buffer. */
 static int hash_one_block(struct inode *inode,
 			  const struct merkle_tree_params *params,
 			  struct block_buffer *cur)
 {
 	struct block_buffer *next = cur + 1;
 	int err;

 	/*
 	 * Safety check to prevent a buffer overflow in case of a filesystem bug
 	 * that allows the file size to change despite deny_write_access(), or a
 	 * bug in the Merkle tree logic itself
 	 */
 	if (WARN_ON_ONCE(next->is_root_hash && next->filled != 0))
 		return -EINVAL;

 	/* Zero-pad the block if it's shorter than the block size. */
 	memset(&cur->data[cur->filled], 0, params->block_size - cur->filled);

 	err = fsverity_hash_block(params, inode, cur->data,
 				  &next->data[next->filled]);
 	if (err)
 		return err;
 	next->filled += params->digest_size;
 	cur->filled = 0;
 	return 0;
 }

 static int write_merkle_tree_block(struct inode *inode, const u8 *buf,
 				   unsigned long index,
 				   const struct merkle_tree_params *params)
 {
 	u64 pos = (u64)index << params->log_blocksize;
 	int err;

 	err = inode->i_sb->s_vop->write_merkle_tree_block(inode, buf, pos,
 							  params->block_size);
 	if (err)
 		fsverity_err(inode, "Error %d writing Merkle tree block %lu",
 			     err, index);
 	return err;
 }

 /*
  * Build the Merkle tree for the given file 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 file *filp,
 			     const struct merkle_tree_params *params,
 			     u8 *root_hash)
 {
 	struct inode *inode = file_inode(filp);
 	const u64 data_size = inode->i_size;
 	const int num_levels = params->num_levels;
 	struct block_buffer _buffers[1 + FS_VERITY_MAX_LEVELS + 1] = {};
 	struct block_buffer *buffers = &_buffers[1];
 	unsigned long level_offset[FS_VERITY_MAX_LEVELS];
 	int level;
 	u64 offset;
 	int err;

 	if (data_size == 0) {
 		/* Empty file is a special case; root hash is all 0's */
 		memset(root_hash, 0, params->digest_size);
 		return 0;
 	}

 	/*
 	 * Allocate the block buffers.  Buffer "-1" is for data blocks.
 	 * Buffers 0 <= level < num_levels are for the actual tree levels.
 	 * Buffer 'num_levels' is for the root hash.
 	 */
 	for (level = -1; level < num_levels; level++) {
 		buffers[level].data = kzalloc(params->block_size, GFP_KERNEL);
 		if (!buffers[level].data) {
 			err = -ENOMEM;
 			goto out;
 		}
 	}
 	buffers[num_levels].data = root_hash;
 	buffers[num_levels].is_root_hash = true;

 	BUILD_BUG_ON(sizeof(level_offset) != sizeof(params->level_start));
 	memcpy(level_offset, params->level_start, sizeof(level_offset));

 	/* Hash each data block, also hashing the tree blocks as they fill up */
 	for (offset = 0; offset < data_size; offset += params->block_size) {
 		ssize_t bytes_read;
 		loff_t pos = offset;

 		buffers[-1].filled = min_t(u64, params->block_size,
 					   data_size - offset);
 		bytes_read = __kernel_read(filp, buffers[-1].data,
 					   buffers[-1].filled, &pos);
 		if (bytes_read < 0) {
 			err = bytes_read;
 			fsverity_err(inode, "Error %d reading file data", err);
 			goto out;
 		}
 		if (bytes_read != buffers[-1].filled) {
 			err = -EINVAL;
 			fsverity_err(inode, "Short read of file data");
 			goto out;
 		}
 		err = hash_one_block(inode, params, &buffers[-1]);
 		if (err)
 			goto out;
 		for (level = 0; level < num_levels; level++) {
 			if (buffers[level].filled + params->digest_size <=
 			    params->block_size) {
 				/* Next block at @level isn't full yet */
 				break;
 			}
 			/* Next block at @level is full */

 			err = hash_one_block(inode, params, &buffers[level]);
 			if (err)
 				goto out;
 			err = write_merkle_tree_block(inode,
 						      buffers[level].data,
 						      level_offset[level],
 						      params);
 			if (err)
 				goto out;
 			level_offset[level]++;
 		}
 		if (fatal_signal_pending(current)) {
 			err = -EINTR;
 			goto out;
 		}
 		cond_resched();
 	}
 	/* Finish all nonempty pending tree blocks. */
 	for (level = 0; level < num_levels; level++) {
 		if (buffers[level].filled != 0) {
 			err = hash_one_block(inode, params, &buffers[level]);
 			if (err)
 				goto out;
 			err = write_merkle_tree_block(inode,
 						      buffers[level].data,
 						      level_offset[level],
 						      params);
 			if (err)
 				goto out;
 		}
 	}
 	/* The root hash was filled by the last call to hash_one_block(). */
 	if (WARN_ON_ONCE(buffers[num_levels].filled != params->digest_size)) {
 		err = -EINVAL;
 		goto out;
 	}
 	err = 0;
 out:
 	for (level = -1; level < num_levels; level++)
 		kfree(buffers[level].data);
 	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 = struct_size(desc, signature, arg->sig_size);
 	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, u64_to_user_ptr(arg->salt_ptr),
 			   arg->salt_size)) {
 		err = -EFAULT;
 		goto out;
 	}
 	desc->salt_size = arg->salt_size;

 	/* Get the builtin signature if the user provided one */
 	if (arg->sig_size &&
 	    copy_from_user(desc->signature, u64_to_user_ptr(arg->sig_ptr),
 			   arg->sig_size)) {
 		err = -EFAULT;
 		goto out;
 	}
 	desc->sig_size = cpu_to_le32(arg->sig_size);

 	desc->data_size = cpu_to_le64(inode->i_size);

 	/* 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;

 	/*
 	 * Start enabling verity on this file, serialized by the inode lock.
 	 * Fail if verity is already enabled or is already being enabled.
 	 */
 	inode_lock(inode);
 	if (IS_VERITY(inode))
 		err = -EEXIST;
 	else
 		err = vops->begin_enable_verity(filp);
 	inode_unlock(inode);
 	if (err)
 		goto out;

 	/*
 	 * Build the Merkle tree.  Don't hold the inode lock during this, since
 	 * on huge files this may take a very long time and we don't want to
 	 * force unrelated syscalls like chown() to block forever.  We don't
 	 * need the inode lock here because deny_write_access() already prevents
 	 * the file from being written to or truncated, and we still serialize
 	 * ->begin_enable_verity() and ->end_enable_verity() using the inode
 	 * lock and only allow one process to be here at a time on a given file.
 	 */
 	BUILD_BUG_ON(sizeof(desc->root_hash) < FS_VERITY_MAX_DIGEST_SIZE);
 	err = build_merkle_tree(filp, &params, desc->root_hash);
 	if (err) {
 		fsverity_err(inode, "Error %d building Merkle tree", err);
 		goto rollback;
 	}

 	/*
 	 * 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);
 	if (IS_ERR(vi)) {
 		err = PTR_ERR(vi);
 		goto rollback;
 	}

 	/*
 	 * Tell the filesystem to finish enabling verity on the file.
 	 * Serialized with ->begin_enable_verity() by the inode lock.
 	 */
 	inode_lock(inode);
 	err = vops->end_enable_verity(filp, desc, desc_size, params.tree_size);
 	inode_unlock(inode);
 	if (err) {
 		fsverity_err(inode, "%ps() failed with err %d",
 			     vops->end_enable_verity, err);
 		fsverity_free_info(vi);
 	} else if (WARN_ON_ONCE(!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:
 	inode_lock(inode);
 	(void)vops->end_enable_verity(filp, NULL, 0, params.tree_size);
 	inode_unlock(inode);
 	goto out;
 }

 /**
  * fsverity_ioctl_enable() - enable verity on a file
  * @filp: file to enable verity on
  * @uarg: user pointer to fsverity_enable_arg
  *
  * 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 (!is_power_of_2(arg.block_size))
 		return -EINVAL;

 	if (arg.salt_size > sizeof_field(struct fsverity_descriptor, salt))
 		return -EMSGSIZE;

 	if (arg.sig_size > FS_VERITY_MAX_SIGNATURE_SIZE)
 		return -EMSGSIZE;

 	/*
 	 * 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 = file_permission(filp, MAY_WRITE);
 	if (err)
 		return err;
 	/*
 	 * __kernel_read() is used while building the Merkle tree.  So, we can't
 	 * allow file descriptors that were opened for ioctl access only, using
 	 * the special nonstandard access mode 3.  O_RDONLY only, please!
 	 */
 	if (!(filp->f_mode & FMODE_READ))
 		return -EBADF;

 	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;

 	err = enable_verity(filp, &arg);

 	/*
 	 * We no longer drop the inode's pagecache after enabling verity.  This
 	 * used to be done to try to avoid a race condition where pages could be
 	 * evicted after being used in the Merkle tree construction, then
 	 * re-instantiated by a concurrent read.  Such pages are unverified, and
 	 * the backing storage could have filled them with different content, so
 	 * they shouldn't be used to fulfill reads once verity is enabled.
 	 *
 	 * But, dropping the pagecache has a big performance impact, and it
 	 * doesn't fully solve the race condition anyway.  So for those reasons,
 	 * and also because this race condition isn't very important relatively
 	 * speaking (especially for small-ish files, where the chance of a page
 	 * being used, evicted, *and* re-instantiated all while enabling verity
 	 * is quite small), we no longer drop the inode's pagecache.
 	 */

 	/*
 	 * allow_write_access() is needed to pair with deny_write_access().
 	 * Regardless, the filesystem won't allow writing to verity files.
 	 */
 	allow_write_access(filp);
 out_drop_write:
 	mnt_drop_write_file(filp);
 	return err;
 }
 EXPORT_SYMBOL_GPL(fsverity_ioctl_enable);
	// SPDX-License-Identifier: GPL-2.0
	/*
	* 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/sched/signal.h>
	#include <linux/uaccess.h>

	struct block_buffer {
	u32 filled;
	bool is_root_hash;
	u8 *data;
	};

	/* Hash a block, writing the result to the next level's pending block buffer. */
	static int hash_one_block(struct inode *inode,
	const struct merkle_tree_params *params,
	struct block_buffer *cur)
	{
	struct block_buffer *next = cur + 1;
	int err;

	/*
	* Safety check to prevent a buffer overflow in case of a filesystem bug
	* that allows the file size to change despite deny_write_access(), or a
	* bug in the Merkle tree logic itself
	*/
	if (WARN_ON_ONCE(next->is_root_hash && next->filled != 0))
	return -EINVAL;

	/* Zero-pad the block if it's shorter than the block size. */
	memset(&cur->data[cur->filled], 0, params->block_size - cur->filled);

	err = fsverity_hash_block(params, inode, cur->data,
	&next->data[next->filled]);
	if (err)
	return err;
	next->filled += params->digest_size;
	cur->filled = 0;
	return 0;
	}

	static int write_merkle_tree_block(struct inode inode, const u8 buf,
	unsigned long index,
	const struct merkle_tree_params *params)
	{
	u64 pos = (u64)index << params->log_blocksize;
	int err;

	err = inode->i_sb->s_vop->write_merkle_tree_block(inode, buf, pos,
	params->block_size);
	if (err)
	fsverity_err(inode, "Error %d writing Merkle tree block %lu",
	err, index);
	return err;
	}

	/*
	* Build the Merkle tree for the given file 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 file *filp,
	const struct merkle_tree_params *params,
	u8 *root_hash)
	{
	struct inode *inode = file_inode(filp);
	const u64 data_size = inode->i_size;
	const int num_levels = params->num_levels;
	struct block_buffer _buffers[1 + FS_VERITY_MAX_LEVELS + 1] = {};
	struct block_buffer *buffers = &_buffers[1];
	unsigned long level_offset[FS_VERITY_MAX_LEVELS];
	int level;
	u64 offset;
	int err;

	if (data_size == 0) {
	/* Empty file is a special case; root hash is all 0's */
	memset(root_hash, 0, params->digest_size);
	return 0;
	}

	/*
	* Allocate the block buffers. Buffer "-1" is for data blocks.
	* Buffers 0 <= level < num_levels are for the actual tree levels.
	* Buffer 'num_levels' is for the root hash.
	*/
	for (level = -1; level < num_levels; level++) {
	buffers[level].data = kzalloc(params->block_size, GFP_KERNEL);
	if (!buffers[level].data) {
	err = -ENOMEM;
	goto out;
	}
	}
	buffers[num_levels].data = root_hash;
	buffers[num_levels].is_root_hash = true;

	BUILD_BUG_ON(sizeof(level_offset) != sizeof(params->level_start));
	memcpy(level_offset, params->level_start, sizeof(level_offset));

	/* Hash each data block, also hashing the tree blocks as they fill up */
	for (offset = 0; offset < data_size; offset += params->block_size) {
	ssize_t bytes_read;
	loff_t pos = offset;

	buffers[-1].filled = min_t(u64, params->block_size,
	data_size - offset);
	bytes_read = __kernel_read(filp, buffers[-1].data,
	buffers[-1].filled, &pos);
	if (bytes_read < 0) {
	err = bytes_read;
	fsverity_err(inode, "Error %d reading file data", err);
	goto out;
	}
	if (bytes_read != buffers[-1].filled) {
	err = -EINVAL;
	fsverity_err(inode, "Short read of file data");
	goto out;
	}
	err = hash_one_block(inode, params, &buffers[-1]);
	if (err)
	goto out;
	for (level = 0; level < num_levels; level++) {
	if (buffers[level].filled + params->digest_size <=
	params->block_size) {
	/* Next block at @level isn't full yet */
	break;
	}
	/* Next block at @level is full */

	err = hash_one_block(inode, params, &buffers[level]);
	if (err)
	goto out;
	err = write_merkle_tree_block(inode,
	buffers[level].data,
	level_offset[level],
	params);
	if (err)
	goto out;
	level_offset[level]++;
	}
	if (fatal_signal_pending(current)) {
	err = -EINTR;
	goto out;
	}
	cond_resched();
	}
	/* Finish all nonempty pending tree blocks. */
	for (level = 0; level < num_levels; level++) {
	if (buffers[level].filled != 0) {
	err = hash_one_block(inode, params, &buffers[level]);
	if (err)
	goto out;
	err = write_merkle_tree_block(inode,
	buffers[level].data,
	level_offset[level],
	params);
	if (err)
	goto out;
	}
	}
	/* The root hash was filled by the last call to hash_one_block(). */
	if (WARN_ON_ONCE(buffers[num_levels].filled != params->digest_size)) {
	err = -EINVAL;
	goto out;
	}
	err = 0;
	out:
	for (level = -1; level < num_levels; level++)
	kfree(buffers[level].data);
	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 = struct_size(desc, signature, arg->sig_size);
	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, u64_to_user_ptr(arg->salt_ptr),
	arg->salt_size)) {
	err = -EFAULT;
	goto out;
	}
	desc->salt_size = arg->salt_size;

	/* Get the builtin signature if the user provided one */
	if (arg->sig_size &&
	copy_from_user(desc->signature, u64_to_user_ptr(arg->sig_ptr),
	arg->sig_size)) {
	err = -EFAULT;
	goto out;
	}
	desc->sig_size = cpu_to_le32(arg->sig_size);

	desc->data_size = cpu_to_le64(inode->i_size);

	/* 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;

	/*
	* Start enabling verity on this file, serialized by the inode lock.
	* Fail if verity is already enabled or is already being enabled.
	*/
	inode_lock(inode);
	if (IS_VERITY(inode))
	err = -EEXIST;
	else
	err = vops->begin_enable_verity(filp);
	inode_unlock(inode);
	if (err)
	goto out;

	/*
	* Build the Merkle tree. Don't hold the inode lock during this, since
	* on huge files this may take a very long time and we don't want to
	* force unrelated syscalls like chown() to block forever. We don't
	* need the inode lock here because deny_write_access() already prevents
	* the file from being written to or truncated, and we still serialize
	* ->begin_enable_verity() and ->end_enable_verity() using the inode
	* lock and only allow one process to be here at a time on a given file.
	*/
	BUILD_BUG_ON(sizeof(desc->root_hash) < FS_VERITY_MAX_DIGEST_SIZE);
	err = build_merkle_tree(filp, &params, desc->root_hash);
	if (err) {
	fsverity_err(inode, "Error %d building Merkle tree", err);
	goto rollback;
	}

	/*
	* 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);
	if (IS_ERR(vi)) {
	err = PTR_ERR(vi);
	goto rollback;
	}

	/*
	* Tell the filesystem to finish enabling verity on the file.
	* Serialized with ->begin_enable_verity() by the inode lock.
	*/
	inode_lock(inode);
	err = vops->end_enable_verity(filp, desc, desc_size, params.tree_size);
	inode_unlock(inode);
	if (err) {
	fsverity_err(inode, "%ps() failed with err %d",
	vops->end_enable_verity, err);
	fsverity_free_info(vi);
	} else if (WARN_ON_ONCE(!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:
	inode_lock(inode);
	(void)vops->end_enable_verity(filp, NULL, 0, params.tree_size);
	inode_unlock(inode);
	goto out;
	}

	/**
	* fsverity_ioctl_enable() - enable verity on a file
	* @filp: file to enable verity on
	* @uarg: user pointer to fsverity_enable_arg
	*
	* 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 (!is_power_of_2(arg.block_size))
	return -EINVAL;

	if (arg.salt_size > sizeof_field(struct fsverity_descriptor, salt))
	return -EMSGSIZE;

	if (arg.sig_size > FS_VERITY_MAX_SIGNATURE_SIZE)
	return -EMSGSIZE;

	/*
	* 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 = file_permission(filp, MAY_WRITE);
	if (err)
	return err;
	/*
	* __kernel_read() is used while building the Merkle tree. So, we can't
	* allow file descriptors that were opened for ioctl access only, using
	* the special nonstandard access mode 3. O_RDONLY only, please!
	*/
	if (!(filp->f_mode & FMODE_READ))
	return -EBADF;

	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;

	err = enable_verity(filp, &arg);

	/*
	* We no longer drop the inode's pagecache after enabling verity. This
	* used to be done to try to avoid a race condition where pages could be
	* evicted after being used in the Merkle tree construction, then
	* re-instantiated by a concurrent read. Such pages are unverified, and
	* the backing storage could have filled them with different content, so
	* they shouldn't be used to fulfill reads once verity is enabled.
	*
	* But, dropping the pagecache has a big performance impact, and it
	* doesn't fully solve the race condition anyway. So for those reasons,
	* and also because this race condition isn't very important relatively
	* speaking (especially for small-ish files, where the chance of a page
	* being used, evicted, and re-instantiated all while enabling verity
	* is quite small), we no longer drop the inode's pagecache.
	*/

	/*
	* allow_write_access() is needed to pair with deny_write_access().
	* Regardless, the filesystem won't allow writing to verity files.
	*/
	allow_write_access(filp);
	out_drop_write:
	mnt_drop_write_file(filp);
	return err;
	}
	EXPORT_SYMBOL_GPL(fsverity_ioctl_enable);