text: // SPDX-License-Identifier: GPL-2.0-only
  /*
   * fs/crypto/hooks.c
   *
   * Encryption hooks for higher-level filesystem operations.
   */
  
  #include <linux/key.h>
  
  #include "fscrypt_private.h"
  
  /**
   * fscrypt_file_open() - prepare to open a possibly-encrypted regular file
   * @inode: the inode being opened
   * @filp: the struct file being set up
   *
   * Currently, an encrypted regular file can only be opened if its encryption key
   * is available; access to the raw encrypted contents is not supported.
   * Therefore, we first set up the inode's encryption key (if not already done)
   * and return an error if it's unavailable.
   *
   * We also verify that if the parent directory (from the path via which the file
   * is being opened) is encrypted, then the inode being opened uses the same
   * encryption policy.  This is needed as part of the enforcement that all files
   * in an encrypted directory tree use the same encryption policy, as a
   * protection against certain types of offline attacks.  Note that this check is
   * needed even when opening an *unencrypted* file, since it's forbidden to have
   * an unencrypted file in an encrypted directory.
   *
   * Return: 0 on success, -ENOKEY if the key is missing, or another -errno code
   */
  int fscrypt_file_open(struct inode *inode, struct file *filp)
  {
  	int err;
  	struct dentry *dir;
  
  	err = fscrypt_require_key(inode);
  	if (err)
  		return err;
  
  	dir = dget_parent(file_dentry(filp));
  	if (IS_ENCRYPTED(d_inode(dir)) &&
  	    !fscrypt_has_permitted_context(d_inode(dir), inode)) {
  		fscrypt_warn(inode,
  			     "Inconsistent encryption context (parent directory: %lu)",
  			     d_inode(dir)->i_ino);
  		err = -EPERM;
  	}
  	dput(dir);
  	return err;
  }
  EXPORT_SYMBOL_GPL(fscrypt_file_open);
  
  int __fscrypt_prepare_link(struct inode *inode, struct inode *dir,
  			   struct dentry *dentry)
  {
  	int err;
  
  	err = fscrypt_require_key(dir);
  	if (err)
  		return err;
  
  	/* ... in case we looked up no-key name before key was added */
  	if (fscrypt_is_nokey_name(dentry))
  		return -ENOKEY;
  
  	if (!fscrypt_has_permitted_context(dir, inode))
  		return -EXDEV;
  
  	return 0;
  }
  EXPORT_SYMBOL_GPL(__fscrypt_prepare_link);
  
  int __fscrypt_prepare_rename(struct inode *old_dir, struct dentry *old_dentry,
  			     struct inode *new_dir, struct dentry *new_dentry,
  			     unsigned int flags)
  {
  	int err;
  
  	err = fscrypt_require_key(old_dir);
  	if (err)
  		return err;
  
  	err = fscrypt_require_key(new_dir);
  	if (err)
  		return err;
  
  	/* ... in case we looked up no-key name(s) before key was added */
  	if (fscrypt_is_nokey_name(old_dentry) ||
  	    fscrypt_is_nokey_name(new_dentry))
  		return -ENOKEY;
  
  	if (old_dir != new_dir) {
  		if (IS_ENCRYPTED(new_dir) &&
  		    !fscrypt_has_permitted_context(new_dir,
  						   d_inode(old_dentry)))
  			return -EXDEV;
  
  		if ((flags & RENAME_EXCHANGE) &&
  		    IS_ENCRYPTED(old_dir) &&
  		    !fscrypt_has_permitted_context(old_dir,
  						   d_inode(new_dentry)))
  			return -EXDEV;
  	}
  	return 0;
  }
  EXPORT_SYMBOL_GPL(__fscrypt_prepare_rename);
  
  int __fscrypt_prepare_lookup(struct inode *dir, struct dentry *dentry,
  			     struct fscrypt_name *fname)
  {
  	int err = fscrypt_setup_filename(dir, &dentry->d_name, 1, fname);
  
  	if (err && err != -ENOENT)
  		return err;
  
  	if (fname->is_nokey_name) {
  		spin_lock(&dentry->d_lock);
  		dentry->d_flags |= DCACHE_NOKEY_NAME;
  		spin_unlock(&dentry->d_lock);
  		d_set_d_op(dentry, &fscrypt_d_ops);
  	}
  	return err;
  }
  EXPORT_SYMBOL_GPL(__fscrypt_prepare_lookup);
  
  /**
   * fscrypt_prepare_setflags() - prepare to change flags with FS_IOC_SETFLAGS
   * @inode: the inode on which flags are being changed
   * @oldflags: the old flags
   * @flags: the new flags
   *
   * The caller should be holding i_rwsem for write.
   *
   * Return: 0 on success; -errno if the flags change isn't allowed or if
   *	   another error occurs.
   */
  int fscrypt_prepare_setflags(struct inode *inode,
  			     unsigned int oldflags, unsigned int flags)
  {
  	struct fscrypt_info *ci;
  	struct fscrypt_master_key *mk;
  	int err;
  
  	/*
  	 * When the CASEFOLD flag is set on an encrypted directory, we must
  	 * derive the secret key needed for the dirhash.  This is only possible
  	 * if the directory uses a v2 encryption policy.
  	 */
  	if (IS_ENCRYPTED(inode) && (flags & ~oldflags & FS_CASEFOLD_FL)) {
  		err = fscrypt_require_key(inode);
  		if (err)
  			return err;
  		ci = inode->i_crypt_info;
  		if (ci->ci_policy.version != FSCRYPT_POLICY_V2)
  			return -EINVAL;
  		mk = ci->ci_master_key->payload.data[0];
  		down_read(&mk->mk_secret_sem);
  		if (is_master_key_secret_present(&mk->mk_secret))
  			err = fscrypt_derive_dirhash_key(ci, mk);
  		else
  			err = -ENOKEY;
  		up_read(&mk->mk_secret_sem);
  		return err;
  	}
  	return 0;
  }
  
  /**
   * fscrypt_prepare_symlink() - prepare to create a possibly-encrypted symlink
   * @dir: directory in which the symlink is being created
   * @target: plaintext symlink target
   * @len: length of @target excluding null terminator
   * @max_len: space the filesystem has available to store the symlink target
   * @disk_link: (out) the on-disk symlink target being prepared
   *
   * This function computes the size the symlink target will require on-disk,
   * stores it in @disk_link->len, and validates it against @max_len.  An
   * encrypted symlink may be longer than the original.
   *
   * Additionally, @disk_link->name is set to @target if the symlink will be
   * unencrypted, but left NULL if the symlink will be encrypted.  For encrypted
   * symlinks, the filesystem must call fscrypt_encrypt_symlink() to create the
   * on-disk target later.  (The reason for the two-step process is that some
   * filesystems need to know the size of the symlink target before creating the
   * inode, e.g. to determine whether it will be a "fast" or "slow" symlink.)
   *
   * Return: 0 on success, -ENAMETOOLONG if the symlink target is too long,
   * -ENOKEY if the encryption key is missing, or another -errno code if a problem
   * occurred while setting up the encryption key.
   */
  int fscrypt_prepare_symlink(struct inode *dir, const char *target,
  			    unsigned int len, unsigned int max_len,
  			    struct fscrypt_str *disk_link)
  {
  	const union fscrypt_policy *policy;
  
  	/*
  	 * To calculate the size of the encrypted symlink target we need to know
  	 * the amount of NUL padding, which is determined by the flags set in
  	 * the encryption policy which will be inherited from the directory.
  	 */
  	policy = fscrypt_policy_to_inherit(dir);
  	if (policy == NULL) {
  		/* Not encrypted */
  		disk_link->name = (unsigned char *)target;
  		disk_link->len = len + 1;
  		if (disk_link->len > max_len)
  			return -ENAMETOOLONG;
  		return 0;
  	}
  	if (IS_ERR(policy))
  		return PTR_ERR(policy);
  
  	/*
  	 * Calculate the size of the encrypted symlink and verify it won't
  	 * exceed max_len.  Note that for historical reasons, encrypted symlink
  	 * targets are prefixed with the ciphertext length, despite this
  	 * actually being redundant with i_size.  This decreases by 2 bytes the
  	 * longest symlink target we can accept.
  	 *
  	 * We could recover 1 byte by not counting a null terminator, but
  	 * counting it (even though it is meaningless for ciphertext) is simpler
  	 * for now since filesystems will assume it is there and subtract it.
  	 */
  	if (!fscrypt_fname_encrypted_size(policy, len,
  					  max_len - sizeof(struct fscrypt_symlink_data),
  					  &disk_link->len))
  		return -ENAMETOOLONG;
  	disk_link->len += sizeof(struct fscrypt_symlink_data);
  
  	disk_link->name = NULL;
  	return 0;
  }
  EXPORT_SYMBOL_GPL(fscrypt_prepare_symlink);
  
  int __fscrypt_encrypt_symlink(struct inode *inode, const char *target,
  			      unsigned int len, struct fscrypt_str *disk_link)
  {
  	int err;
  	struct qstr iname = QSTR_INIT(target, len);
  	struct fscrypt_symlink_data *sd;
  	unsigned int ciphertext_len;
  
  	/*
  	 * fscrypt_prepare_new_inode() should have already set up the new
  	 * symlink inode's encryption key.  We don't wait until now to do it,
  	 * since we may be in a filesystem transaction now.
  	 */
  	if (WARN_ON_ONCE(!fscrypt_has_encryption_key(inode)))
  		return -ENOKEY;
  
  	if (disk_link->name) {
  		/* filesystem-provided buffer */
  		sd = (struct fscrypt_symlink_data *)disk_link->name;
  	} else {
  		sd = kmalloc(disk_link->len, GFP_NOFS);
  		if (!sd)
  			return -ENOMEM;
  	}
  	ciphertext_len = disk_link->len - sizeof(*sd);
  	sd->len = cpu_to_le16(ciphertext_len);
  
  	err = fscrypt_fname_encrypt(inode, &iname, sd->encrypted_path,
  				    ciphertext_len);
  	if (err)
  		goto err_free_sd;
  
  	/*
  	 * Null-terminating the ciphertext doesn't make sense, but we still
  	 * count the null terminator in the length, so we might as well
  	 * initialize it just in case the filesystem writes it out.
  	 */
  	sd->encrypted_path[ciphertext_len] = '\0';
  
  	/* Cache the plaintext symlink target for later use by get_link() */
  	err = -ENOMEM;
  	inode->i_link = kmemdup(target, len + 1, GFP_NOFS);
  	if (!inode->i_link)
  		goto err_free_sd;
  
  	if (!disk_link->name)
  		disk_link->name = (unsigned char *)sd;
  	return 0;
  
  err_free_sd:
  	if (!disk_link->name)
  		kfree(sd);
  	return err;
  }
  EXPORT_SYMBOL_GPL(__fscrypt_encrypt_symlink);
  
  /**
   * fscrypt_get_symlink() - get the target of an encrypted symlink
   * @inode: the symlink inode
   * @caddr: the on-disk contents of the symlink
   * @max_size: size of @caddr buffer
   * @done: if successful, will be set up to free the returned target if needed
   *
   * If the symlink's encryption key is available, we decrypt its target.
   * Otherwise, we encode its target for presentation.
   *
   * This may sleep, so the filesystem must have dropped out of RCU mode already.
   *
   * Return: the presentable symlink target or an ERR_PTR()
   */
  const char *fscrypt_get_symlink(struct inode *inode, const void *caddr,
  				unsigned int max_size,
  				struct delayed_call *done)
  {
  	const struct fscrypt_symlink_data *sd;
  	struct fscrypt_str cstr, pstr;
  	bool has_key;
  	int err;
  
  	/* This is for encrypted symlinks only */
  	if (WARN_ON(!IS_ENCRYPTED(inode)))
  		return ERR_PTR(-EINVAL);
  
  	/* If the decrypted target is already cached, just return it. */
  	pstr.name = READ_ONCE(inode->i_link);
  	if (pstr.name)
  		return pstr.name;
  
  	/*
  	 * Try to set up the symlink's encryption key, but we can continue
  	 * regardless of whether the key is available or not.
  	 */
  	err = fscrypt_get_encryption_info(inode);
  	if (err)
  		return ERR_PTR(err);
  	has_key = fscrypt_has_encryption_key(inode);
  
  	/*
  	 * For historical reasons, encrypted symlink targets are prefixed with
  	 * the ciphertext length, even though this is redundant with i_size.
  	 */
  
  	if (max_size < sizeof(*sd))
  		return ERR_PTR(-EUCLEAN);
  	sd = caddr;
  	cstr.name = (unsigned char *)sd->encrypted_path;
  	cstr.len = le16_to_cpu(sd->len);
  
  	if (cstr.len == 0)
  		return ERR_PTR(-EUCLEAN);
  
  	if (cstr.len + sizeof(*sd) - 1 > max_size)
  		return ERR_PTR(-EUCLEAN);
  
  	err = fscrypt_fname_alloc_buffer(cstr.len, &pstr);
  	if (err)
  		return ERR_PTR(err);
  
  	err = fscrypt_fname_disk_to_usr(inode, 0, 0, &cstr, &pstr);
  	if (err)
  		goto err_kfree;
  
  	err = -EUCLEAN;
  	if (pstr.name[0] == '\0')
  		goto err_kfree;
  
  	pstr.name[pstr.len] = '\0';
  
  	/*
  	 * Cache decrypted symlink targets in i_link for later use.  Don't cache
  	 * symlink targets encoded without the key, since those become outdated
  	 * once the key is added.  This pairs with the READ_ONCE() above and in
  	 * the VFS path lookup code.
  	 */
  	if (!has_key ||
  	    cmpxchg_release(&inode->i_link, NULL, pstr.name) != NULL)
  		set_delayed_call(done, kfree_link, pstr.name);
  
  	return pstr.name;
  
  err_kfree:
  	kfree(pstr.name);
  	return ERR_PTR(err);
  }
  EXPORT_SYMBOL_GPL(fscrypt_get_symlink);
  