linux/fs/crypto/keysetup_v1.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Key setup for v1 encryption policies
 *
 * Copyright 2015, 2019 Google LLC
 */

/*
 * This file implements compatibility functions for the original encryption
 * policy version ("v1"), including:
 *
 * - Deriving per-file encryption keys using the AES-128-ECB based KDF
 *   (rather than the new method of using HKDF-SHA512)
 *
 * - Retrieving fscrypt master keys from process-subscribed keyrings
 *   (rather than the new method of using a filesystem-level keyring)
 *
 * - Handling policies with the DIRECT_KEY flag set using a master key table
 *   (rather than the new method of implementing DIRECT_KEY with per-mode keys
 *    managed alongside the master keys in the filesystem-level keyring)
 */

#include <crypto/skcipher.h>
#include <crypto/utils.h>
#include <keys/user-type.h>
#include <linux/hashtable.h>
#include <linux/scatterlist.h>

#include "fscrypt_private.h"

/* Table of keys referenced by DIRECT_KEY policies */
static DEFINE_HASHTABLE(fscrypt_direct_keys, 6); /* 6 bits = 64 buckets */
static DEFINE_SPINLOCK(fscrypt_direct_keys_lock);

/*
 * v1 key derivation function.  This generates the derived key by encrypting the
 * master key with AES-128-ECB using the nonce as the AES key.  This provides a
 * unique derived key with sufficient entropy for each inode.  However, it's
 * nonstandard, non-extensible, doesn't evenly distribute the entropy from the
 * master key, and is trivially reversible: an attacker who compromises a
 * derived key can "decrypt" it to get back to the master key, then derive any
 * other key.  For all new code, use HKDF instead.
 *
 * The master key must be at least as long as the derived key.  If the master
 * key is longer, then only the first 'derived_keysize' bytes are used.
 */
static int derive_key_aes(const u8 *master_key,
			  const u8 nonce[FSCRYPT_FILE_NONCE_SIZE],
			  u8 *derived_key, unsigned int derived_keysize)
{ … }

/*
 * Search the current task's subscribed keyrings for a "logon" key with
 * description prefix:descriptor, and if found acquire a read lock on it and
 * return a pointer to its validated payload in *payload_ret.
 */
static struct key *
find_and_lock_process_key(const char *prefix,
			  const u8 descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE],
			  unsigned int min_keysize,
			  const struct fscrypt_key **payload_ret)
{ … }

/* Master key referenced by DIRECT_KEY policy */
struct fscrypt_direct_key { … };

static void free_direct_key(struct fscrypt_direct_key *dk)
{ … }

void fscrypt_put_direct_key(struct fscrypt_direct_key *dk)
{ … }

/*
 * Find/insert the given key into the fscrypt_direct_keys table.  If found, it
 * is returned with elevated refcount, and 'to_insert' is freed if non-NULL.  If
 * not found, 'to_insert' is inserted and returned if it's non-NULL; otherwise
 * NULL is returned.
 */
static struct fscrypt_direct_key *
find_or_insert_direct_key(struct fscrypt_direct_key *to_insert,
			  const u8 *raw_key,
			  const struct fscrypt_inode_info *ci)
{ … }

/* Prepare to encrypt directly using the master key in the given mode */
static struct fscrypt_direct_key *
fscrypt_get_direct_key(const struct fscrypt_inode_info *ci, const u8 *raw_key)
{ … }

/* v1 policy, DIRECT_KEY: use the master key directly */
static int setup_v1_file_key_direct(struct fscrypt_inode_info *ci,
				    const u8 *raw_master_key)
{ … }

/* v1 policy, !DIRECT_KEY: derive the file's encryption key */
static int setup_v1_file_key_derived(struct fscrypt_inode_info *ci,
				     const u8 *raw_master_key)
{ … }

int fscrypt_setup_v1_file_key(struct fscrypt_inode_info *ci,
			      const u8 *raw_master_key)
{ … }

int
fscrypt_setup_v1_file_key_via_subscribed_keyrings(struct fscrypt_inode_info *ci)
{ … }