// 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)
{
int res = 0;
struct skcipher_request *req = NULL;
DECLARE_CRYPTO_WAIT(wait);
struct scatterlist src_sg, dst_sg;
struct crypto_skcipher *tfm = crypto_alloc_skcipher("ecb(aes)", 0, 0);
if (IS_ERR(tfm)) {
res = PTR_ERR(tfm);
tfm = NULL;
goto out;
}
crypto_skcipher_set_flags(tfm, CRYPTO_TFM_REQ_FORBID_WEAK_KEYS);
req = skcipher_request_alloc(tfm, GFP_KERNEL);
if (!req) {
res = -ENOMEM;
goto out;
}
skcipher_request_set_callback(req,
CRYPTO_TFM_REQ_MAY_BACKLOG | CRYPTO_TFM_REQ_MAY_SLEEP,
crypto_req_done, &wait);
res = crypto_skcipher_setkey(tfm, nonce, FSCRYPT_FILE_NONCE_SIZE);
if (res < 0)
goto out;
sg_init_one(&src_sg, master_key, derived_keysize);
sg_init_one(&dst_sg, derived_key, derived_keysize);
skcipher_request_set_crypt(req, &src_sg, &dst_sg, derived_keysize,
NULL);
res = crypto_wait_req(crypto_skcipher_encrypt(req), &wait);
out:
skcipher_request_free(req);
crypto_free_skcipher(tfm);
return res;
}
/*
* 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)
{
char *description;
struct key *key;
const struct user_key_payload *ukp;
const struct fscrypt_key *payload;
description = kasprintf(GFP_KERNEL, "%s%*phN", prefix,
FSCRYPT_KEY_DESCRIPTOR_SIZE, descriptor);
if (!description)
return ERR_PTR(-ENOMEM);
key = request_key(&key_type_logon, description, NULL);
kfree(description);
if (IS_ERR(key))
return key;
down_read(&key->sem);
ukp = user_key_payload_locked(key);
if (!ukp) /* was the key revoked before we acquired its semaphore? */
goto invalid;
payload = (const struct fscrypt_key *)ukp->data;
if (ukp->datalen != sizeof(struct fscrypt_key) ||
payload->size < 1 || payload->size > FSCRYPT_MAX_KEY_SIZE) {
fscrypt_warn(NULL,
"key with description '%s' has invalid payload",
key->description);
goto invalid;
}
if (payload->size < min_keysize) {
fscrypt_warn(NULL,
"key with description '%s' is too short (got %u bytes, need %u+ bytes)",
key->description, payload->size, min_keysize);
goto invalid;
}
*payload_ret = payload;
return key;
invalid:
up_read(&key->sem);
key_put(key);
return ERR_PTR(-ENOKEY);
}
/* Master key referenced by DIRECT_KEY policy */
struct fscrypt_direct_key {
struct super_block *dk_sb;
struct hlist_node dk_node;
refcount_t dk_refcount;
const struct fscrypt_mode *dk_mode;
struct fscrypt_prepared_key dk_key;
u8 dk_descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE];
u8 dk_raw[FSCRYPT_MAX_KEY_SIZE];
};
static void free_direct_key(struct fscrypt_direct_key *dk)
{
if (dk) {
fscrypt_destroy_prepared_key(dk->dk_sb, &dk->dk_key);
kfree_sensitive(dk);
}
}
void fscrypt_put_direct_key(struct fscrypt_direct_key *dk)
{
if (!refcount_dec_and_lock(&dk->dk_refcount, &fscrypt_direct_keys_lock))
return;
hash_del(&dk->dk_node);
spin_unlock(&fscrypt_direct_keys_lock);
free_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)
{
unsigned long hash_key;
struct fscrypt_direct_key *dk;
/*
* Careful: to avoid potentially leaking secret key bytes via timing
* information, we must key the hash table by descriptor rather than by
* raw key, and use crypto_memneq() when comparing raw keys.
*/
BUILD_BUG_ON(sizeof(hash_key) > FSCRYPT_KEY_DESCRIPTOR_SIZE);
memcpy(&hash_key, ci->ci_policy.v1.master_key_descriptor,
sizeof(hash_key));
spin_lock(&fscrypt_direct_keys_lock);
hash_for_each_possible(fscrypt_direct_keys, dk, dk_node, hash_key) {
if (memcmp(ci->ci_policy.v1.master_key_descriptor,
dk->dk_descriptor, FSCRYPT_KEY_DESCRIPTOR_SIZE) != 0)
continue;
if (ci->ci_mode != dk->dk_mode)
continue;
if (!fscrypt_is_key_prepared(&dk->dk_key, ci))
continue;
if (crypto_memneq(raw_key, dk->dk_raw, ci->ci_mode->keysize))
continue;
/* using existing tfm with same (descriptor, mode, raw_key) */
refcount_inc(&dk->dk_refcount);
spin_unlock(&fscrypt_direct_keys_lock);
free_direct_key(to_insert);
return dk;
}
if (to_insert)
hash_add(fscrypt_direct_keys, &to_insert->dk_node, hash_key);
spin_unlock(&fscrypt_direct_keys_lock);
return to_insert;
}
/* 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)
{
struct fscrypt_direct_key *dk;
int err;
/* Is there already a tfm for this key? */
dk = find_or_insert_direct_key(NULL, raw_key, ci);
if (dk)
return dk;
/* Nope, allocate one. */
dk = kzalloc(sizeof(*dk), GFP_KERNEL);
if (!dk)
return ERR_PTR(-ENOMEM);
dk->dk_sb = ci->ci_inode->i_sb;
refcount_set(&dk->dk_refcount, 1);
dk->dk_mode = ci->ci_mode;
err = fscrypt_prepare_key(&dk->dk_key, raw_key, ci);
if (err)
goto err_free_dk;
memcpy(dk->dk_descriptor, ci->ci_policy.v1.master_key_descriptor,
FSCRYPT_KEY_DESCRIPTOR_SIZE);
memcpy(dk->dk_raw, raw_key, ci->ci_mode->keysize);
return find_or_insert_direct_key(dk, raw_key, ci);
err_free_dk:
free_direct_key(dk);
return ERR_PTR(err);
}
/* 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)
{
struct fscrypt_direct_key *dk;
dk = fscrypt_get_direct_key(ci, raw_master_key);
if (IS_ERR(dk))
return PTR_ERR(dk);
ci->ci_direct_key = dk;
ci->ci_enc_key = dk->dk_key;
return 0;
}
/* 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)
{
u8 *derived_key;
int err;
/*
* This cannot be a stack buffer because it will be passed to the
* scatterlist crypto API during derive_key_aes().
*/
derived_key = kmalloc(ci->ci_mode->keysize, GFP_KERNEL);
if (!derived_key)
return -ENOMEM;
err = derive_key_aes(raw_master_key, ci->ci_nonce,
derived_key, ci->ci_mode->keysize);
if (err)
goto out;
err = fscrypt_set_per_file_enc_key(ci, derived_key);
out:
kfree_sensitive(derived_key);
return err;
}
int fscrypt_setup_v1_file_key(struct fscrypt_inode_info *ci,
const u8 *raw_master_key)
{
if (ci->ci_policy.v1.flags & FSCRYPT_POLICY_FLAG_DIRECT_KEY)
return setup_v1_file_key_direct(ci, raw_master_key);
else
return setup_v1_file_key_derived(ci, raw_master_key);
}
int
fscrypt_setup_v1_file_key_via_subscribed_keyrings(struct fscrypt_inode_info *ci)
{
const struct super_block *sb = ci->ci_inode->i_sb;
struct key *key;
const struct fscrypt_key *payload;
int err;
key = find_and_lock_process_key(FSCRYPT_KEY_DESC_PREFIX,
ci->ci_policy.v1.master_key_descriptor,
ci->ci_mode->keysize, &payload);
if (key == ERR_PTR(-ENOKEY) && sb->s_cop->legacy_key_prefix) {
key = find_and_lock_process_key(sb->s_cop->legacy_key_prefix,
ci->ci_policy.v1.master_key_descriptor,
ci->ci_mode->keysize, &payload);
}
if (IS_ERR(key))
return PTR_ERR(key);
err = fscrypt_setup_v1_file_key(ci, payload->raw);
up_read(&key->sem);
key_put(key);
return err;
}
Codebase Browser
linux