Contributors: 11
Author Tokens Token Proportion Commits Commit Proportion
Eric Biggers 965 83.84% 20 58.82%
Jaegeuk Kim 128 11.12% 4 11.76%
Satya Tangirala 27 2.35% 1 2.94%
Daniel Walter 18 1.56% 1 2.94%
Josef Whiter 6 0.52% 1 2.94%
David Howells 2 0.17% 2 5.88%
Herbert Xu 1 0.09% 1 2.94%
Waiman Long 1 0.09% 1 2.94%
Theodore Y. Ts'o 1 0.09% 1 2.94%
Greg Kroah-Hartman 1 0.09% 1 2.94%
Kees Cook 1 0.09% 1 2.94%
Total 1151 34 


// 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/aes.h>
#include <crypto/utils.h>
#include <keys/user-type.h>
#include <linux/hashtable.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);

/*
 * 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 > sizeof(payload->raw)) {
		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_RAW_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_obj(*dk);
	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.
 *
 * The v1 key derivation function generates the derived key by encrypting the
 * master key with AES-128-ECB using the file's 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 ci->ci_mode->keysize bytes are used.
 */
static int setup_v1_file_key_derived(struct fscrypt_inode_info *ci,
				     const u8 *raw_master_key)
{
	const unsigned int derived_keysize = ci->ci_mode->keysize;
	u8 derived_key[FSCRYPT_MAX_RAW_KEY_SIZE];
	struct aes_enckey aes;
	int err;

	if (WARN_ON_ONCE(derived_keysize > FSCRYPT_MAX_RAW_KEY_SIZE ||
			 derived_keysize % AES_BLOCK_SIZE != 0))
		return -EINVAL;

	static_assert(FSCRYPT_FILE_NONCE_SIZE == AES_KEYSIZE_128);
	aes_prepareenckey(&aes, ci->ci_nonce, FSCRYPT_FILE_NONCE_SIZE);
	for (unsigned int i = 0; i < derived_keysize; i += AES_BLOCK_SIZE)
		aes_encrypt(&aes, &derived_key[i], &raw_master_key[i]);

	err = fscrypt_set_per_file_enc_key(ci, derived_key);

	memzero_explicit(derived_key, derived_keysize);
	/* No need to zeroize 'aes', as its key is not secret. */
	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;
}