xref: /fs/ext4/crypto_key.c

/*
 * linux/fs/ext4/crypto_key.c
 *
 * Copyright (C) 2015, Google, Inc.
 *
 * This contains encryption key functions for ext4
 *
 * Written by Michael Halcrow, Ildar Muslukhov, and Uday Savagaonkar, 2015.
 */

#include <keys/encrypted-type.h>
#include <keys/user-type.h>
#include <linux/random.h>
#include <linux/scatterlist.h>
#include <uapi/linux/keyctl.h>

#include "ext4.h"
#include "xattr.h"

static void derive_crypt_complete(struct crypto_async_request *req, int rc)
{
	struct ext4_completion_result *ecr = req->data;

	if (rc == -EINPROGRESS)
		return;

	ecr->res = rc;
	complete(&ecr->completion);
}

/**
 * ext4_derive_key_v1() - Derive a key using AES-128-ECB
 * @deriving_key: Encryption key used for derivation.
 * @source_key:   Source key to which to apply derivation.
 * @derived_key:  Derived key.
 *
 * Return: 0 on success, -errno on failure
 */
static int ext4_derive_key_v1(const char deriving_key[EXT4_AES_128_ECB_KEY_SIZE],
			      const char source_key[EXT4_AES_256_XTS_KEY_SIZE],
			      char derived_key[EXT4_AES_256_XTS_KEY_SIZE])
{
	int res = 0;
	struct ablkcipher_request *req = NULL;
	DECLARE_EXT4_COMPLETION_RESULT(ecr);
	struct scatterlist src_sg, dst_sg;
	struct crypto_ablkcipher *tfm = crypto_alloc_ablkcipher("ecb(aes)", 0,
								0);

	if (IS_ERR(tfm)) {
		res = PTR_ERR(tfm);
		tfm = NULL;
		goto out;
	}
	crypto_ablkcipher_set_flags(tfm, CRYPTO_TFM_REQ_WEAK_KEY);
	req = ablkcipher_request_alloc(tfm, GFP_NOFS);
	if (!req) {
		res = -ENOMEM;
		goto out;
	}
	ablkcipher_request_set_callback(req,
			CRYPTO_TFM_REQ_MAY_BACKLOG | CRYPTO_TFM_REQ_MAY_SLEEP,
			derive_crypt_complete, &ecr);
	res = crypto_ablkcipher_setkey(tfm, deriving_key,
				       EXT4_AES_128_ECB_KEY_SIZE);
	if (res < 0)
		goto out;
	sg_init_one(&src_sg, source_key, EXT4_AES_256_XTS_KEY_SIZE);
	sg_init_one(&dst_sg, derived_key, EXT4_AES_256_XTS_KEY_SIZE);
	ablkcipher_request_set_crypt(req, &src_sg, &dst_sg,
				     EXT4_AES_256_XTS_KEY_SIZE, NULL);
	res = crypto_ablkcipher_encrypt(req);
	if (res == -EINPROGRESS || res == -EBUSY) {
		wait_for_completion(&ecr.completion);
		res = ecr.res;
	}

out:
	if (req)
		ablkcipher_request_free(req);
	if (tfm)
		crypto_free_ablkcipher(tfm);
	return res;
}

/**
 * ext4_derive_key_v2() - Derive a key non-reversibly
 * @nonce: the nonce associated with the file
 * @master_key: the master key referenced by the file
 * @derived_key: (output) the resulting derived key
 *
 * This function computes the following:
 *	 derived_key[0:127]   = AES-256-ENCRYPT(master_key[0:255], nonce)
 *	 derived_key[128:255] = AES-256-ENCRYPT(master_key[0:255], nonce ^ 0x01)
 *	 derived_key[256:383] = AES-256-ENCRYPT(master_key[256:511], nonce)
 *	 derived_key[384:511] = AES-256-ENCRYPT(master_key[256:511], nonce ^ 0x01)
 *
 * 'nonce ^ 0x01' denotes flipping the low order bit of the last byte.
 *
 * Unlike the v1 algorithm, the v2 algorithm is "non-reversible", meaning that
 * compromising a derived key does not also compromise the master key.
 *
 * Return: 0 on success, -errno on failure
 */
static int ext4_derive_key_v2(const char nonce[EXT4_KEY_DERIVATION_NONCE_SIZE],
			      const char master_key[EXT4_MAX_KEY_SIZE],
			      char derived_key[EXT4_MAX_KEY_SIZE])
{
	const int noncelen = EXT4_KEY_DERIVATION_NONCE_SIZE;
	struct crypto_cipher *tfm;
	int err;
	int i;

	/*
	 * Since we only use each transform for a small number of encryptions,
	 * requesting just "aes" turns out to be significantly faster than
	 * "ecb(aes)", by about a factor of two.
	 */
	tfm = crypto_alloc_cipher("aes", 0, 0);
	if (IS_ERR(tfm))
		return PTR_ERR(tfm);

	BUILD_BUG_ON(4 * EXT4_KEY_DERIVATION_NONCE_SIZE != EXT4_MAX_KEY_SIZE);
	BUILD_BUG_ON(2 * EXT4_AES_256_ECB_KEY_SIZE != EXT4_MAX_KEY_SIZE);
	for (i = 0; i < 2; i++) {
		memcpy(derived_key, nonce, noncelen);
		memcpy(derived_key + noncelen, nonce, noncelen);
		derived_key[2 * noncelen - 1] ^= 0x01;
		err = crypto_cipher_setkey(tfm, master_key,
					   EXT4_AES_256_ECB_KEY_SIZE);
		if (err)
			break;
		crypto_cipher_encrypt_one(tfm, derived_key, derived_key);
		crypto_cipher_encrypt_one(tfm, derived_key + noncelen,
					  derived_key + noncelen);
		master_key += EXT4_AES_256_ECB_KEY_SIZE;
		derived_key += 2 * noncelen;
	}
	crypto_free_cipher(tfm);
	return err;
}

/**
 * ext4_derive_key() - Derive a per-file key from a nonce and master key
 * @ctx: the encryption context associated with the file
 * @master_key: the master key referenced by the file
 * @derived_key: (output) the resulting derived key
 *
 * Return: 0 on success, -errno on failure
 */
static int ext4_derive_key(const struct ext4_encryption_context *ctx,
			   const char master_key[EXT4_MAX_KEY_SIZE],
			   char derived_key[EXT4_MAX_KEY_SIZE])
{
	BUILD_BUG_ON(EXT4_AES_128_ECB_KEY_SIZE != EXT4_KEY_DERIVATION_NONCE_SIZE);
	BUILD_BUG_ON(EXT4_AES_256_XTS_KEY_SIZE != EXT4_MAX_KEY_SIZE);

	/*
	 * Although the key derivation algorithm is logically independent of the
	 * choice of encryption modes, in this kernel it is bundled with HEH
	 * encryption of filenames, which is another crypto improvement that
	 * requires an on-disk format change and requires userspace to specify
	 * different encryption policies.
	 */
	if (ctx->filenames_encryption_mode == EXT4_ENCRYPTION_MODE_AES_256_HEH)
		return ext4_derive_key_v2(ctx->nonce, master_key, derived_key);
	else
		return ext4_derive_key_v1(ctx->nonce, master_key, derived_key);
}

void ext4_free_crypt_info(struct ext4_crypt_info *ci)
{
	if (!ci)
		return;

	crypto_free_ablkcipher(ci->ci_ctfm);
	kmem_cache_free(ext4_crypt_info_cachep, ci);
}

void ext4_free_encryption_info(struct inode *inode,
			       struct ext4_crypt_info *ci)
{
	struct ext4_inode_info *ei = EXT4_I(inode);
	struct ext4_crypt_info *prev;

	if (ci == NULL)
		ci = ACCESS_ONCE(ei->i_crypt_info);
	if (ci == NULL)
		return;
	prev = cmpxchg(&ei->i_crypt_info, ci, NULL);
	if (prev != ci)
		return;

	ext4_free_crypt_info(ci);
}

int ext4_get_encryption_info(struct inode *inode)
{
	struct ext4_inode_info *ei = EXT4_I(inode);
	struct ext4_crypt_info *crypt_info;
	char full_key_descriptor[EXT4_KEY_DESC_PREFIX_SIZE +
				 (EXT4_KEY_DESCRIPTOR_SIZE * 2) + 1];
	struct key *keyring_key = NULL;
	struct ext4_encryption_key *master_key;
	struct ext4_encryption_context ctx;
	const struct user_key_payload *ukp;
	struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
	struct crypto_ablkcipher *ctfm;
	const char *cipher_str;
	char raw_key[EXT4_MAX_KEY_SIZE];
	char mode;
	int res;

	if (ei->i_crypt_info)
		return 0;

	res = ext4_init_crypto();
	if (res)
		return res;

	res = ext4_xattr_get(inode, EXT4_XATTR_INDEX_ENCRYPTION,
				 EXT4_XATTR_NAME_ENCRYPTION_CONTEXT,
				 &ctx, sizeof(ctx));
	if (res < 0) {
		if (!DUMMY_ENCRYPTION_ENABLED(sbi))
			return res;
		ctx.contents_encryption_mode = EXT4_ENCRYPTION_MODE_AES_256_XTS;
		ctx.filenames_encryption_mode =
			EXT4_ENCRYPTION_MODE_AES_256_CTS;
		ctx.flags = 0;
	} else if (res != sizeof(ctx))
		return -EINVAL;
	res = 0;

	crypt_info = kmem_cache_alloc(ext4_crypt_info_cachep, GFP_KERNEL);
	if (!crypt_info)
		return -ENOMEM;

	crypt_info->ci_flags = ctx.flags;
	crypt_info->ci_data_mode = ctx.contents_encryption_mode;
	crypt_info->ci_filename_mode = ctx.filenames_encryption_mode;
	crypt_info->ci_ctfm = NULL;
	memcpy(crypt_info->ci_master_key, ctx.master_key_descriptor,
	       sizeof(crypt_info->ci_master_key));
	if (S_ISREG(inode->i_mode))
		mode = crypt_info->ci_data_mode;
	else if (S_ISDIR(inode->i_mode) || S_ISLNK(inode->i_mode))
		mode = crypt_info->ci_filename_mode;
	else
		BUG();
	switch (mode) {
	case EXT4_ENCRYPTION_MODE_AES_256_XTS:
		cipher_str = "xts(aes)";
		break;
	case EXT4_ENCRYPTION_MODE_AES_256_CTS:
		cipher_str = "cts(cbc(aes))";
		break;
	case EXT4_ENCRYPTION_MODE_AES_256_HEH:
		cipher_str = "heh(aes)";
		break;
	case EXT4_ENCRYPTION_MODE_SPECK128_256_XTS:
		cipher_str = "xts(speck128)";
		break;
	case EXT4_ENCRYPTION_MODE_SPECK128_256_CTS:
		cipher_str = "cts(cbc(speck128))";
		break;
	default:
		printk_once(KERN_WARNING
			    "ext4: unsupported key mode %d (ino %u)\n",
			    mode, (unsigned) inode->i_ino);
		res = -ENOKEY;
		goto out;
	}
	if (DUMMY_ENCRYPTION_ENABLED(sbi)) {
		memset(raw_key, 0x42, EXT4_AES_256_XTS_KEY_SIZE);
		goto got_key;
	}
	memcpy(full_key_descriptor, EXT4_KEY_DESC_PREFIX,
	       EXT4_KEY_DESC_PREFIX_SIZE);
	sprintf(full_key_descriptor + EXT4_KEY_DESC_PREFIX_SIZE,
		"%*phN", EXT4_KEY_DESCRIPTOR_SIZE,
		ctx.master_key_descriptor);
	full_key_descriptor[EXT4_KEY_DESC_PREFIX_SIZE +
			    (2 * EXT4_KEY_DESCRIPTOR_SIZE)] = '\0';
	keyring_key = request_key(&key_type_logon, full_key_descriptor, NULL);
	if (IS_ERR(keyring_key)) {
		res = PTR_ERR(keyring_key);
		keyring_key = NULL;
		goto out;
	}
	if (keyring_key->type != &key_type_logon) {
		printk_once(KERN_WARNING
			    "ext4: key type must be logon\n");
		res = -ENOKEY;
		goto out;
	}
	down_read(&keyring_key->sem);
	ukp = user_key_payload(keyring_key);
	if (!ukp) {
		/* key was revoked before we acquired its semaphore */
		res = -EKEYREVOKED;
		up_read(&keyring_key->sem);
		goto out;
	}
	if (ukp->datalen != sizeof(struct ext4_encryption_key)) {
		res = -EINVAL;
		up_read(&keyring_key->sem);
		goto out;
	}
	master_key = (struct ext4_encryption_key *)ukp->data;
	BUILD_BUG_ON(EXT4_AES_128_ECB_KEY_SIZE !=
		     EXT4_KEY_DERIVATION_NONCE_SIZE);
	if (master_key->size != EXT4_AES_256_XTS_KEY_SIZE) {
		printk_once(KERN_WARNING
			    "ext4: key size incorrect: %d\n",
			    master_key->size);
		res = -ENOKEY;
		up_read(&keyring_key->sem);
		goto out;
	}
	res = ext4_derive_key(&ctx, master_key->raw, raw_key);
	up_read(&keyring_key->sem);
	if (res)
		goto out;
got_key:
	ctfm = crypto_alloc_ablkcipher(cipher_str, 0, 0);
	if (!ctfm || IS_ERR(ctfm)) {
		res = ctfm ? PTR_ERR(ctfm) : -ENOMEM;
		printk(KERN_DEBUG
		       "%s: error %d (inode %u) allocating crypto tfm\n",
		       __func__, res, (unsigned) inode->i_ino);
		goto out;
	}
	crypt_info->ci_ctfm = ctfm;
	crypto_ablkcipher_clear_flags(ctfm, ~0);
	crypto_tfm_set_flags(crypto_ablkcipher_tfm(ctfm),
			     CRYPTO_TFM_REQ_WEAK_KEY);
	res = crypto_ablkcipher_setkey(ctfm, raw_key,
				       ext4_encryption_key_size(mode));
	if (res)
		goto out;

	if (cmpxchg(&ei->i_crypt_info, NULL, crypt_info) == NULL)
		crypt_info = NULL;
out:
	if (res == -ENOKEY)
		res = 0;
	key_put(keyring_key);
	ext4_free_crypt_info(crypt_info);
	memzero_explicit(raw_key, sizeof(raw_key));
	return res;
}

int ext4_has_encryption_key(struct inode *inode)
{
	struct ext4_inode_info *ei = EXT4_I(inode);

	return (ei->i_crypt_info != NULL);
}