fscrypt: refactor key setup code in preparation for v2 policies

[linux-block.git] / fs / crypto / keyinfo.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Key setup facility for FS encryption support.
 *
 * Copyright (C) 2015, Google, Inc.
 *
 * Originally written by Michael Halcrow, Ildar Muslukhov, and Uday Savagaonkar.
 * Heavily modified since then.
 */

#include <keys/user-type.h>
#include <linux/hashtable.h>
#include <linux/scatterlist.h>
#include <crypto/aes.h>
#include <crypto/algapi.h>
#include <crypto/sha.h>
#include <crypto/skcipher.h>
#include "fscrypt_private.h"

static struct crypto_shash *essiv_hash_tfm;

/* 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[FS_KEY_DERIVATION_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_NOFS);
	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, FS_KEY_DERIVATION_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_NOFS, "%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);
}

static struct fscrypt_mode available_modes[] = {
	[FSCRYPT_MODE_AES_256_XTS] = {
		.friendly_name = "AES-256-XTS",
		.cipher_str = "xts(aes)",
		.keysize = 64,
		.ivsize = 16,
	},
	[FSCRYPT_MODE_AES_256_CTS] = {
		.friendly_name = "AES-256-CTS-CBC",
		.cipher_str = "cts(cbc(aes))",
		.keysize = 32,
		.ivsize = 16,
	},
	[FSCRYPT_MODE_AES_128_CBC] = {
		.friendly_name = "AES-128-CBC",
		.cipher_str = "cbc(aes)",
		.keysize = 16,
		.ivsize = 16,
		.needs_essiv = true,
	},
	[FSCRYPT_MODE_AES_128_CTS] = {
		.friendly_name = "AES-128-CTS-CBC",
		.cipher_str = "cts(cbc(aes))",
		.keysize = 16,
		.ivsize = 16,
	},
	[FSCRYPT_MODE_ADIANTUM] = {
		.friendly_name = "Adiantum",
		.cipher_str = "adiantum(xchacha12,aes)",
		.keysize = 32,
		.ivsize = 32,
	},
};

static struct fscrypt_mode *
select_encryption_mode(const struct fscrypt_info *ci, const struct inode *inode)
{
	if (!fscrypt_valid_enc_modes(ci->ci_data_mode, ci->ci_filename_mode)) {
		fscrypt_warn(inode,
			     "Unsupported encryption modes (contents mode %d, filenames mode %d)",
			     ci->ci_data_mode, ci->ci_filename_mode);
		return ERR_PTR(-EINVAL);
	}

	if (S_ISREG(inode->i_mode))
		return &available_modes[ci->ci_data_mode];

	if (S_ISDIR(inode->i_mode) || S_ISLNK(inode->i_mode))
		return &available_modes[ci->ci_filename_mode];

	WARN_ONCE(1, "fscrypt: filesystem tried to load encryption info for inode %lu, which is not encryptable (file type %d)\n",
		  inode->i_ino, (inode->i_mode & S_IFMT));
	return ERR_PTR(-EINVAL);
}

/* Create a symmetric cipher object for the given encryption mode and key */
static struct crypto_skcipher *
fscrypt_allocate_skcipher(struct fscrypt_mode *mode, const u8 *raw_key,
			  const struct inode *inode)
{
	struct crypto_skcipher *tfm;
	int err;

	tfm = crypto_alloc_skcipher(mode->cipher_str, 0, 0);
	if (IS_ERR(tfm)) {
		if (PTR_ERR(tfm) == -ENOENT) {
			fscrypt_warn(inode,
				     "Missing crypto API support for %s (API name: \"%s\")",
				     mode->friendly_name, mode->cipher_str);
			return ERR_PTR(-ENOPKG);
		}
		fscrypt_err(inode, "Error allocating '%s' transform: %ld",
			    mode->cipher_str, PTR_ERR(tfm));
		return tfm;
	}
	if (unlikely(!mode->logged_impl_name)) {
		/*
		 * fscrypt performance can vary greatly depending on which
		 * crypto algorithm implementation is used.  Help people debug
		 * performance problems by logging the ->cra_driver_name the
		 * first time a mode is used.  Note that multiple threads can
		 * race here, but it doesn't really matter.
		 */
		mode->logged_impl_name = true;
		pr_info("fscrypt: %s using implementation \"%s\"\n",
			mode->friendly_name,
			crypto_skcipher_alg(tfm)->base.cra_driver_name);
	}
	crypto_skcipher_set_flags(tfm, CRYPTO_TFM_REQ_FORBID_WEAK_KEYS);
	err = crypto_skcipher_setkey(tfm, raw_key, mode->keysize);
	if (err)
		goto err_free_tfm;

	return tfm;

err_free_tfm:
	crypto_free_skcipher(tfm);
	return ERR_PTR(err);
}

/* Master key referenced by DIRECT_KEY policy */
struct fscrypt_direct_key {
	struct hlist_node		dk_node;
	refcount_t			dk_refcount;
	const struct fscrypt_mode	*dk_mode;
	struct crypto_skcipher		*dk_ctfm;
	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) {
		crypto_free_skcipher(dk->dk_ctfm);
		kzfree(dk);
	}
}

static void 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_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_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_master_key_descriptor, dk->dk_descriptor,
			   FSCRYPT_KEY_DESCRIPTOR_SIZE) != 0)
			continue;
		if (ci->ci_mode != dk->dk_mode)
			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_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_NOFS);
	if (!dk)
		return ERR_PTR(-ENOMEM);
	refcount_set(&dk->dk_refcount, 1);
	dk->dk_mode = ci->ci_mode;
	dk->dk_ctfm = fscrypt_allocate_skcipher(ci->ci_mode, raw_key,
						ci->ci_inode);
	if (IS_ERR(dk->dk_ctfm)) {
		err = PTR_ERR(dk->dk_ctfm);
		dk->dk_ctfm = NULL;
		goto err_free_dk;
	}
	memcpy(dk->dk_descriptor, ci->ci_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);
}

static int derive_essiv_salt(const u8 *key, int keysize, u8 *salt)
{
	struct crypto_shash *tfm = READ_ONCE(essiv_hash_tfm);

	/* init hash transform on demand */
	if (unlikely(!tfm)) {
		struct crypto_shash *prev_tfm;

		tfm = crypto_alloc_shash("sha256", 0, 0);
		if (IS_ERR(tfm)) {
			if (PTR_ERR(tfm) == -ENOENT) {
				fscrypt_warn(NULL,
					     "Missing crypto API support for SHA-256");
				return -ENOPKG;
			}
			fscrypt_err(NULL,
				    "Error allocating SHA-256 transform: %ld",
				    PTR_ERR(tfm));
			return PTR_ERR(tfm);
		}
		prev_tfm = cmpxchg(&essiv_hash_tfm, NULL, tfm);
		if (prev_tfm) {
			crypto_free_shash(tfm);
			tfm = prev_tfm;
		}
	}

	{
		SHASH_DESC_ON_STACK(desc, tfm);
		desc->tfm = tfm;

		return crypto_shash_digest(desc, key, keysize, salt);
	}
}

static int init_essiv_generator(struct fscrypt_info *ci, const u8 *raw_key,
				int keysize)
{
	int err;
	struct crypto_cipher *essiv_tfm;
	u8 salt[SHA256_DIGEST_SIZE];

	if (WARN_ON(ci->ci_mode->ivsize != AES_BLOCK_SIZE))
		return -EINVAL;

	essiv_tfm = crypto_alloc_cipher("aes", 0, 0);
	if (IS_ERR(essiv_tfm))
		return PTR_ERR(essiv_tfm);

	ci->ci_essiv_tfm = essiv_tfm;

	err = derive_essiv_salt(raw_key, keysize, salt);
	if (err)
		goto out;

	/*
	 * Using SHA256 to derive the salt/key will result in AES-256 being
	 * used for IV generation. File contents encryption will still use the
	 * configured keysize (AES-128) nevertheless.
	 */
	err = crypto_cipher_setkey(essiv_tfm, salt, sizeof(salt));
	if (err)
		goto out;

out:
	memzero_explicit(salt, sizeof(salt));
	return err;
}

/* Given the per-file key, set up the file's crypto transform object(s) */
static int fscrypt_set_derived_key(struct fscrypt_info *ci,
				   const u8 *derived_key)
{
	struct fscrypt_mode *mode = ci->ci_mode;
	struct crypto_skcipher *ctfm;
	int err;

	ctfm = fscrypt_allocate_skcipher(mode, derived_key, ci->ci_inode);
	if (IS_ERR(ctfm))
		return PTR_ERR(ctfm);

	ci->ci_ctfm = ctfm;

	if (mode->needs_essiv) {
		err = init_essiv_generator(ci, derived_key, mode->keysize);
		if (err) {
			fscrypt_warn(ci->ci_inode,
				     "Error initializing ESSIV generator: %d",
				     err);
			return err;
		}
	}
	return 0;
}

/* v1 policy, DIRECT_KEY: use the master key directly */
static int setup_v1_file_key_direct(struct fscrypt_info *ci,
				    const u8 *raw_master_key)
{
	const struct fscrypt_mode *mode = ci->ci_mode;
	struct fscrypt_direct_key *dk;

	if (!fscrypt_mode_supports_direct_key(mode)) {
		fscrypt_warn(ci->ci_inode,
			     "Direct key mode not allowed with %s",
			     mode->friendly_name);
		return -EINVAL;
	}

	if (ci->ci_data_mode != ci->ci_filename_mode) {
		fscrypt_warn(ci->ci_inode,
			     "Direct key mode not allowed with different contents and filenames modes");
		return -EINVAL;
	}

	/* ESSIV implies 16-byte IVs which implies !DIRECT_KEY */
	if (WARN_ON(mode->needs_essiv))
		return -EINVAL;

	dk = fscrypt_get_direct_key(ci, raw_master_key);
	if (IS_ERR(dk))
		return PTR_ERR(dk);
	ci->ci_direct_key = dk;
	ci->ci_ctfm = dk->dk_ctfm;
	return 0;
}

/* v1 policy, !DIRECT_KEY: derive the file's encryption key */
static int setup_v1_file_key_derived(struct fscrypt_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_NOFS);
	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_derived_key(ci, derived_key);
out:
	kzfree(derived_key);
	return err;
}

static int fscrypt_setup_v1_file_key(struct fscrypt_info *ci,
				     const u8 *raw_master_key)
{
	if (ci->ci_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);
}

static int fscrypt_setup_v1_file_key_via_subscribed_keyrings(
						struct fscrypt_info *ci)
{
	struct key *key;
	const struct fscrypt_key *payload;
	int err;

	key = find_and_lock_process_key(FSCRYPT_KEY_DESC_PREFIX,
					ci->ci_master_key_descriptor,
					ci->ci_mode->keysize, &payload);
	if (key == ERR_PTR(-ENOKEY) && ci->ci_inode->i_sb->s_cop->key_prefix) {
		key = find_and_lock_process_key(ci->ci_inode->i_sb->s_cop->key_prefix,
						ci->ci_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;
}

/*
 * Find the master key, then set up the inode's actual encryption key.
 */
static int setup_file_encryption_key(struct fscrypt_info *ci)
{
	return fscrypt_setup_v1_file_key_via_subscribed_keyrings(ci);
}

static void put_crypt_info(struct fscrypt_info *ci)
{
	if (!ci)
		return;

	if (ci->ci_direct_key) {
		put_direct_key(ci->ci_direct_key);
	} else {
		crypto_free_skcipher(ci->ci_ctfm);
		crypto_free_cipher(ci->ci_essiv_tfm);
	}
	kmem_cache_free(fscrypt_info_cachep, ci);
}

int fscrypt_get_encryption_info(struct inode *inode)
{
	struct fscrypt_info *crypt_info;
	struct fscrypt_context ctx;
	struct fscrypt_mode *mode;
	int res;

	if (fscrypt_has_encryption_key(inode))
		return 0;

	res = fscrypt_initialize(inode->i_sb->s_cop->flags);
	if (res)
		return res;

	res = inode->i_sb->s_cop->get_context(inode, &ctx, sizeof(ctx));
	if (res < 0) {
		if (!fscrypt_dummy_context_enabled(inode) ||
		    IS_ENCRYPTED(inode)) {
			fscrypt_warn(inode,
				     "Error %d getting encryption context",
				     res);
			return res;
		}
		/* Fake up a context for an unencrypted directory */
		memset(&ctx, 0, sizeof(ctx));
		ctx.format = FS_ENCRYPTION_CONTEXT_FORMAT_V1;
		ctx.contents_encryption_mode = FSCRYPT_MODE_AES_256_XTS;
		ctx.filenames_encryption_mode = FSCRYPT_MODE_AES_256_CTS;
		memset(ctx.master_key_descriptor, 0x42,
		       FSCRYPT_KEY_DESCRIPTOR_SIZE);
	} else if (res != sizeof(ctx)) {
		fscrypt_warn(inode,
			     "Unknown encryption context size (%d bytes)", res);
		return -EINVAL;
	}

	if (ctx.format != FS_ENCRYPTION_CONTEXT_FORMAT_V1) {
		fscrypt_warn(inode, "Unknown encryption context version (%d)",
			     ctx.format);
		return -EINVAL;
	}

	if (ctx.flags & ~FSCRYPT_POLICY_FLAGS_VALID) {
		fscrypt_warn(inode, "Unknown encryption context flags (0x%02x)",
			     ctx.flags);
		return -EINVAL;
	}

	crypt_info = kmem_cache_zalloc(fscrypt_info_cachep, GFP_NOFS);
	if (!crypt_info)
		return -ENOMEM;

	crypt_info->ci_inode = inode;

	crypt_info->ci_flags = ctx.flags;
	crypt_info->ci_data_mode = ctx.contents_encryption_mode;
	crypt_info->ci_filename_mode = ctx.filenames_encryption_mode;
	memcpy(crypt_info->ci_master_key_descriptor, ctx.master_key_descriptor,
	       FSCRYPT_KEY_DESCRIPTOR_SIZE);
	memcpy(crypt_info->ci_nonce, ctx.nonce, FS_KEY_DERIVATION_NONCE_SIZE);

	mode = select_encryption_mode(crypt_info, inode);
	if (IS_ERR(mode)) {
		res = PTR_ERR(mode);
		goto out;
	}
	WARN_ON(mode->ivsize > FSCRYPT_MAX_IV_SIZE);
	crypt_info->ci_mode = mode;

	res = setup_file_encryption_key(crypt_info);
	if (res)
		goto out;

	if (cmpxchg_release(&inode->i_crypt_info, NULL, crypt_info) == NULL)
		crypt_info = NULL;
out:
	if (res == -ENOKEY)
		res = 0;
	put_crypt_info(crypt_info);
	return res;
}
EXPORT_SYMBOL(fscrypt_get_encryption_info);

/**
 * fscrypt_put_encryption_info - free most of an inode's fscrypt data
 *
 * Free the inode's fscrypt_info.  Filesystems must call this when the inode is
 * being evicted.  An RCU grace period need not have elapsed yet.
 */
void fscrypt_put_encryption_info(struct inode *inode)
{
	put_crypt_info(inode->i_crypt_info);
	inode->i_crypt_info = NULL;
}
EXPORT_SYMBOL(fscrypt_put_encryption_info);

/**
 * fscrypt_free_inode - free an inode's fscrypt data requiring RCU delay
 *
 * Free the inode's cached decrypted symlink target, if any.  Filesystems must
 * call this after an RCU grace period, just before they free the inode.
 */
void fscrypt_free_inode(struct inode *inode)
{
	if (IS_ENCRYPTED(inode) && S_ISLNK(inode->i_mode)) {
		kfree(inode->i_link);
		inode->i_link = NULL;
	}
}
EXPORT_SYMBOL(fscrypt_free_inode);