fs/crypto/keysetup.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 <crypto/aes.h>
#include <crypto/sha.h>
#include <crypto/skcipher.h>
#include <linux/key.h>

#include "fscrypt_private.h"

static struct crypto_shash *essiv_hash_tfm;

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 union fscrypt_policy *policy,
		       const struct inode *inode)
{
	BUILD_BUG_ON(ARRAY_SIZE(available_modes) != FSCRYPT_MODE_MAX + 1);

	if (S_ISREG(inode->i_mode))
		return &available_modes[fscrypt_policy_contents_mode(policy)];

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

	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 */
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);
}

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) */
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;
}

static int setup_per_mode_key(struct fscrypt_info *ci,
			      struct fscrypt_master_key *mk)
{
	struct fscrypt_mode *mode = ci->ci_mode;
	u8 mode_num = mode - available_modes;
	struct crypto_skcipher *tfm, *prev_tfm;
	u8 mode_key[FSCRYPT_MAX_KEY_SIZE];
	int err;

	if (WARN_ON(mode_num >= ARRAY_SIZE(mk->mk_mode_keys)))
		return -EINVAL;

	/* pairs with cmpxchg() below */
	tfm = READ_ONCE(mk->mk_mode_keys[mode_num]);
	if (likely(tfm != NULL))
		goto done;

	BUILD_BUG_ON(sizeof(mode_num) != 1);
	err = fscrypt_hkdf_expand(&mk->mk_secret.hkdf,
				  HKDF_CONTEXT_PER_MODE_KEY,
				  &mode_num, sizeof(mode_num),
				  mode_key, mode->keysize);
	if (err)
		return err;
	tfm = fscrypt_allocate_skcipher(mode, mode_key, ci->ci_inode);
	memzero_explicit(mode_key, mode->keysize);
	if (IS_ERR(tfm))
		return PTR_ERR(tfm);

	/* pairs with READ_ONCE() above */
	prev_tfm = cmpxchg(&mk->mk_mode_keys[mode_num], NULL, tfm);
	if (prev_tfm != NULL) {
		crypto_free_skcipher(tfm);
		tfm = prev_tfm;
	}
done:
	ci->ci_ctfm = tfm;
	return 0;
}

static int fscrypt_setup_v2_file_key(struct fscrypt_info *ci,
				     struct fscrypt_master_key *mk)
{
	u8 derived_key[FSCRYPT_MAX_KEY_SIZE];
	int err;

	if (ci->ci_policy.v2.flags & FSCRYPT_POLICY_FLAG_DIRECT_KEY) {
		/*
		 * DIRECT_KEY: instead of deriving per-file keys, the per-file
		 * nonce will be included in all the IVs.  But unlike v1
		 * policies, for v2 policies in this case we don't encrypt with
		 * the master key directly but rather derive a per-mode key.
		 * This ensures that the master key is consistently used only
		 * for HKDF, avoiding key reuse issues.
		 */
		if (!fscrypt_mode_supports_direct_key(ci->ci_mode)) {
			fscrypt_warn(ci->ci_inode,
				     "Direct key flag not allowed with %s",
				     ci->ci_mode->friendly_name);
			return -EINVAL;
		}
		return setup_per_mode_key(ci, mk);
	}

	err = fscrypt_hkdf_expand(&mk->mk_secret.hkdf,
				  HKDF_CONTEXT_PER_FILE_KEY,
				  ci->ci_nonce, FS_KEY_DERIVATION_NONCE_SIZE,
				  derived_key, ci->ci_mode->keysize);
	if (err)
		return err;

	err = fscrypt_set_derived_key(ci, derived_key);
	memzero_explicit(derived_key, ci->ci_mode->keysize);
	return err;
}

/*
 * Find the master key, then set up the inode's actual encryption key.
 *
 * If the master key is found in the filesystem-level keyring, then the
 * corresponding 'struct key' is returned in *master_key_ret with
 * ->mk_secret_sem read-locked.  This is needed to ensure that only one task
 * links the fscrypt_info into ->mk_decrypted_inodes (as multiple tasks may race
 * to create an fscrypt_info for the same inode), and to synchronize the master
 * key being removed with a new inode starting to use it.
 */
static int setup_file_encryption_key(struct fscrypt_info *ci,
				     struct key **master_key_ret)
{
	struct key *key;
	struct fscrypt_master_key *mk = NULL;
	struct fscrypt_key_specifier mk_spec;
	int err;

	switch (ci->ci_policy.version) {
	case FSCRYPT_POLICY_V1:
		mk_spec.type = FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR;
		memcpy(mk_spec.u.descriptor,
		       ci->ci_policy.v1.master_key_descriptor,
		       FSCRYPT_KEY_DESCRIPTOR_SIZE);
		break;
	case FSCRYPT_POLICY_V2:
		mk_spec.type = FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER;
		memcpy(mk_spec.u.identifier,
		       ci->ci_policy.v2.master_key_identifier,
		       FSCRYPT_KEY_IDENTIFIER_SIZE);
		break;
	default:
		WARN_ON(1);
		return -EINVAL;
	}

	key = fscrypt_find_master_key(ci->ci_inode->i_sb, &mk_spec);
	if (IS_ERR(key)) {
		if (key != ERR_PTR(-ENOKEY) ||
		    ci->ci_policy.version != FSCRYPT_POLICY_V1)
			return PTR_ERR(key);

		/*
		 * As a legacy fallback for v1 policies, search for the key in
		 * the current task's subscribed keyrings too.  Don't move this
		 * to before the search of ->s_master_keys, since users
		 * shouldn't be able to override filesystem-level keys.
		 */
		return fscrypt_setup_v1_file_key_via_subscribed_keyrings(ci);
	}

	mk = key->payload.data[0];
	down_read(&mk->mk_secret_sem);

	/* Has the secret been removed (via FS_IOC_REMOVE_ENCRYPTION_KEY)? */
	if (!is_master_key_secret_present(&mk->mk_secret)) {
		err = -ENOKEY;
		goto out_release_key;
	}

	/*
	 * Require that the master key be at least as long as the derived key.
	 * Otherwise, the derived key cannot possibly contain as much entropy as
	 * that required by the encryption mode it will be used for.  For v1
	 * policies it's also required for the KDF to work at all.
	 */
	if (mk->mk_secret.size < ci->ci_mode->keysize) {
		fscrypt_warn(NULL,
			     "key with %s %*phN is too short (got %u bytes, need %u+ bytes)",
			     master_key_spec_type(&mk_spec),
			     master_key_spec_len(&mk_spec), (u8 *)&mk_spec.u,
			     mk->mk_secret.size, ci->ci_mode->keysize);
		err = -ENOKEY;
		goto out_release_key;
	}

	switch (ci->ci_policy.version) {
	case FSCRYPT_POLICY_V1:
		err = fscrypt_setup_v1_file_key(ci, mk->mk_secret.raw);
		break;
	case FSCRYPT_POLICY_V2:
		err = fscrypt_setup_v2_file_key(ci, mk);
		break;
	default:
		WARN_ON(1);
		err = -EINVAL;
		break;
	}
	if (err)
		goto out_release_key;

	*master_key_ret = key;
	return 0;

out_release_key:
	up_read(&mk->mk_secret_sem);
	key_put(key);
	return err;
}

static void put_crypt_info(struct fscrypt_info *ci)
{
	struct key *key;

	if (!ci)
		return;

	if (ci->ci_direct_key) {
		fscrypt_put_direct_key(ci->ci_direct_key);
	} else if ((ci->ci_ctfm != NULL || ci->ci_essiv_tfm != NULL) &&
		   !fscrypt_is_direct_key_policy(&ci->ci_policy)) {
		crypto_free_skcipher(ci->ci_ctfm);
		crypto_free_cipher(ci->ci_essiv_tfm);
	}

	key = ci->ci_master_key;
	if (key) {
		struct fscrypt_master_key *mk = key->payload.data[0];

		/*
		 * Remove this inode from the list of inodes that were unlocked
		 * with the master key.
		 *
		 * In addition, if we're removing the last inode from a key that
		 * already had its secret removed, invalidate the key so that it
		 * gets removed from ->s_master_keys.
		 */
		spin_lock(&mk->mk_decrypted_inodes_lock);
		list_del(&ci->ci_master_key_link);
		spin_unlock(&mk->mk_decrypted_inodes_lock);
		if (refcount_dec_and_test(&mk->mk_refcount))
			key_invalidate(key);
		key_put(key);
	}
	kmem_cache_free(fscrypt_info_cachep, ci);
}

int fscrypt_get_encryption_info(struct inode *inode)
{
	struct fscrypt_info *crypt_info;
	union fscrypt_context ctx;
	struct fscrypt_mode *mode;
	struct key *master_key = NULL;
	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.version = FSCRYPT_CONTEXT_V1;
		ctx.v1.contents_encryption_mode = FSCRYPT_MODE_AES_256_XTS;
		ctx.v1.filenames_encryption_mode = FSCRYPT_MODE_AES_256_CTS;
		memset(ctx.v1.master_key_descriptor, 0x42,
		       FSCRYPT_KEY_DESCRIPTOR_SIZE);
		res = sizeof(ctx.v1);
	}

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

	crypt_info->ci_inode = inode;

	res = fscrypt_policy_from_context(&crypt_info->ci_policy, &ctx, res);
	if (res) {
		fscrypt_warn(inode,
			     "Unrecognized or corrupt encryption context");
		goto out;
	}

	switch (ctx.version) {
	case FSCRYPT_CONTEXT_V1:
		memcpy(crypt_info->ci_nonce, ctx.v1.nonce,
		       FS_KEY_DERIVATION_NONCE_SIZE);
		break;
	case FSCRYPT_CONTEXT_V2:
		memcpy(crypt_info->ci_nonce, ctx.v2.nonce,
		       FS_KEY_DERIVATION_NONCE_SIZE);
		break;
	default:
		WARN_ON(1);
		res = -EINVAL;
		goto out;
	}

	if (!fscrypt_supported_policy(&crypt_info->ci_policy, inode)) {
		res = -EINVAL;
		goto out;
	}

	mode = select_encryption_mode(&crypt_info->ci_policy, 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, &master_key);
	if (res)
		goto out;

	if (cmpxchg_release(&inode->i_crypt_info, NULL, crypt_info) == NULL) {
		if (master_key) {
			struct fscrypt_master_key *mk =
				master_key->payload.data[0];

			refcount_inc(&mk->mk_refcount);
			crypt_info->ci_master_key = key_get(master_key);
			spin_lock(&mk->mk_decrypted_inodes_lock);
			list_add(&crypt_info->ci_master_key_link,
				 &mk->mk_decrypted_inodes);
			spin_unlock(&mk->mk_decrypted_inodes_lock);
		}
		crypt_info = NULL;
	}
	res = 0;
out:
	if (master_key) {
		struct fscrypt_master_key *mk = master_key->payload.data[0];

		up_read(&mk->mk_secret_sem);
		key_put(master_key);
	}
	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);

/**
 * fscrypt_drop_inode - check whether the inode's master key has been removed
 *
 * Filesystems supporting fscrypt must call this from their ->drop_inode()
 * method so that encrypted inodes are evicted as soon as they're no longer in
 * use and their master key has been removed.
 *
 * Return: 1 if fscrypt wants the inode to be evicted now, otherwise 0
 */
int fscrypt_drop_inode(struct inode *inode)
{
	const struct fscrypt_info *ci = READ_ONCE(inode->i_crypt_info);
	const struct fscrypt_master_key *mk;

	/*
	 * If ci is NULL, then the inode doesn't have an encryption key set up
	 * so it's irrelevant.  If ci_master_key is NULL, then the master key
	 * was provided via the legacy mechanism of the process-subscribed
	 * keyrings, so we don't know whether it's been removed or not.
	 */
	if (!ci || !ci->ci_master_key)
		return 0;
	mk = ci->ci_master_key->payload.data[0];

	/*
	 * With proper, non-racy use of FS_IOC_REMOVE_ENCRYPTION_KEY, all inodes
	 * protected by the key were cleaned by sync_filesystem().  But if
	 * userspace is still using the files, inodes can be dirtied between
	 * then and now.  We mustn't lose any writes, so skip dirty inodes here.
	 */
	if (inode->i_state & I_DIRTY_ALL)
		return 0;

	/*
	 * Note: since we aren't holding ->mk_secret_sem, the result here can
	 * immediately become outdated.  But there's no correctness problem with
	 * unnecessarily evicting.  Nor is there a correctness problem with not
	 * evicting while iput() is racing with the key being removed, since
	 * then the thread removing the key will either evict the inode itself
	 * or will correctly detect that it wasn't evicted due to the race.
	 */
	return !is_master_key_secret_present(&mk->mk_secret);
}
EXPORT_SYMBOL_GPL(fscrypt_drop_inode);