fscrypt: invoke crypto API for ESSIV handling

Instead of open-coding the calculations for ESSIV handling, use an ESSIV
skcipher which does all of this under the hood.  ESSIV was added to the
crypto API in v5.4.

This is based on a patch from Ard Biesheuvel, but reworked to apply
after all the fscrypt changes that went into v5.4.

Tested with 'kvm-xfstests -c ext4,f2fs -g encrypt', including the
ciphertext verification tests for v1 and v2 encryption policies.

Originally-from: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>

4 files changed, 11 insertions, 114 deletions

diff --git a/fs/crypto/crypto.c b/fs/crypto/crypto.c
index 32a7ad0098cc..6bc3e4f1e657 100644
--- a/fs/crypto/crypto.c
+++ b/fs/crypto/crypto.c
@@ -27,7 +27,6 @@
 #include <linux/ratelimit.h>
 #include <linux/dcache.h>
 #include <linux/namei.h>
-#include <crypto/aes.h>
 #include <crypto/skcipher.h>
 #include "fscrypt_private.h"
 
@@ -143,9 +142,6 @@ void fscrypt_generate_iv(union fscrypt_iv *iv, u64 lblk_num,
 
 	if (fscrypt_is_direct_key_policy(&ci->ci_policy))
 		memcpy(iv->nonce, ci->ci_nonce, FS_KEY_DERIVATION_NONCE_SIZE);
-
-	if (ci->ci_essiv_tfm != NULL)
-		crypto_cipher_encrypt_one(ci->ci_essiv_tfm, iv->raw, iv->raw);
 }
 
 /* Encrypt or decrypt a single filesystem block of file contents */
diff --git a/fs/crypto/fscrypt_private.h b/fs/crypto/fscrypt_private.h
index e84efc01512e..76c64297ce18 100644
--- a/fs/crypto/fscrypt_private.h
+++ b/fs/crypto/fscrypt_private.h
@@ -164,12 +164,6 @@ struct fscrypt_info {
 	struct crypto_skcipher *ci_ctfm;
 
 	/*
-	 * Cipher for ESSIV IV generation.  Only set for CBC contents
-	 * encryption, otherwise is NULL.
-	 */
-	struct crypto_cipher *ci_essiv_tfm;
-
-	/*
 	 * Encryption mode used for this inode.  It corresponds to either the
 	 * contents or filenames encryption mode, depending on the inode type.
 	 */
@@ -444,7 +438,6 @@ struct fscrypt_mode {
 	int keysize;
 	int ivsize;
 	bool logged_impl_name;
-	bool needs_essiv;
 };
 
 static inline bool
diff --git a/fs/crypto/keysetup.c b/fs/crypto/keysetup.c
index d71c2d6dd162..8eb5a0e762ec 100644
--- a/fs/crypto/keysetup.c
+++ b/fs/crypto/keysetup.c
@@ -8,15 +8,11 @@
  * 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",
@@ -31,11 +27,10 @@ static struct fscrypt_mode available_modes[] = {
 		.ivsize = 16,
 	},
 	[FSCRYPT_MODE_AES_128_CBC] = {
-		.friendly_name = "AES-128-CBC",
-		.cipher_str = "cbc(aes)",
+		.friendly_name = "AES-128-CBC-ESSIV",
+		.cipher_str = "essiv(cbc(aes),sha256)",
 		.keysize = 16,
 		.ivsize = 16,
-		.needs_essiv = true,
 	},
 	[FSCRYPT_MODE_AES_128_CTS] = {
 		.friendly_name = "AES-128-CTS-CBC",
@@ -111,97 +106,16 @@ err_free_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) */
+/* Given the per-file key, set up the file's crypto transform object */
 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);
+	struct crypto_skcipher *tfm;
 
-	ci->ci_ctfm = ctfm;
+	tfm = fscrypt_allocate_skcipher(ci->ci_mode, derived_key, ci->ci_inode);
+	if (IS_ERR(tfm))
+		return PTR_ERR(tfm);
 
-	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;
-		}
-	}
+	ci->ci_ctfm = tfm;
 	return 0;
 }
 
@@ -388,13 +302,11 @@ static void put_crypt_info(struct fscrypt_info *ci)
 	if (!ci)
 		return;
 
-	if (ci->ci_direct_key) {
+	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)) {
+	else if (ci->ci_ctfm != 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) {
diff --git a/fs/crypto/keysetup_v1.c b/fs/crypto/keysetup_v1.c
index ad1a36c370c3..5298ef22aa85 100644
--- a/fs/crypto/keysetup_v1.c
+++ b/fs/crypto/keysetup_v1.c
@@ -270,10 +270,6 @@ static int setup_v1_file_key_direct(struct fscrypt_info *ci,
 		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);