[dm-devel] [RFC PATCH v2] md/dm-crypt - reuse eboiv skcipher for IV generation

[Ard Biesheuvel]

Instead of instantiating a separate cipher to perform the encryption
needed to produce the IV, reuse the skcipher used for the block data
and invoke it one additional time for each block to encrypt a zero
vector and use the output as the IV.

For CBC mode, this is equivalent to using the bare block cipher, but
without the risk of ending up with a non-time invariant implementation
of AES when the skcipher itself is time variant (e.g., arm64 without
Crypto Extensions has a NEON based time invariant implementation of
cbc(aes) but no time invariant implementation of the core cipher other
than aes-ti, which is not enabled by default)

This approach is a compromise between dm-crypt API flexibility and
reducing dependence on parts of the crypto API that should not usually
be exposed to other subsystems, such as the bare cipher API.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
---
 drivers/md/dm-crypt.c | 70 ++++++++++++++-----------------------------
 1 file changed, 22 insertions(+), 48 deletions(-)

diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c
index d5216bcc4649..48cd76c88d77 100644
--- a/drivers/md/dm-crypt.c
+++ b/drivers/md/dm-crypt.c
@@ -120,10 +120,6 @@ struct iv_tcw_private {
 	u8 *whitening;
 };
 
-struct iv_eboiv_private {
-	struct crypto_cipher *tfm;
-};
-
 /*
  * Crypt: maps a linear range of a block device
  * and encrypts / decrypts at the same time.
@@ -163,7 +159,6 @@ struct crypt_config {
 		struct iv_benbi_private benbi;
 		struct iv_lmk_private lmk;
 		struct iv_tcw_private tcw;
-		struct iv_eboiv_private eboiv;
 	} iv_gen_private;
 	u64 iv_offset;
 	unsigned int iv_size;
@@ -847,65 +842,47 @@ static int crypt_iv_random_gen(struct crypt_config *cc, u8 *iv,
 	return 0;
 }
 
-static void crypt_iv_eboiv_dtr(struct crypt_config *cc)
-{
-	struct iv_eboiv_private *eboiv = &cc->iv_gen_private.eboiv;
-
-	crypto_free_cipher(eboiv->tfm);
-	eboiv->tfm = NULL;
-}
-
 static int crypt_iv_eboiv_ctr(struct crypt_config *cc, struct dm_target *ti,
 			    const char *opts)
 {
-	struct iv_eboiv_private *eboiv = &cc->iv_gen_private.eboiv;
-	struct crypto_cipher *tfm;
-
-	tfm = crypto_alloc_cipher(cc->cipher, 0, 0);
-	if (IS_ERR(tfm)) {
-		ti->error = "Error allocating crypto tfm for EBOIV";
-		return PTR_ERR(tfm);
+	if (test_bit(CRYPT_MODE_INTEGRITY_AEAD, &cc->cipher_flags)) {
+		ti->error = "AEAD transforms not supported for EBOIV";
+		return -EINVAL;
 	}
 
-	if (crypto_cipher_blocksize(tfm) != cc->iv_size) {
+	if (crypto_skcipher_blocksize(any_tfm(cc)) != cc->iv_size) {
 		ti->error = "Block size of EBOIV cipher does "
 			    "not match IV size of block cipher";
-		crypto_free_cipher(tfm);
 		return -EINVAL;
 	}
 
-	eboiv->tfm = tfm;
 	return 0;
 }
 
-static int crypt_iv_eboiv_init(struct crypt_config *cc)
+static int crypt_iv_eboiv_gen(struct crypt_config *cc, u8 *iv,
+			    struct dm_crypt_request *dmreq)
 {
-	struct iv_eboiv_private *eboiv = &cc->iv_gen_private.eboiv;
+	u8 buf[MAX_CIPHER_BLOCKSIZE] __aligned(__alignof__(__le64));
+	struct skcipher_request *req;
+	struct scatterlist src, dst;
+	struct crypto_wait wait;
 	int err;
 
-	err = crypto_cipher_setkey(eboiv->tfm, cc->key, cc->key_size);
-	if (err)
-		return err;
-
-	return 0;
-}
-
-static int crypt_iv_eboiv_wipe(struct crypt_config *cc)
-{
-	/* Called after cc->key is set to random key in crypt_wipe() */
-	return crypt_iv_eboiv_init(cc);
-}
+	req = skcipher_request_alloc(any_tfm(cc), GFP_KERNEL | GFP_NOFS);
+	if (!req)
+		return -ENOMEM;
 
-static int crypt_iv_eboiv_gen(struct crypt_config *cc, u8 *iv,
-			    struct dm_crypt_request *dmreq)
-{
-	struct iv_eboiv_private *eboiv = &cc->iv_gen_private.eboiv;
+	memset(buf, 0, cc->iv_size);
+	*(__le64 *)buf = cpu_to_le64(dmreq->iv_sector * cc->sector_size);
 
-	memset(iv, 0, cc->iv_size);
-	*(__le64 *)iv = cpu_to_le64(dmreq->iv_sector * cc->sector_size);
-	crypto_cipher_encrypt_one(eboiv->tfm, iv, iv);
+	sg_init_one(&src, page_address(ZERO_PAGE(0)), cc->iv_size);
+	sg_init_one(&dst, iv, cc->iv_size);
+	skcipher_request_set_crypt(req, &src, &dst, cc->iv_size, buf);
+	skcipher_request_set_callback(req, 0, crypto_req_done, &wait);
+	err = crypto_wait_req(crypto_skcipher_encrypt(req), &wait);
+	skcipher_request_free(req);
 
-	return 0;
+	return err;
 }
 
 static const struct crypt_iv_operations crypt_iv_plain_ops = {
@@ -962,9 +939,6 @@ static struct crypt_iv_operations crypt_iv_random_ops = {
 
 static struct crypt_iv_operations crypt_iv_eboiv_ops = {
 	.ctr	   = crypt_iv_eboiv_ctr,
-	.dtr	   = crypt_iv_eboiv_dtr,
-	.init	   = crypt_iv_eboiv_init,
-	.wipe	   = crypt_iv_eboiv_wipe,
 	.generator = crypt_iv_eboiv_gen
 };
 
-- 
2.17.1