fscrypt: add Adiantum support

Add support for the Adiantum encryption mode to fscrypt.  Adiantum is a
tweakable, length-preserving encryption mode with security provably
reducible to that of XChaCha12 and AES-256, subject to a security bound.
It's also a true wide-block mode, unlike XTS.  See the paper
"Adiantum: length-preserving encryption for entry-level processors"
(https://eprint.iacr.org/2018/720.pdf) for more details.  Also see
commit 059c2a4d8e16 ("crypto: adiantum - add Adiantum support").

On sufficiently long messages, Adiantum's bottlenecks are XChaCha12 and
the NH hash function.  These algorithms are fast even on processors
without dedicated crypto instructions.  Adiantum makes it feasible to
enable storage encryption on low-end mobile devices that lack AES
instructions; currently such devices are unencrypted.  On ARM Cortex-A7,
on 4096-byte messages Adiantum encryption is about 4 times faster than
AES-256-XTS encryption; decryption is about 5 times faster.

In fscrypt, Adiantum is suitable for encrypting both file contents and
names.  With filenames, it fixes a known weakness: when two filenames in
a directory share a common prefix of >= 16 bytes, with CTS-CBC their
encrypted filenames share a common prefix too, leaking information.
Adiantum does not have this problem.

Since Adiantum also accepts long tweaks (IVs), it's also safe to use the
master key directly for Adiantum encryption rather than deriving
per-file keys, provided that the per-file nonce is included in the IVs
and the master key isn't used for any other encryption mode.  This
configuration saves memory and improves performance.  A new fscrypt
policy flag is added to allow users to opt-in to this configuration.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

1 files changed, 12 insertions, 10 deletions

diff --git a/fs/crypto/fname.c b/fs/crypto/fname.c
index d7a0f682ca12..7ff40a73dbec 100644
--- a/fs/crypto/fname.c
+++ b/fs/crypto/fname.c
@@ -40,10 +40,11 @@ int fname_encrypt(struct inode *inode, const struct qstr *iname,
 {
 	struct skcipher_request *req = NULL;
 	DECLARE_CRYPTO_WAIT(wait);
-	struct crypto_skcipher *tfm = inode->i_crypt_info->ci_ctfm;
-	int res = 0;
-	char iv[FS_CRYPTO_BLOCK_SIZE];
+	struct fscrypt_info *ci = inode->i_crypt_info;
+	struct crypto_skcipher *tfm = ci->ci_ctfm;
+	union fscrypt_iv iv;
 	struct scatterlist sg;
+	int res;
 
 	/*
 	 * Copy the filename to the output buffer for encrypting in-place and
@@ -55,7 +56,7 @@ int fname_encrypt(struct inode *inode, const struct qstr *iname,
 	memset(out + iname->len, 0, olen - iname->len);
 
 	/* Initialize the IV */
-	memset(iv, 0, FS_CRYPTO_BLOCK_SIZE);
+	fscrypt_generate_iv(&iv, 0, ci);
 
 	/* Set up the encryption request */
 	req = skcipher_request_alloc(tfm, GFP_NOFS);
@@ -65,7 +66,7 @@ int fname_encrypt(struct inode *inode, const struct qstr *iname,
 			CRYPTO_TFM_REQ_MAY_BACKLOG | CRYPTO_TFM_REQ_MAY_SLEEP,
 			crypto_req_done, &wait);
 	sg_init_one(&sg, out, olen);
-	skcipher_request_set_crypt(req, &sg, &sg, olen, iv);
+	skcipher_request_set_crypt(req, &sg, &sg, olen, &iv);
 
 	/* Do the encryption */
 	res = crypto_wait_req(crypto_skcipher_encrypt(req), &wait);
@@ -94,9 +95,10 @@ static int fname_decrypt(struct inode *inode,
 	struct skcipher_request *req = NULL;
 	DECLARE_CRYPTO_WAIT(wait);
 	struct scatterlist src_sg, dst_sg;
-	struct crypto_skcipher *tfm = inode->i_crypt_info->ci_ctfm;
-	int res = 0;
-	char iv[FS_CRYPTO_BLOCK_SIZE];
+	struct fscrypt_info *ci = inode->i_crypt_info;
+	struct crypto_skcipher *tfm = ci->ci_ctfm;
+	union fscrypt_iv iv;
+	int res;
 
 	/* Allocate request */
 	req = skcipher_request_alloc(tfm, GFP_NOFS);
@@ -107,12 +109,12 @@ static int fname_decrypt(struct inode *inode,
 		crypto_req_done, &wait);
 
 	/* Initialize IV */
-	memset(iv, 0, FS_CRYPTO_BLOCK_SIZE);
+	fscrypt_generate_iv(&iv, 0, ci);
 
 	/* Create decryption request */
 	sg_init_one(&src_sg, iname->name, iname->len);
 	sg_init_one(&dst_sg, oname->name, oname->len);
-	skcipher_request_set_crypt(req, &src_sg, &dst_sg, iname->len, iv);
+	skcipher_request_set_crypt(req, &src_sg, &dst_sg, iname->len, &iv);
 	res = crypto_wait_req(crypto_skcipher_decrypt(req), &wait);
 	skcipher_request_free(req);
 	if (res < 0) {