fscrypt: use HMAC-SHA512 library for HKDF

For the HKDF-SHA512 key derivation needed by fscrypt, just use the
HMAC-SHA512 library functions directly.  These functions were introduced
in v6.17, and they provide simple and efficient direct support for
HMAC-SHA512.  This ends up being quite a bit simpler and more efficient
than using crypto/hkdf.c, as it avoids the generic crypto layer:

- The HMAC library can't fail, so callers don't need to handle errors
- No inefficient indirect calls
- No inefficient and error-prone dynamic allocations
- No inefficient and error-prone loading of algorithm by name
- Less stack usage

Benchmarks on x86_64 show that deriving a per-file key gets about 30%
faster, and FS_IOC_ADD_ENCRYPTION_KEY gets nearly twice as fast.

The only small downside is the HKDF-Expand logic gets duplicated again.
Then again, even considering that, the new fscrypt_hkdf_expand() is only
7 lines longer than the version that called hkdf_expand().  Later we
could add HKDF support to lib/crypto/, but for now let's just do this.

Link: https://lore.kernel.org/r/20250906035913.1141532-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>

1 files changed, 40 insertions, 69 deletions

diff --git a/fs/crypto/hkdf.c b/fs/crypto/hkdf.c
index b1ef506cd341..706f56d0076e 100644
--- a/fs/crypto/hkdf.c
+++ b/fs/crypto/hkdf.c
@@ -1,5 +1,9 @@
 // SPDX-License-Identifier: GPL-2.0
 /*
+ * Implementation of HKDF ("HMAC-based Extract-and-Expand Key Derivation
+ * Function"), aka RFC 5869.  See also the original paper (Krawczyk 2010):
+ * "Cryptographic Extraction and Key Derivation: The HKDF Scheme".
+ *
  * This is used to derive keys from the fscrypt master keys (or from the
  * "software secrets" which hardware derives from the fscrypt master keys, in
  * the case that the fscrypt master keys are hardware-wrapped keys).
@@ -7,10 +11,6 @@
  * Copyright 2019 Google LLC
  */
 
-#include <crypto/hash.h>
-#include <crypto/hkdf.h>
-#include <crypto/sha2.h>
-
 #include "fscrypt_private.h"
 
 /*
@@ -24,7 +24,6 @@
  * HKDF-SHA512 being much faster than HKDF-SHA256, as the longer digest size of
  * SHA-512 causes HKDF-Expand to only need to do one iteration rather than two.
  */
-#define HKDF_HMAC_ALG		"hmac(sha512)"
 #define HKDF_HASHLEN		SHA512_DIGEST_SIZE
 
 /*
@@ -44,54 +43,24 @@
  */
 
 /*
- * Compute HKDF-Extract using the given master key as the input keying material,
- * and prepare an HMAC transform object keyed by the resulting pseudorandom key.
- *
- * Afterwards, the keyed HMAC transform object can be used for HKDF-Expand many
- * times without having to recompute HKDF-Extract each time.
+ * Compute HKDF-Extract using 'master_key' as the input keying material, and
+ * prepare the resulting HMAC key in 'hkdf'.  Afterwards, 'hkdf' can be used for
+ * HKDF-Expand many times without having to recompute HKDF-Extract each time.
  */
-int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
-		      unsigned int master_key_size)
+void fscrypt_init_hkdf(struct hmac_sha512_key *hkdf, const u8 *master_key,
+		       unsigned int master_key_size)
 {
-	struct crypto_shash *hmac_tfm;
 	static const u8 default_salt[HKDF_HASHLEN];
 	u8 prk[HKDF_HASHLEN];
-	int err;
-
-	hmac_tfm = crypto_alloc_shash(HKDF_HMAC_ALG, 0, FSCRYPT_CRYPTOAPI_MASK);
-	if (IS_ERR(hmac_tfm)) {
-		fscrypt_err(NULL, "Error allocating " HKDF_HMAC_ALG ": %ld",
-			    PTR_ERR(hmac_tfm));
-		return PTR_ERR(hmac_tfm);
-	}
-
-	if (WARN_ON_ONCE(crypto_shash_digestsize(hmac_tfm) != sizeof(prk))) {
-		err = -EINVAL;
-		goto err_free_tfm;
-	}
-
-	err = hkdf_extract(hmac_tfm, master_key, master_key_size,
-			   default_salt, HKDF_HASHLEN, prk);
-	if (err)
-		goto err_free_tfm;
-
-	err = crypto_shash_setkey(hmac_tfm, prk, sizeof(prk));
-	if (err)
-		goto err_free_tfm;
 
-	hkdf->hmac_tfm = hmac_tfm;
-	goto out;
-
-err_free_tfm:
-	crypto_free_shash(hmac_tfm);
-out:
+	hmac_sha512_usingrawkey(default_salt, sizeof(default_salt),
+				master_key, master_key_size, prk);
+	hmac_sha512_preparekey(hkdf, prk, sizeof(prk));
 	memzero_explicit(prk, sizeof(prk));
-	return err;
 }
 
 /*
- * HKDF-Expand (RFC 5869 section 2.3).  This expands the pseudorandom key, which
- * was already keyed into 'hkdf->hmac_tfm' by fscrypt_init_hkdf(), into 'okmlen'
+ * HKDF-Expand (RFC 5869 section 2.3).  Expand the HMAC key 'hkdf' into 'okmlen'
  * bytes of output keying material parameterized by the application-specific
  * 'info' of length 'infolen' bytes, prefixed by "fscrypt\0" and the 'context'
  * byte.  This is thread-safe and may be called by multiple threads in parallel.
@@ -100,30 +69,32 @@ out:
  * adds to its application-specific info strings to guarantee that it doesn't
  * accidentally repeat an info string when using HKDF for different purposes.)
  */
-int fscrypt_hkdf_expand(const struct fscrypt_hkdf *hkdf, u8 context,
-			const u8 *info, unsigned int infolen,
-			u8 *okm, unsigned int okmlen)
-{
-	SHASH_DESC_ON_STACK(desc, hkdf->hmac_tfm);
-	u8 *full_info;
-	int err;
-
-	full_info = kzalloc(infolen + 9, GFP_KERNEL);
-	if (!full_info)
-		return -ENOMEM;
-	desc->tfm = hkdf->hmac_tfm;
-
-	memcpy(full_info, "fscrypt\0", 8);
-	full_info[8] = context;
-	memcpy(full_info + 9, info, infolen);
-
-	err = hkdf_expand(hkdf->hmac_tfm, full_info, infolen + 9,
-			  okm, okmlen);
-	kfree_sensitive(full_info);
-	return err;
-}
-
-void fscrypt_destroy_hkdf(struct fscrypt_hkdf *hkdf)
+void fscrypt_hkdf_expand(const struct hmac_sha512_key *hkdf, u8 context,
+			 const u8 *info, unsigned int infolen,
+			 u8 *okm, unsigned int okmlen)
 {
-	crypto_free_shash(hkdf->hmac_tfm);
+	struct hmac_sha512_ctx ctx;
+	u8 counter = 1;
+	u8 tmp[HKDF_HASHLEN];
+
+	WARN_ON_ONCE(okmlen > 255 * HKDF_HASHLEN);
+
+	for (unsigned int i = 0; i < okmlen; i += HKDF_HASHLEN) {
+		hmac_sha512_init(&ctx, hkdf);
+		if (i != 0)
+			hmac_sha512_update(&ctx, &okm[i - HKDF_HASHLEN],
+					   HKDF_HASHLEN);
+		hmac_sha512_update(&ctx, "fscrypt\0", 8);
+		hmac_sha512_update(&ctx, &context, 1);
+		hmac_sha512_update(&ctx, info, infolen);
+		hmac_sha512_update(&ctx, &counter, 1);
+		if (okmlen - i < HKDF_HASHLEN) {
+			hmac_sha512_final(&ctx, tmp);
+			memcpy(&okm[i], tmp, okmlen - i);
+			memzero_explicit(tmp, sizeof(tmp));
+		} else {
+			hmac_sha512_final(&ctx, &okm[i]);
+		}
+		counter++;
+	}
 }