[PATCH v2 2/2] tpm: Use HMAC-SHA256 library instead of open-coded HMAC

[Eric Biggers <ebiggers@kernel.org>]

Now that there are easy-to-use HMAC-SHA256 library functions, use these
in tpm2-sessions.c instead of open-coding the HMAC algorithm.

Note that the new implementation correctly handles keys longer than 64
bytes (SHA256_BLOCK_SIZE), whereas the old implementation handled such
keys incorrectly.  But it doesn't appear that such keys were being used.

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
---
 drivers/char/tpm/tpm2-sessions.c | 98 +++++++++-----------------------
 1 file changed, 27 insertions(+), 71 deletions(-)

diff --git a/drivers/char/tpm/tpm2-sessions.c b/drivers/char/tpm/tpm2-sessions.c
index 5fbd62ee50903..6d03c224e6b21 100644
--- a/drivers/char/tpm/tpm2-sessions.c
+++ b/drivers/char/tpm/tpm2-sessions.c
@@ -67,12 +67,11 @@
 #include <linux/random.h>
 #include <linux/scatterlist.h>
 #include <linux/unaligned.h>
 #include <crypto/kpp.h>
 #include <crypto/ecdh.h>
-#include <crypto/hash.h>
-#include <crypto/hmac.h>
+#include <crypto/sha2.h>
 #include <crypto/utils.h>
 
 /* maximum number of names the TPM must remember for authorization */
 #define AUTH_MAX_NAMES	3
 
@@ -383,55 +382,10 @@ EXPORT_SYMBOL_GPL(tpm_buf_append_hmac_session);
 #ifdef CONFIG_TCG_TPM2_HMAC
 
 static int tpm2_create_primary(struct tpm_chip *chip, u32 hierarchy,
 			       u32 *handle, u8 *name);
 
-/*
- * It turns out the crypto hmac(sha256) is hard for us to consume
- * because it assumes a fixed key and the TPM seems to change the key
- * on every operation, so we weld the hmac init and final functions in
- * here to give it the same usage characteristics as a regular hash
- */
-static void tpm2_hmac_init(struct sha256_ctx *sctx, u8 *key, u32 key_len)
-{
-	u8 pad[SHA256_BLOCK_SIZE];
-	int i;
-
-	sha256_init(sctx);
-	for (i = 0; i < sizeof(pad); i++) {
-		if (i < key_len)
-			pad[i] = key[i];
-		else
-			pad[i] = 0;
-		pad[i] ^= HMAC_IPAD_VALUE;
-	}
-	sha256_update(sctx, pad, sizeof(pad));
-}
-
-static void tpm2_hmac_final(struct sha256_ctx *sctx, u8 *key, u32 key_len,
-			    u8 *out)
-{
-	u8 pad[SHA256_BLOCK_SIZE];
-	int i;
-
-	for (i = 0; i < sizeof(pad); i++) {
-		if (i < key_len)
-			pad[i] = key[i];
-		else
-			pad[i] = 0;
-		pad[i] ^= HMAC_OPAD_VALUE;
-	}
-
-	/* collect the final hash;  use out as temporary storage */
-	sha256_final(sctx, out);
-
-	sha256_init(sctx);
-	sha256_update(sctx, pad, sizeof(pad));
-	sha256_update(sctx, out, SHA256_DIGEST_SIZE);
-	sha256_final(sctx, out);
-}
-
 /*
  * assume hash sha256 and nonces u, v of size SHA256_DIGEST_SIZE but
  * otherwise standard tpm2_KDFa.  Note output is in bytes not bits.
  */
 static void tpm2_KDFa(u8 *key, u32 key_len, const char *label, u8 *u,
@@ -439,20 +393,20 @@ static void tpm2_KDFa(u8 *key, u32 key_len, const char *label, u8 *u,
 {
 	u32 counter = 1;
 	const __be32 bits = cpu_to_be32(bytes * 8);
 
 	while (bytes > 0) {
-		struct sha256_ctx sctx;
+		struct hmac_sha256_ctx hctx;
 		__be32 c = cpu_to_be32(counter);
 
-		tpm2_hmac_init(&sctx, key, key_len);
-		sha256_update(&sctx, (u8 *)&c, sizeof(c));
-		sha256_update(&sctx, label, strlen(label)+1);
-		sha256_update(&sctx, u, SHA256_DIGEST_SIZE);
-		sha256_update(&sctx, v, SHA256_DIGEST_SIZE);
-		sha256_update(&sctx, (u8 *)&bits, sizeof(bits));
-		tpm2_hmac_final(&sctx, key, key_len, out);
+		hmac_sha256_init_usingrawkey(&hctx, key, key_len);
+		hmac_sha256_update(&hctx, (u8 *)&c, sizeof(c));
+		hmac_sha256_update(&hctx, label, strlen(label) + 1);
+		hmac_sha256_update(&hctx, u, SHA256_DIGEST_SIZE);
+		hmac_sha256_update(&hctx, v, SHA256_DIGEST_SIZE);
+		hmac_sha256_update(&hctx, (u8 *)&bits, sizeof(bits));
+		hmac_sha256_final(&hctx, out);
 
 		bytes -= SHA256_DIGEST_SIZE;
 		counter++;
 		out += SHA256_DIGEST_SIZE;
 	}
@@ -592,10 +546,11 @@ void tpm_buf_fill_hmac_session(struct tpm_chip *chip, struct tpm_buf *buf)
 	off_t offset_s = TPM_HEADER_SIZE, offset_p;
 	u8 *hmac = NULL;
 	u32 attrs;
 	u8 cphash[SHA256_DIGEST_SIZE];
 	struct sha256_ctx sctx;
+	struct hmac_sha256_ctx hctx;
 
 	if (!auth)
 		return;
 
 	/* save the command code in BE format */
@@ -703,18 +658,18 @@ void tpm_buf_fill_hmac_session(struct tpm_chip *chip, struct tpm_buf *buf)
 		sha256_update(&sctx, &buf->data[offset_s],
 			      tpm_buf_length(buf) - offset_s);
 	sha256_final(&sctx, cphash);
 
 	/* now calculate the hmac */
-	tpm2_hmac_init(&sctx, auth->session_key, sizeof(auth->session_key)
-		       + auth->passphrase_len);
-	sha256_update(&sctx, cphash, sizeof(cphash));
-	sha256_update(&sctx, auth->our_nonce, sizeof(auth->our_nonce));
-	sha256_update(&sctx, auth->tpm_nonce, sizeof(auth->tpm_nonce));
-	sha256_update(&sctx, &auth->attrs, 1);
-	tpm2_hmac_final(&sctx, auth->session_key, sizeof(auth->session_key)
-			+ auth->passphrase_len, hmac);
+	hmac_sha256_init_usingrawkey(&hctx, auth->session_key,
+				     sizeof(auth->session_key) +
+					     auth->passphrase_len);
+	hmac_sha256_update(&hctx, cphash, sizeof(cphash));
+	hmac_sha256_update(&hctx, auth->our_nonce, sizeof(auth->our_nonce));
+	hmac_sha256_update(&hctx, auth->tpm_nonce, sizeof(auth->tpm_nonce));
+	hmac_sha256_update(&hctx, &auth->attrs, 1);
+	hmac_sha256_final(&hctx, hmac);
 }
 EXPORT_SYMBOL(tpm_buf_fill_hmac_session);
 
 /**
  * tpm_buf_check_hmac_response() - check the TPM return HMAC for correctness
@@ -750,10 +705,11 @@ int tpm_buf_check_hmac_response(struct tpm_chip *chip, struct tpm_buf *buf,
 	struct tpm2_auth *auth = chip->auth;
 	off_t offset_s, offset_p;
 	u8 rphash[SHA256_DIGEST_SIZE];
 	u32 attrs, cc;
 	struct sha256_ctx sctx;
+	struct hmac_sha256_ctx hctx;
 	u16 tag = be16_to_cpu(head->tag);
 	int parm_len, len, i, handles;
 
 	if (!auth)
 		return rc;
@@ -819,19 +775,19 @@ int tpm_buf_check_hmac_response(struct tpm_chip *chip, struct tpm_buf *buf,
 	sha256_update(&sctx, (u8 *)&auth->ordinal, sizeof(auth->ordinal));
 	sha256_update(&sctx, &buf->data[offset_p], parm_len);
 	sha256_final(&sctx, rphash);
 
 	/* now calculate the hmac */
-	tpm2_hmac_init(&sctx, auth->session_key, sizeof(auth->session_key)
-		       + auth->passphrase_len);
-	sha256_update(&sctx, rphash, sizeof(rphash));
-	sha256_update(&sctx, auth->tpm_nonce, sizeof(auth->tpm_nonce));
-	sha256_update(&sctx, auth->our_nonce, sizeof(auth->our_nonce));
-	sha256_update(&sctx, &auth->attrs, 1);
+	hmac_sha256_init_usingrawkey(&hctx, auth->session_key,
+				     sizeof(auth->session_key) +
+					     auth->passphrase_len);
+	hmac_sha256_update(&hctx, rphash, sizeof(rphash));
+	hmac_sha256_update(&hctx, auth->tpm_nonce, sizeof(auth->tpm_nonce));
+	hmac_sha256_update(&hctx, auth->our_nonce, sizeof(auth->our_nonce));
+	hmac_sha256_update(&hctx, &auth->attrs, 1);
 	/* we're done with the rphash, so put our idea of the hmac there */
-	tpm2_hmac_final(&sctx, auth->session_key, sizeof(auth->session_key)
-			+ auth->passphrase_len, rphash);
+	hmac_sha256_final(&hctx, rphash);
 	if (crypto_memneq(rphash, &buf->data[offset_s], SHA256_DIGEST_SIZE)) {
 		dev_err(&chip->dev, "TPM: HMAC check failed\n");
 		goto out;
 	}
 	rc = 0;
-- 
2.50.1