tpm: Compare HMAC values in constant time

In tpm_buf_check_hmac_response(), compare the HMAC values in constant
time using crypto_memneq() instead of in variable time using memcmp().

This is worthwhile to follow best practices and to be consistent with
MAC comparisons elsewhere in the kernel.  However, in this driver the
side channel seems to have been benign: the HMAC input data is
guaranteed to always be unique, which makes the usual MAC forgery via
timing side channel not possible.  Specifically, the HMAC input data in
tpm_buf_check_hmac_response() includes the "our_nonce" field, which was
generated by the kernel earlier, remains under the control of the
kernel, and is unique for each call to tpm_buf_check_hmac_response().

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>

Author: Eric Biggers

drivers/char/tpm/Kconfig

@@ -33,6 +33,7 @@ config TCG_TPM2_HMAC
 	select CRYPTO_ECDH
 	select CRYPTO_LIB_AESCFB
 	select CRYPTO_LIB_SHA256
+	select CRYPTO_LIB_UTILS
 	help
 	  Setting this causes us to deploy a scheme which uses request
 	  and response HMACs in addition to encryption for

drivers/char/tpm/tpm2-sessions.c

@@ -71,6 +71,7 @@
 #include <crypto/ecdh.h>
 #include <crypto/hash.h>
 #include <crypto/hmac.h>
+#include <crypto/utils.h>
 
 /* maximum number of names the TPM must remember for authorization */
 #define AUTH_MAX_NAMES	3
@@ -829,12 +830,11 @@ int tpm_buf_check_hmac_response(struct tpm_chip *chip, struct tpm_buf *buf,
 	/* 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);
-	if (memcmp(rphash, &buf->data[offset_s], SHA256_DIGEST_SIZE) == 0) {
-		rc = 0;
-	} else {
+	if (crypto_memneq(rphash, &buf->data[offset_s], SHA256_DIGEST_SIZE)) {
 		dev_err(&chip->dev, "TPM: HMAC check failed\n");
 		goto out;
 	}
+	rc = 0;
 
 	/* now do response decryption */
 	if (auth->attrs & TPM2_SA_ENCRYPT) {