cr50: Add a separate seed for kek, that resets on TPM clear.

This is so that U2F registrations are invalidated after
the device goes through powerwash.

TEST=test_that <..> firmware_Cr50U2fPowerwash, manual tests
BRANCH=none
BUG=b:112604850

Change-Id: I94257ec71adc7d49dcb676f0b1dc9aa1151116bd
Signed-off-by: Louis Collard <louiscollard@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1308238
Reviewed-by: Vadim Bendebury <vbendeb@chromium.org>
Reviewed-by: Andrey Pronin <apronin@chromium.org>

3 files changed, 66 insertions, 1 deletions

diff --git a/board/cr50/tpm2/platform.c b/board/cr50/tpm2/platform.c
index 964fe45dda..407a71f94d 100644
--- a/board/cr50/tpm2/platform.c
+++ b/board/cr50/tpm2/platform.c
@@ -10,6 +10,7 @@
 #include "pinweaver.h"
 #include "tpm_nvmem.h"
 #include "trng.h"
+#include "u2f_impl.h"
 #include "util.h"
 #include "version.h"
 
@@ -89,3 +90,9 @@ BOOL _plat__ShallSurviveOwnerClear(uint32_t  index)
 {
 	return index == HR_NV_INDEX + FWMP_NV_INDEX;
 }
+
+void _plat__OwnerClearCallback(void)
+{
+	// Invalidate existing u2f registrations.
+	u2f_gen_kek_seed(0 /* commit */);
+}
diff --git a/board/cr50/u2f.c b/board/cr50/u2f.c
index f379e66566..4cb09f9663 100644
--- a/board/cr50/u2f.c
+++ b/board/cr50/u2f.c
@@ -13,6 +13,7 @@
 #include "registers.h"
 #include "signed_header.h"
 #include "system.h"
+#include "tpm_nvmem_ops.h"
 #include "tpm_vendor_cmds.h"
 #include "u2f.h"
 #include "u2f_impl.h"
@@ -63,6 +64,7 @@ enum u2f_mode {
 };
 
 static uint32_t salt[8];
+static uint32_t salt_kek[8];
 static uint8_t u2f_mode = MODE_UNSET;
 static const uint8_t k_salt = NVMEM_VAR_U2F_SALT;
 
@@ -83,6 +85,39 @@ static int load_state(void)
 		memcpy(salt, tuple_val(t_salt), sizeof(salt));
 	}
 
+	if (read_tpm_nvmem_hidden(
+		TPM_HIDDEN_U2F_KEK,
+		sizeof(salt_kek), salt_kek) ==
+	    tpm_read_not_found) {
+		/*
+		 * Not found means that we have not used u2f before,
+		 * or not used it with updated fw that resets kek seed
+		 * on TPM clear.
+		 */
+		if (t_salt) {
+			/*
+			 * We have previously used u2f, and may have
+			 * existing registrations; we don't want to
+			 * invalidate these, so preserve the existing
+			 * seed as a one-off. It will be changed on
+			 * next TPM clear.
+			 */
+			memcpy(salt_kek, salt, sizeof(salt_kek));
+		} else {
+			/*
+			 * We have never used u2f before - generate
+			 * new seed.
+			 */
+			if (!DCRYPTO_ladder_random(salt_kek))
+				return 0;
+		}
+		if (write_tpm_nvmem_hidden(
+			TPM_HIDDEN_U2F_KEK,
+			sizeof(salt_kek), salt_kek, 1 /* commit */) !=
+		    tpm_write_created)
+			return 0;
+	}
+
 	return 1;
 }
 
@@ -174,7 +209,7 @@ int u2f_gen_kek(const uint8_t *origin, uint8_t *kek, size_t key_len)
 
 	if (key_len != sizeof(buf))
 		return EC_ERROR_UNKNOWN;
-	if (!_derive_key(U2F_WRAP, salt, buf))
+	if (!_derive_key(U2F_WRAP, salt_kek, buf))
 		return EC_ERROR_UNKNOWN;
 	memcpy(kek, buf, key_len);
 
@@ -201,6 +236,19 @@ int g2f_individual_keypair(p256_int *d, p256_int *pk_x, p256_int *pk_y)
 	return EC_SUCCESS;
 }
 
+int u2f_gen_kek_seed(int commit)
+{
+	if (!DCRYPTO_ladder_random(salt_kek))
+		return EC_ERROR_HW_INTERNAL;
+
+	if (write_tpm_nvmem_hidden(
+		TPM_HIDDEN_U2F_KEK, sizeof(salt_kek), salt_kek, commit) ==
+	    tpm_write_fail)
+		return EC_ERROR_UNKNOWN;
+
+	return EC_SUCCESS;
+}
+
 /* ---- Send/receive U2F APDU over TPM vendor commands ---- */
 
 enum vendor_cmd_rc vc_u2f_apdu(enum vendor_cmd_cc code, void *body,
diff --git a/include/u2f_impl.h b/include/u2f_impl.h
index 6a14f5a151..0effc2dd8d 100644
--- a/include/u2f_impl.h
+++ b/include/u2f_impl.h
@@ -88,6 +88,16 @@ int u2f_gen_kek(const uint8_t *origin, uint8_t *kek, size_t key_len);
  */
 int g2f_individual_keypair(p256_int *d, p256_int *pk_x, p256_int *pk_y);
 
+/***
+ * Generates and persists to nvram a new seed that will be used to
+ * derive kek in future calls to u2f_gen_kek().
+ *
+ * @param commit whether to commit nvram changes before returning.
+ * @return EC_SUCCESS if seed was successfully created
+ * (and persisted if requested).
+ */
+int u2f_gen_kek_seed(int commit);
+
 /* ---- protocol extensions ---- */
 
 /* Use non-standard extensions to the U2F protocol */