TBB: add non-volatile counter support
This patch adds support for non-volatile counter authentication to
the Authentication Module. This method consists of matching the
counter values provided in the certificates with the ones stored
in the platform. If the value from the certificate is lower than
the platform, the boot process is aborted. This mechanism protects
the system against rollback.
The TBBR CoT has been updated to include this method as part of the
authentication process. Two counters are used: one for the trusted
world images and another for the non trusted world images.
** NEW PLATFORM APIs (mandatory when TBB is enabled) **
int plat_get_nv_ctr(void *cookie, unsigned int *nv_ctr);
This API returns the non-volatile counter value stored
in the platform. The cookie in the first argument may be
used to select the counter in case the platform provides
more than one (i.e. TBSA compliant platforms must provide
trusted and non-trusted counters). This cookie is specified
in the CoT.
int plat_set_nv_ctr(void *cookie, unsigned int nv_ctr);
This API sets a new counter value. The cookie may be
used to select the counter to be updated.
An implementation of these new APIs for ARM platforms is also
provided. The values are obtained from the Trusted Non-Volatile
Counters peripheral. The cookie is used to pass the extension OID.
This OID may be interpreted by the platform to know which counter
must return. On Juno, The trusted and non-trusted counter values
have been tied to 31 and 223, respectively, and cannot be modified.
** IMPORTANT **
THIS PATCH BREAKS THE BUILD WHEN TRUSTED_BOARD_BOOT IS ENABLED. THE
NEW PLATFORM APIs INTRODUCED IN THIS PATCH MUST BE IMPLEMENTED IN
ORDER TO SUCCESSFULLY BUILD TF.
Change-Id: Ic943b76b25f2a37f490eaaab6d87b4a8b3cbc89a
diff --git a/drivers/auth/auth_mod.c b/drivers/auth/auth_mod.c
index bdd3c5a..4184556 100644
--- a/drivers/auth/auth_mod.c
+++ b/drivers/auth/auth_mod.c
@@ -40,6 +40,9 @@
#include <stdint.h>
#include <string.h>
+/* ASN.1 tags */
+#define ASN1_INTEGER 0x02
+
#define return_if_error(rc) \
do { \
if (rc != 0) { \
@@ -227,6 +230,83 @@
}
/*
+ * Authenticate by Non-Volatile counter
+ *
+ * To protect the system against rollback, the platform includes a non-volatile
+ * counter whose value can only be increased. All certificates include a counter
+ * value that should not be lower than the value stored in the platform. If the
+ * value is larger, the counter in the platform must be updated to the new
+ * value.
+ *
+ * Return: 0 = success, Otherwise = error
+ */
+static int auth_nvctr(const auth_method_param_nv_ctr_t *param,
+ const auth_img_desc_t *img_desc,
+ void *img, unsigned int img_len)
+{
+ char *p;
+ void *data_ptr = NULL;
+ unsigned int data_len, len, i;
+ unsigned int cert_nv_ctr, plat_nv_ctr;
+ int rc = 0;
+
+ /* Get the counter value from current image. The AM expects the IPM
+ * to return the counter value as a DER encoded integer */
+ rc = img_parser_get_auth_param(img_desc->img_type, param->cert_nv_ctr,
+ img, img_len, &data_ptr, &data_len);
+ return_if_error(rc);
+
+ /* Parse the DER encoded integer */
+ assert(data_ptr);
+ p = (char *)data_ptr;
+ if (*p != ASN1_INTEGER) {
+ /* Invalid ASN.1 integer */
+ return 1;
+ }
+ p++;
+
+ /* NV-counters are unsigned integers up to 32-bit */
+ len = (unsigned int)(*p & 0x7f);
+ if ((*p & 0x80) || (len > 4)) {
+ return 1;
+ }
+ p++;
+
+ /* Check the number is not negative */
+ if (*p & 0x80) {
+ return 1;
+ }
+
+ /* Convert to unsigned int. This code is for a little-endian CPU */
+ cert_nv_ctr = 0;
+ for (i = 0; i < len; i++) {
+ cert_nv_ctr = (cert_nv_ctr << 8) | *p++;
+ }
+
+ /* Get the counter from the platform */
+ rc = plat_get_nv_ctr(param->plat_nv_ctr->cookie, &plat_nv_ctr);
+ return_if_error(rc);
+
+ if (cert_nv_ctr < plat_nv_ctr) {
+ /* Invalid NV-counter */
+ return 1;
+ } else if (cert_nv_ctr > plat_nv_ctr) {
+ if (img_desc->parent == NULL) {
+ /* This certificate has been signed with the ROT key.
+ * Update the platform counter value */
+ rc = plat_set_nv_ctr(param->plat_nv_ctr->cookie,
+ cert_nv_ctr);
+ return_if_error(rc);
+ } else {
+ /* Secondary certificates cannot modify the counter */
+ return 1;
+ }
+ }
+
+ return 0;
+}
+
+/*
* Return the parent id in the output parameter '*parent_id'
*
* Return value:
@@ -310,6 +390,10 @@
rc = auth_signature(&auth_method->param.sig,
img_desc, img_ptr, img_len);
break;
+ case AUTH_METHOD_NV_CTR:
+ rc = auth_nvctr(&auth_method->param.nv_ctr,
+ img_desc, img_ptr, img_len);
+ break;
default:
/* Unknown authentication method */
rc = 1;