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hv: Derive decryption key from Seed for Trusty to decrypt attestation keybox

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CSE FW uses an AEK (Attestation keybox Encryption Key) to encrypt the keybox
with AES-256-GCM algorithm before sending it to Android/Trusty. This key is
derived from the latest platform Seed by CSE FW with KDF (key derivation function)
HMAC-SHA256. After Trusty retrieves this encrypted keybox over HECI/MEI driver,
Trusty needs the same AEKkey to decrypt it. Hence, before Trusty launches,
Hypervisor derives the same AEK key from Platform Seed with the same algorithm
and the same derivation parameters, then sends this AEK along with Trusty vSeed
to Trusty world memory.

Since Platform Seed is only visible to Hypervisor and it must not be
sent to any guest VM, only Hypervisor can derive this AEK from this
Platform Seed, just like previous per-Trusty virtual Seed derivation.
Please note that Android Attestation Keybox is shared in a single hardware
platform, so all the Trusty instance/world can get the same AEK for
decryption even if there are multiple Android User OS/VMs running
on top of Hypervisor.

v1 --> v2:
	Add detailed description why we need the patch to derive an extra key

v2 --> v3:
	Convert API descriptions to Doxygen

Tracked-On: #1812
Reviewed-by: Bing Zhu <bing.zhu@intel.com>
Reviewed-by: Kai Wang <kai.z.wang@intel.com>
Signed-off-by: Chen Gang G <gang.g.chen@intel.com>
Acked-by: Bing Zhu <bing.zhu@intel.com>
This commit is contained in:
Chen, Gang G
2018-11-14 13:32:56 +08:00
committed by lijinxia
parent 7978188c1d
commit fc9ec5d88f
6 changed files with 153 additions and 46 deletions
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57
hypervisor/arch/x86/trusty.c
View File

@@ -5,7 +5,7 @@
*/
#include <hypervisor.h>
#include <hkdf_wrap.h>
#include <crypto_api.h>
#define ACRN_DBG_TRUSTY 6U
@@ -334,6 +334,54 @@ void switch_world(struct acrn_vcpu *vcpu, int next_world)
arch->cur_context = next_world;
}
static int32_t get_max_svn_index(void)
{
uint32_t i, max_svn_idx = 0U;
if ((g_key_info.num_seeds == 0U) ||
(g_key_info.num_seeds > BOOTLOADER_SEED_MAX_ENTRIES)) {
return -1;
}
for (i = 1U; i < g_key_info.num_seeds; i++) {
if (g_key_info.dseed_list[i].cse_svn >
g_key_info.dseed_list[i-1].cse_svn) {
max_svn_idx = i;
}
}
return max_svn_idx;
}
static bool derive_aek(uint8_t *attkb_key)
{
const int8_t salt[] = "Attestation Keybox Encryption Key";
const uint8_t *ikm;
uint32_t ikm_len;
int32_t max_svn_idx;
if (!attkb_key) {
return false;
}
max_svn_idx = get_max_svn_index();
if (max_svn_idx < 0) {
return false;
}
ikm = g_key_info.dseed_list[max_svn_idx].seed;
/* only the low 32 bits of seed are valid */
ikm_len = 32;
if (hmac_sha256(attkb_key, ikm, ikm_len,
(const uint8_t *)salt, sizeof(salt)) != 1) {
pr_err("%s: failed to derive key!\n", __func__);
return false;
}
return true;
}
/* Put key_info and trusty_startup_param in the first Page of Trusty
* runtime memory
*/
@@ -368,6 +416,13 @@ static bool setup_trusty_info(struct acrn_vcpu *vcpu,
key_info->dseed_list[i].cse_svn = g_key_info.dseed_list[i].cse_svn;
}
/* Derive decryption key of attestation keybox from dseed */
if (!derive_aek(key_info->attkb_enc_key)) {
(void)memset(key_info, 0U, sizeof(struct trusty_key_info));
pr_err("%s: derive key of att keybox failed!", __func__);
return false;
}
/* Prepare trusty startup param */
mem->first_page.data.startup_param.size_of_this_struct =
sizeof(struct trusty_startup_param);