C++ AIDL on-device GateKeeper / SharedSecret impl
Because Keystore2 always requires valid auth tokens for user keys, this
copies and extends the existing in-guest GateKeeper implementation to
negotiate a shared secret key with Keymint in order to generate a
session key for signing auth tokens.
This implementation also uses AIDL rather than HIDL to interact with the
framework. It's also packaged in an APEX.
The files are cobbled together from a few sources:
- SoftGateKeeper.h is based on hardware/interfaces/gatekeeper/1.0/software/SoftGateKeeper.h
- GateKeeper.{cpp,h} are based on device/google/cuttlefish/guest/hals/gatekeeper/remote/remote_gatekeeper.{cpp,h}
- SharedSecret.{cpp,h} are based on device/google/cuttlefish/guest/hals/keymint/remote/remote_shared_secret.{cpp,h}
- Apex files are based on device/google/cuttlefish/guest/hals/keymint/rust/
Keymint modifications to use BOOT_TIME are lifted from
https://android-review.git.corp.google.com/c/platform/hardware/interfaces/+/2856649/6..8/security/keymint/aidl/default/ta/clock.rs#38
Bug: 332376454
Change-Id: I81845d5e6370bdddb1a24d67437964e03a5fb243
Test: Run with rust-nonsecure keymint on Cuttlefish
diff --git a/gatekeeper/aidl/software/SharedSecret.cpp b/gatekeeper/aidl/software/SharedSecret.cpp
new file mode 100644
index 0000000..f693700
--- /dev/null
+++ b/gatekeeper/aidl/software/SharedSecret.cpp
@@ -0,0 +1,131 @@
+/*
+ * Copyright 2024, The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "SharedSecret.h"
+
+#include <algorithm>
+#include <cstring>
+#include <mutex>
+#include <vector>
+
+#include <openssl/rand.h>
+
+#include <KeyMintUtils.h>
+#include <aidl/android/hardware/security/sharedsecret/BnSharedSecret.h>
+#include <aidl/android/hardware/security/sharedsecret/SharedSecretParameters.h>
+#include <android-base/logging.h>
+#include <keymaster/android_keymaster_messages.h>
+#include <keymaster/android_keymaster_utils.h>
+#include <keymaster/km_openssl/ckdf.h>
+#include <keymaster/km_openssl/hmac.h>
+
+namespace aidl::android::hardware::security::sharedsecret {
+
+::ndk::ScopedAStatus SoftSharedSecret::getSharedSecretParameters(
+ SharedSecretParameters* out_params) {
+ std::lock_guard lock(mutex_);
+ if (seed_.empty()) {
+ seed_.resize(32, 0);
+ }
+ out_params->seed = seed_;
+ if (nonce_.empty()) {
+ nonce_.resize(32, 0);
+ RAND_bytes(nonce_.data(), 32);
+ }
+ out_params->nonce = nonce_;
+ LOG(INFO) << "Presented shared secret parameters with seed size " << out_params->seed.size()
+ << " and nonce size " << out_params->nonce.size();
+ return ::ndk::ScopedAStatus::ok();
+}
+
+::ndk::ScopedAStatus SoftSharedSecret::computeSharedSecret(
+ const std::vector<SharedSecretParameters>& params, std::vector<uint8_t>* sharing_check) {
+ std::lock_guard lock(mutex_);
+ LOG(INFO) << "Computing shared secret";
+ // Reimplemented based on SoftKeymasterEnforcement, which does not expose
+ // enough functionality to satisfy the GateKeeper interface
+ keymaster::KeymasterKeyBlob key_agreement_key;
+ if (key_agreement_key.Reset(32) == nullptr) {
+ LOG(ERROR) << "key agreement key memory allocation failed";
+ return keymint::km_utils::kmError2ScopedAStatus(KM_ERROR_MEMORY_ALLOCATION_FAILED);
+ }
+ // Matching:
+ // - kFakeAgreementKey in system/keymaster/km_openssl/soft_keymaster_enforcement.cpp
+ // - Keys::kak in hardware/interfaces/security/keymint/aidl/default/ta/soft.rs
+ std::memset(key_agreement_key.writable_data(), 0, 32);
+ keymaster::KeymasterBlob label((uint8_t*)KEY_AGREEMENT_LABEL, strlen(KEY_AGREEMENT_LABEL));
+ if (label.data == nullptr) {
+ LOG(ERROR) << "label memory allocation failed";
+ return keymint::km_utils::kmError2ScopedAStatus(KM_ERROR_MEMORY_ALLOCATION_FAILED);
+ }
+
+ static_assert(sizeof(keymaster_blob_t) == sizeof(keymaster::KeymasterBlob));
+
+ bool found_mine = false;
+ std::vector<keymaster::KeymasterBlob> context_blobs;
+ for (const auto& param : params) {
+ auto& seed_blob = context_blobs.emplace_back();
+ if (seed_blob.Reset(param.seed.size()) == nullptr) {
+ LOG(ERROR) << "seed memory allocation failed";
+ return keymint::km_utils::kmError2ScopedAStatus(KM_ERROR_MEMORY_ALLOCATION_FAILED);
+ }
+ std::copy(param.seed.begin(), param.seed.end(), seed_blob.writable_data());
+ auto& nonce_blob = context_blobs.emplace_back();
+ if (nonce_blob.Reset(param.nonce.size()) == nullptr) {
+ LOG(ERROR) << "Nonce memory allocation failed";
+ return keymint::km_utils::kmError2ScopedAStatus(KM_ERROR_MEMORY_ALLOCATION_FAILED);
+ }
+ std::copy(param.nonce.begin(), param.nonce.end(), nonce_blob.writable_data());
+ if (param.seed == seed_ && param.nonce == nonce_) {
+ found_mine = true;
+ }
+ }
+ if (!found_mine) {
+ LOG(ERROR) << "Did not receive my own shared secret parameter back";
+ return keymint::km_utils::kmError2ScopedAStatus(KM_ERROR_INVALID_ARGUMENT);
+ }
+ auto context_blobs_ptr = reinterpret_cast<keymaster_blob_t*>(context_blobs.data());
+ if (hmac_key_.Reset(32) == nullptr) {
+ LOG(ERROR) << "hmac key allocation failed";
+ return keymint::km_utils::kmError2ScopedAStatus(KM_ERROR_MEMORY_ALLOCATION_FAILED);
+ }
+ auto error = keymaster::ckdf(key_agreement_key, label, context_blobs_ptr, context_blobs.size(),
+ &hmac_key_);
+ if (error != KM_ERROR_OK) {
+ LOG(ERROR) << "CKDF failed";
+ return keymint::km_utils::kmError2ScopedAStatus(error);
+ }
+
+ keymaster::HmacSha256 hmac_impl;
+ if (!hmac_impl.Init(hmac_key_.key_material, hmac_key_.key_material_size)) {
+ LOG(ERROR) << "hmac initialization failed";
+ return ::ndk::ScopedAStatus::fromExceptionCode(EX_ILLEGAL_STATE);
+ }
+ sharing_check->clear();
+ sharing_check->resize(32, 0);
+ if (!hmac_impl.Sign((const uint8_t*)KEY_CHECK_LABEL, strlen(KEY_CHECK_LABEL),
+ sharing_check->data(), sharing_check->size())) {
+ return ::ndk::ScopedAStatus::fromExceptionCode(EX_ILLEGAL_STATE);
+ }
+ return ::ndk::ScopedAStatus::ok();
+}
+
+keymaster::KeymasterKeyBlob SoftSharedSecret::HmacKey() const {
+ std::lock_guard lock(mutex_);
+ return hmac_key_;
+}
+
+} // namespace aidl::android::hardware::security::sharedsecret