zircon/system/ulib/kms-stateless/kms-stateless.cpp

#include "kms-stateless/kms-stateless.h"

#include <fbl/string_buffer.h>
#include <fbl/unique_fd.h>
#include <fbl/unique_ptr.h>
#include <fcntl.h>
#include <lib/fdio/watcher.h>
#include <ramdevice-client/ramdisk.h>
#include <stdint.h>
#include <stdio.h>

#include "tee-client-api/tee_client_api.h"

namespace kms_stateless {
namespace {

const size_t kDerivedKeySize = 16;
const char kDeviceClass[] = "/dev/class/tee";
// Path estimated to be "/dev/class/tee/XXX".
const size_t kMaxPathLen = 64;

// UUID of the keysafe TA.
const TEEC_UUID kKeysafeTaUuid = {
    0x808032e0,
    0xfd9e,
    0x4e6f,
    {0x88, 0x96, 0x54, 0x47, 0x35, 0xc9, 0x84, 0x80}};
// Command ID of the SignHash function of the TA.
const uint32_t kKeysafeGetHardwareDerivedKeyCmdID = 5;

// Wrapper around TEEC_Session to ensure correct deletion.
class ScopedTeecSession {
public:
    ScopedTeecSession(TEEC_Session session_) {
        session = session_;
    }
    TEEC_Result invokeCommand(uint32_t commandID, TEEC_Operation* operation) {
        return TEEC_InvokeCommand(&session, commandID, operation, nullptr);
    }
    ~ScopedTeecSession() { TEEC_CloseSession(&session); }

private:
    TEEC_Session session;
};

// Wrapper around TEEC_Context to ensure correct deletion.
class ScopedTeecContext {
public:
    ScopedTeecContext()
        : context_({0}), initialized_(false) {}
    ~ScopedTeecContext() {
        if (initialized_) {
            TEEC_FinalizeContext(&context_);
        }
    }

    TEEC_Result initialize(const char* device_path) {
        TEEC_Result result = TEEC_InitializeContext(device_path, &context_);
        if (result == TEEC_SUCCESS) {
            initialized_ = true;
        }
        return result;
    }

    fbl::unique_ptr<ScopedTeecSession> openSession() {
        TEEC_Session session = {0, 0};
        TEEC_Result result = TEEC_OpenSession(
            &context_, &session,
            &kKeysafeTaUuid, TEEC_LOGIN_PUBLIC, 0 /* connectionData*/,
            nullptr /* operation */, nullptr /* returnOrigin */);
        if (result != TEEC_SUCCESS) {
            fprintf(stderr, "TEE Unable to open session. Error: %X\n", result);
            return fbl::unique_ptr<ScopedTeecSession>();
        }
        fbl::unique_ptr<ScopedTeecSession> session_ptr =
            std::make_unique<ScopedTeecSession>(session);
        return session_ptr;
    }

private:
    TEEC_Context context_;
    bool initialized_;
};

// Gets a hardware derived key from a tee device at |device_path|.
//
// Arguments:
//    device_path: The path to the tee device.
//    key_info: The key information as part of the input to the key derivation function.
//    key_info_size: The size of |key_info|.
//    key_buffer: The caller allocated buffer to store the derived key.
//    key_size: The size of the derived key.
//
// Returns true if the operation succeed, false otherwise.
bool GetKeyFromTeeDevice(const char* device_path, uint8_t* key_info, size_t key_info_size,
                         uint8_t* key_buffer, size_t* key_size, size_t key_buffer_size) {
    ScopedTeecContext scoped_teec_context;
    TEEC_Result result = scoped_teec_context.initialize(device_path);
    if (result != TEEC_SUCCESS) {
        fprintf(stderr, "Failed to initialize TEE context: %X\n", result);
        return false;
    }
    fbl::unique_ptr<ScopedTeecSession> session_ptr = scoped_teec_context.openSession();
    if (!session_ptr.get()) {
        fprintf(stderr, "Failed to open TEE Session to Keysafe!\n");
        return false;
    }

    TEEC_Operation op;
    op.started = 0;

    op.params[0].tmpref.buffer = key_info;
    op.params[0].tmpref.size = key_info_size;
    op.params[3].tmpref.buffer = key_buffer;
    op.params[3].tmpref.size = key_buffer_size;

    op.paramTypes =
        TEEC_PARAM_TYPES(TEEC_MEMREF_TEMP_INPUT, TEEC_NONE, TEEC_NONE, TEEC_MEMREF_TEMP_OUTPUT);
    op.imp = {0};

    result = session_ptr->invokeCommand(kKeysafeGetHardwareDerivedKeyCmdID, &op);

    if (result == TEEC_ERROR_SHORT_BUFFER) {
        fprintf(stderr, "Output buffer for hardware derived key too small!\n");
        *key_size = op.params[0].tmpref.size;
        return false;
    }

    if (result != TEEC_SUCCESS) {
        fprintf(stderr, "Failed to get hardware derived key: result=%u\n", result);
        return false;
    }

    *key_size = op.params[3].tmpref.size;
    return true;
}

// The structure to pass as cookie to fdio_watch_directory callback function.
struct WatchTeeArgs {
    // The callback function called when a hardware key is successfully derived.
    GetHardwareDerivedKeyCallback callback;
    uint8_t* key_info;
    size_t key_info_size;
};

// Callback function called when a TEE device is found.
zx_status_t WatchTee(int dirfd, int event, const char* filename, void* cookie) {
    fbl::StringBuffer<kMaxPathLen> device_path;
    device_path.Append(kDeviceClass).Append("/").Append(filename);
    // Hardware derived key is expected to be 128-bit AES key.
    fbl::unique_ptr<uint8_t[]> key_buffer(new uint8_t[kDerivedKeySize]);
    size_t key_size = 0;
    WatchTeeArgs* args = reinterpret_cast<WatchTeeArgs*>(cookie);
    if (!GetKeyFromTeeDevice(device_path.c_str(), std::move(args->key_info), args->key_info_size,
                             key_buffer.get(), &key_size, kDerivedKeySize)) {
        fprintf(stderr, "Failed to get hardware derived key from TEE!\n");
        return ZX_ERR_IO;
    }
    if (key_size != kDerivedKeySize) {
        fprintf(stderr, "The hardware derived key is of wrong size!\n");
        return ZX_ERR_IO;
    }
    zx_status_t status = args->callback(std::move(key_buffer), key_size);
    if (status == ZX_OK) {
        return ZX_ERR_STOP;
    } else {
        fprintf(stderr, "Get hardware key callback function returns error!\n");
        return status;
    }
}

} // namespace

zx_status_t GetHardwareDerivedKey(GetHardwareDerivedKeyCallback callback,
                                  uint8_t key_info[kExpectedKeyInfoSize]) {
    if (wait_for_device(kDeviceClass, ZX_SEC(5)) != ZX_OK) {
        fprintf(stderr, "Error waiting for tee device directory!\n");
        return ZX_ERR_IO;
    }
    fbl::unique_fd dirfd(open(kDeviceClass, O_RDONLY));
    if (!dirfd.is_valid()) {
        fprintf(stderr, "Failed to open tee device directory!\n");
        return ZX_ERR_IO;
    }
    WatchTeeArgs args = {
        std::move(callback),
        std::move(key_info),
        kExpectedKeyInfoSize
    };
    zx_status_t watch_status = fdio_watch_directory(
        dirfd.get(), WatchTee, ZX_SEC(5), reinterpret_cast<void*>(&args));
    if (watch_status != ZX_ERR_STOP) {
        fprintf(stderr, "Failed to get hardware derived key!\n");
        return watch_status;
    }
    return ZX_OK;
}

} // namespace kms_stateless