drivers/firmware/efi/libstub/tpm.c

/*
 * TPM handling.
 *
 * Copyright (C) 2016 CoreOS, Inc
 * Copyright (C) 2017 Google, Inc.
 *     Matthew Garrett <mjg59@google.com>
 *
 * This file is part of the Linux kernel, and is made available under the
 * terms of the GNU General Public License version 2.
 */
#include <linux/efi.h>
#include <asm/efi.h>

#include "efistub.h"

static const efi_char16_t efi_MemoryOverWriteRequest_name[] = {
	'M', 'e', 'm', 'o', 'r', 'y', 'O', 'v', 'e', 'r', 'w', 'r', 'i', 't',
	'e', 'R', 'e', 'q', 'u', 'e', 's', 't', 'C', 'o', 'n', 't', 'r', 'o',
	'l', 0
};

#define MEMORY_ONLY_RESET_CONTROL_GUID \
	EFI_GUID(0xe20939be, 0x32d4, 0x41be, 0xa1, 0x50, 0x89, 0x7f, 0x85, 0xd4, 0x98, 0x29)

#define get_efi_var(name, vendor, ...) \
	efi_call_runtime(get_variable, \
			 (efi_char16_t *)(name), (efi_guid_t *)(vendor), \
			 __VA_ARGS__)

#define set_efi_var(name, vendor, ...) \
	efi_call_runtime(set_variable, \
			 (efi_char16_t *)(name), (efi_guid_t *)(vendor), \
			 __VA_ARGS__)

/*
 * Enable reboot attack mitigation. This requests that the firmware clear the
 * RAM on next reboot before proceeding with boot, ensuring that any secrets
 * are cleared. If userland has ensured that all secrets have been removed
 * from RAM before reboot it can simply reset this variable.
 */
void efi_enable_reset_attack_mitigation(efi_system_table_t *sys_table_arg)
{
	u8 val = 1;
	efi_guid_t var_guid = MEMORY_ONLY_RESET_CONTROL_GUID;
	efi_status_t status;
	unsigned long datasize = 0;

	status = get_efi_var(efi_MemoryOverWriteRequest_name, &var_guid,
			     NULL, &datasize, NULL);

	if (status == EFI_NOT_FOUND)
		return;

	set_efi_var(efi_MemoryOverWriteRequest_name, &var_guid,
		    EFI_VARIABLE_NON_VOLATILE |
		    EFI_VARIABLE_BOOTSERVICE_ACCESS |
		    EFI_VARIABLE_RUNTIME_ACCESS, sizeof(val), &val);
}