drivers/firmware/efi/libstub/tpm.c

   1 /*
   2  * TPM handling.
   3  *
   4  * Copyright (C) 2016 CoreOS, Inc
   5  * Copyright (C) 2017 Google, Inc.
   6  *     Matthew Garrett <mjg59@google.com>
   7  *
   8  * This file is part of the Linux kernel, and is made available under the
   9  * terms of the GNU General Public License version 2.
  10  */
  11 #include <linux/efi.h>
  12 #include <asm/efi.h>
  13
  14 #include "efistub.h"
  15
  16 static const efi_char16_t efi_MemoryOverWriteRequest_name[] = {
  17         'M', 'e', 'm', 'o', 'r', 'y', 'O', 'v', 'e', 'r', 'w', 'r', 'i', 't',
  18         'e', 'R', 'e', 'q', 'u', 'e', 's', 't', 'C', 'o', 'n', 't', 'r', 'o',
  19         'l', 0
  20 };
  21
  22 #define MEMORY_ONLY_RESET_CONTROL_GUID \
  23         EFI_GUID(0xe20939be, 0x32d4, 0x41be, 0xa1, 0x50, 0x89, 0x7f, 0x85, 0xd4, 0x98, 0x29)
  24
  25 #define get_efi_var(name, vendor, ...) \
  26         efi_call_runtime(get_variable, \
  27                          (efi_char16_t *)(name), (efi_guid_t *)(vendor), \
  28                          __VA_ARGS__)
  29
  30 #define set_efi_var(name, vendor, ...) \
  31         efi_call_runtime(set_variable, \
  32                          (efi_char16_t *)(name), (efi_guid_t *)(vendor), \
  33                          __VA_ARGS__)
  34
  35 /*
  36  * Enable reboot attack mitigation. This requests that the firmware clear the
  37  * RAM on next reboot before proceeding with boot, ensuring that any secrets
  38  * are cleared. If userland has ensured that all secrets have been removed
  39  * from RAM before reboot it can simply reset this variable.
  40  */
  41 void efi_enable_reset_attack_mitigation(efi_system_table_t *sys_table_arg)
  42 {
  43         u8 val = 1;
  44         efi_guid_t var_guid = MEMORY_ONLY_RESET_CONTROL_GUID;
  45         efi_status_t status;
  46         unsigned long datasize = 0;
  47
  48         status = get_efi_var(efi_MemoryOverWriteRequest_name, &var_guid,
  49                              NULL, &datasize, NULL);
  50
  51         if (status == EFI_NOT_FOUND)
  52                 return;
  53
  54         set_efi_var(efi_MemoryOverWriteRequest_name, &var_guid,
  55                     EFI_VARIABLE_NON_VOLATILE |
  56                     EFI_VARIABLE_BOOTSERVICE_ACCESS |
  57                     EFI_VARIABLE_RUNTIME_ACCESS, sizeof(val), &val);
  58 }