xref: /openbmc/linux/drivers/firmware/efi/libstub/tpm.c (revision 68198dca)
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 }
59