Commit ccc829ba authored by Matthew Garrett's avatar Matthew Garrett Committed by Ingo Molnar
Browse files

efi/libstub: Enable reset attack mitigation 



If a machine is reset while secrets are present in RAM, it may be
possible for code executed after the reboot to extract those secrets
from untouched memory. The Trusted Computing Group specified a mechanism
for requesting that the firmware clear all RAM on reset before booting
another OS. This is done by setting the MemoryOverwriteRequestControl
variable at startup. If userspace can ensure that all secrets are
removed as part of a controlled shutdown, it can reset this variable to
0 before triggering a hardware reboot.

Signed-off-by: default avatarMatthew Garrett <mjg59@google.com>
Signed-off-by: default avatarArd Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-efi@vger.kernel.org
Link: http://lkml.kernel.org/r/20170825155019.6740-2-ard.biesheuvel@linaro.org


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 3cb9bc85

arch/x86/boot/compressed/eboot.c

+3 −0
Original line number Diff line number Diff line
@@ -997,6 +997,9 @@ struct boot_params *efi_main(struct efi_config *c, 
	if (boot_params->secure_boot == efi_secureboot_mode_unset)

		boot_params->secure_boot = efi_get_secureboot(sys_table);



	/* Ask the firmware to clear memory on unclean shutdown */

	efi_enable_reset_attack_mitigation(sys_table);



	setup_graphics(boot_params);



	setup_efi_pci(boot_params);

drivers/firmware/efi/Kconfig

+10 −0
Original line number Diff line number Diff line
@@ -151,6 +151,16 @@ config APPLE_PROPERTIES 


	  If unsure, say Y if you have a Mac.  Otherwise N.



config RESET_ATTACK_MITIGATION

	bool "Reset memory attack mitigation"

	depends on EFI_STUB

	help

	  Request that the firmware clear the contents of RAM after a reboot

	  using the TCG Platform Reset Attack Mitigation specification. This

	  protects against an attacker forcibly rebooting the system while it

	  still contains secrets in RAM, booting another OS and extracting the

	  secrets.



endmenu



config UEFI_CPER

drivers/firmware/efi/libstub/Makefile

+1 −0
Original line number Diff line number Diff line
@@ -30,6 +30,7 @@ OBJECT_FILES_NON_STANDARD := y 
KCOV_INSTRUMENT			:= n



lib-y				:= efi-stub-helper.o gop.o secureboot.o

lib-$(CONFIG_RESET_ATTACK_MITIGATION) += tpm.o



# include the stub's generic dependencies from lib/ when building for ARM/arm64

arm-deps := fdt_rw.c fdt_ro.c fdt_wip.c fdt.c fdt_empty_tree.c fdt_sw.c sort.c

drivers/firmware/efi/libstub/arm-stub.c

+3 −0
Original line number Diff line number Diff line
@@ -192,6 +192,9 @@ unsigned long efi_entry(void *handle, efi_system_table_t *sys_table, 
		goto fail_free_cmdline;

	}



	/* Ask the firmware to clear memory on unclean shutdown */

	efi_enable_reset_attack_mitigation(sys_table);



	secure_boot = efi_get_secureboot(sys_table);



	/*

drivers/firmware/efi/libstub/tpm.c

0 → 100644
+58 −0
Original line number Diff line number Diff line 
/*

 * 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);

}

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