From ccc829ba3624beb9a703fc995d016b836d9eead8 Mon Sep 17 00:00:00 2001 From: Matthew Garrett Date: Fri, 25 Aug 2017 16:50:15 +0100 Subject: 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: Matthew Garrett Signed-off-by: Ard Biesheuvel Cc: Linus Torvalds Cc: Matt Fleming Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: linux-efi@vger.kernel.org Link: http://lkml.kernel.org/r/20170825155019.6740-2-ard.biesheuvel@linaro.org Signed-off-by: Ingo Molnar --- drivers/firmware/efi/Kconfig | 10 ++++++ drivers/firmware/efi/libstub/Makefile | 1 + drivers/firmware/efi/libstub/arm-stub.c | 3 ++ drivers/firmware/efi/libstub/tpm.c | 58 +++++++++++++++++++++++++++++++++ 4 files changed, 72 insertions(+) create mode 100644 drivers/firmware/efi/libstub/tpm.c (limited to 'drivers/firmware') diff --git a/drivers/firmware/efi/Kconfig b/drivers/firmware/efi/Kconfig index 394db40ed374..2b4c39fdfa91 100644 --- a/drivers/firmware/efi/Kconfig +++ b/drivers/firmware/efi/Kconfig @@ -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 diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile index cf81e6cf5ae8..dedf9bde44db 100644 --- a/drivers/firmware/efi/libstub/Makefile +++ b/drivers/firmware/efi/libstub/Makefile @@ -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 diff --git a/drivers/firmware/efi/libstub/arm-stub.c b/drivers/firmware/efi/libstub/arm-stub.c index 8181ac179d14..1cb2d1c070c3 100644 --- a/drivers/firmware/efi/libstub/arm-stub.c +++ b/drivers/firmware/efi/libstub/arm-stub.c @@ -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); /* diff --git a/drivers/firmware/efi/libstub/tpm.c b/drivers/firmware/efi/libstub/tpm.c new file mode 100644 index 000000000000..6224cdbc9669 --- /dev/null +++ b/drivers/firmware/efi/libstub/tpm.c @@ -0,0 +1,58 @@ +/* + * TPM handling. + * + * Copyright (C) 2016 CoreOS, Inc + * Copyright (C) 2017 Google, Inc. + * Matthew Garrett + * + * This file is part of the Linux kernel, and is made available under the + * terms of the GNU General Public License version 2. + */ +#include +#include + +#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); +} -- cgit v1.2.3