[PATCH 5.19 049/207] efi: libstub: check Shim mode using MokSBStateRT

Greg Kroah-Hartman
2022-09-26T11:41:30Z

From: Ard Biesheuvel <ardb@kernel.org>

commit 5f56a74cc0a6d9b9f8ba89cea29cd7c4774cb2b1 upstream.

We currently check the MokSBState variable to decide whether we should
treat UEFI secure boot as being disabled, even if the firmware thinks
otherwise. This is used by shim to indicate that it is not checking
signatures on boot images. In the kernel, we use this to relax lockdown
policies.

However, in cases where shim is not even being used, we don't want this
variable to interfere with lockdown, given that the variable may be
non-volatile and therefore persist across a reboot. This means setting
it once will persistently disable lockdown checks on a given system.

So switch to the mirrored version of this variable, called MokSBStateRT,
which is supposed to be volatile, and this is something we can check.

Cc: <stable@vger.kernel.org> # v4.19+
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Peter Jones <pjones@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/firmware/efi/libstub/secureboot.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)

--- a/drivers/firmware/efi/libstub/secureboot.c
+++ b/drivers/firmware/efi/libstub/secureboot.c
@@ -14,7 +14,7 @@

/* SHIM variables */
static const efi_guid_t shim_guid = EFI_SHIM_LOCK_GUID;
-static const efi_char16_t shim_MokSBState_name[] = L"MokSBState";
+static const efi_char16_t shim_MokSBState_name[] = L"MokSBStateRT";

static efi_status_t get_var(efi_char16_t *name, efi_guid_t *vendor, u32 *attr,
unsigned long *data_size, void *data)
@@ -43,8 +43,8 @@ enum efi_secureboot_mode efi_get_secureb

/*
* See if a user has put the shim into insecure mode. If so, and if the
- * variable doesn't have the runtime attribute set, we might as well
- * honor that.
+ * variable doesn't have the non-volatile attribute set, we might as
+ * well honor that.
*/
size = sizeof(moksbstate);
status = get_efi_var(shim_MokSBState_name, &shim_guid,
@@ -53,7 +53,7 @@ enum efi_secureboot_mode efi_get_secureb
/* If it fails, we don't care why. Default to secure */
if (status != EFI_SUCCESS)
goto secure_boot_enabled;
- if (!(attr & EFI_VARIABLE_RUNTIME_ACCESS) && moksbstate == 1)
+ if (!(attr & EFI_VARIABLE_NON_VOLATILE) && moksbstate == 1)
return efi_secureboot_mode_disabled;