efi: libstub: check Shim mode using MokSBStateRT

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: <[email protected]> # v4.19+
Signed-off-by: Ard Biesheuvel <[email protected]>
Reviewed-by: Ilias Apalodimas <[email protected]>
Reviewed-by: Peter Jones <[email protected]>

Author: ardbiesheuvel

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_secureboot(void)
 
 	/*
 	 * 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_secureboot(void)
 	/* 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;
 
 secure_boot_enabled: