[PATCH 2/2] efi: introduce EFI_KERNEL_LOCK_DOWN_IN_SECURE_BOOT

[Hamza Mahfooz]

Add a kernel configuration option to lock down the kernel, to restrict
userspace's ability to modify the running kernel when Secure Boot is
enabled.

Signed-off-by: Hamza Mahfooz <hamzamahfooz at linux.microsoft.com>
---
 drivers/firmware/efi/Kconfig | 10 ++++++++++
 drivers/firmware/efi/efi.c   |  9 +++++++++
 2 files changed, 19 insertions(+)

diff --git a/drivers/firmware/efi/Kconfig b/drivers/firmware/efi/Kconfig
index 5fe61b9ab5f9..4e827354e919 100644
--- a/drivers/firmware/efi/Kconfig
+++ b/drivers/firmware/efi/Kconfig
@@ -248,6 +248,16 @@ config EFI_DISABLE_RUNTIME
 
 	  This default can be overridden by using the efi=runtime option.
 
+config EFI_KERNEL_LOCK_DOWN_IN_SECURE_BOOT
+	bool "Lock down the kernel in EFI Secure Boot mode"
+	default n
+	depends on EFI
+	depends on SECURITY_LOCKDOWN_LSM
+	select SECURITY_LOCKDOWN_LSM_EARLY
+	help
+	  Enabling this option results in kernel lockdown being
+	  set in integrity mode if EFI Secure Boot is enabled.
+
 config EFI_COCO_SECRET
 	bool "EFI Confidential Computing Secret Area Support"
 	help
diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
index 7309394b8fc9..b7a5fc79b065 100644
--- a/drivers/firmware/efi/efi.c
+++ b/drivers/firmware/efi/efi.c
@@ -427,6 +427,15 @@ static int __init efisubsys_init(void)
 		}
 	}
 
+#ifdef CONFIG_EFI_KERNEL_LOCK_DOWN_IN_SECURE_BOOT
+	if (efi_rt_services_supported(EFI_RT_SUPPORTED_GET_VARIABLE)) {
+		if (efi_get_secureboot_mode(efi.get_variable) ==
+		    efi_secureboot_mode_enabled)
+			security_lock_kernel_down("EFI Secure Boot",
+						  LOCKDOWN_INTEGRITY_MAX);
+	}
+#endif
+
 	if (efi_rt_services_supported(EFI_RT_SUPPORTED_TIME_SERVICES))
 		platform_device_register_simple("rtc-efi", 0, NULL, 0);
 
-- 
2.49.0