[PATCH 12/12] Add option to automatically set trusted_kernel when in Secure Boot mode

[Matthew Garrett]

UEFI Secure Boot provides a mechanism for ensuring that the firmware will
only load signed bootloaders and kernels. Certain use cases may also
require that the kernel prevent userspace from inserting untrusted kernel
code at runtime. Add a configuration option that enforces this automatically
when enabled.

Signed-off-by: Matthew Garrett <matthew.garrett@xxxxxxxxxx>
---
 Documentation/x86/zero-page.txt       |  2 ++
 arch/x86/Kconfig                      | 13 +++++++++++++
 arch/x86/boot/compressed/eboot.c      | 33 +++++++++++++++++++++++++++++++++
 arch/x86/include/uapi/asm/bootparam.h |  3 ++-
 arch/x86/kernel/setup.c               |  6 ++++++
 5 files changed, 56 insertions(+), 1 deletion(-)

diff --git a/Documentation/x86/zero-page.txt b/Documentation/x86/zero-page.txt
index 82fbdbc..a811210 100644
--- a/Documentation/x86/zero-page.txt
+++ b/Documentation/x86/zero-page.txt
@@ -30,6 +30,8 @@ Offset	Proto	Name		Meaning
 1E9/001	ALL	eddbuf_entries	Number of entries in eddbuf (below)
 1EA/001	ALL	edd_mbr_sig_buf_entries	Number of entries in edd_mbr_sig_buffer
 				(below)
+1EB/001	ALL     kbd_status      Numlock is enabled
+1EC/001	ALL     secure_boot	Secure boot is enabled in the firmware
 1EF/001	ALL	sentinel	Used to detect broken bootloaders
 290/040	ALL	edd_mbr_sig_buffer EDD MBR signatures
 2D0/A00	ALL	e820_map	E820 memory map table
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index b7d31ca..0f9ee6f 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1695,6 +1695,19 @@ config EFI_MIXED
 
 	   If unsure, say N.
 
+config EFI_SECURE_BOOT_TRUSTED_KERNEL
+        def_bool n
+	depends on SECURITY_TRUSTED_KERNEL
+	depends on EFI
+	prompt "Automatically set trusted_kernel with UEFI Secure Boot"
+	---help---
+	  UEFI Secure Boot provides a mechanism for ensuring that the
+	  firmware will only load signed bootloaders and kernels. Certain
+	  use cases may also require that the kernel restrict any userspace
+	  mechanism that could insert untrusted code into the kernel.
+	  Say Y here to automatically enable trusted_kernel enforcement
+	  when a system boots with UEFI Secure Boot enabled.
+
 config SECCOMP
 	def_bool y
 	prompt "Enable seccomp to safely compute untrusted bytecode"
diff --git a/arch/x86/boot/compressed/eboot.c b/arch/x86/boot/compressed/eboot.c
index ef17683..4def2aa 100644
--- a/arch/x86/boot/compressed/eboot.c
+++ b/arch/x86/boot/compressed/eboot.c
@@ -12,6 +12,7 @@
 #include <asm/efi.h>
 #include <asm/setup.h>
 #include <asm/desc.h>
+#include <asm/bootparam_utils.h>
 
 #include "../string.h"
 #include "eboot.h"
@@ -1030,6 +1031,36 @@ void setup_graphics(struct boot_params *boot_params)
 	}
 }
 
+static int get_secure_boot(void)
+{
+	u8 sb, setup;
+	unsigned long datasize = sizeof(sb);
+	efi_guid_t var_guid = EFI_GLOBAL_VARIABLE_GUID;
+	efi_status_t status;
+
+	status = efi_call_phys5(sys_table->runtime->get_variable,
+				L"SecureBoot", &var_guid, NULL, &datasize, &sb);
+
+	if (status != EFI_SUCCESS)
+		return 0;
+
+	if (sb == 0)
+		return 0;
+
+
+	status = efi_call_phys5(sys_table->runtime->get_variable,
+				L"SetupMode", &var_guid, NULL, &datasize,
+				&setup);
+
+	if (status != EFI_SUCCESS)
+		return 0;
+
+	if (setup == 1)
+		return 0;
+
+	return 1;
+}
+
 /*
  * Because the x86 boot code expects to be passed a boot_params we
  * need to create one ourselves (usually the bootloader would create
@@ -1406,6 +1437,8 @@ struct boot_params *efi_main(struct efi_config *c,
 	else
 		setup_boot_services32(efi_early);
 
+	boot_params->secure_boot = get_secure_boot();
+
 	setup_graphics(boot_params);
 
 	setup_efi_pci(boot_params);
diff --git a/arch/x86/include/uapi/asm/bootparam.h b/arch/x86/include/uapi/asm/bootparam.h
index 44e6dd7..3ddf415 100644
--- a/arch/x86/include/uapi/asm/bootparam.h
+++ b/arch/x86/include/uapi/asm/bootparam.h
@@ -134,7 +134,8 @@ struct boot_params {
 	__u8  eddbuf_entries;				/* 0x1e9 */
 	__u8  edd_mbr_sig_buf_entries;			/* 0x1ea */
 	__u8  kbd_status;				/* 0x1eb */
-	__u8  _pad5[3];					/* 0x1ec */
+	__u8  secure_boot;				/* 0x1ec */
+	__u8  _pad5[2];					/* 0x1ed */
 	/*
 	 * The sentinel is set to a nonzero value (0xff) in header.S.
 	 *
diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
index 98dc931..980e308 100644
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -50,6 +50,7 @@
 #include <linux/init_ohci1394_dma.h>
 #include <linux/kvm_para.h>
 #include <linux/dma-contiguous.h>
+#include <linux/security.h>
 
 #include <linux/errno.h>
 #include <linux/kernel.h>
@@ -1165,6 +1166,11 @@ void __init setup_arch(char **cmdline_p)
 
 	io_delay_init();
 
+#ifdef CONFIG_EFI_SECURE_BOOT_TRUSTED_KERNEL
+	if (boot_params.secure_boot)
+		set_trusted_kernel(true);
+#endif
+
 	/*
 	 * Parse the ACPI tables for possible boot-time SMP configuration.
 	 */
-- 
2.1.0

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