efi: Lock down the kernel if booted in secure boot mode

jira LE-2629
feature Additional SecureBoot patches for dynamic lockdown
commit 78c8af872660c31779951583b6f1ebf283d95985
commit-source https://salsa.debian.org/kernel-team/linux.git

Based on an earlier patch by David Howells, who wrote the following
description:

> 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 all kernel modules also be signed.  Add a configuration option
> that to lock down the kernel - which includes requiring validly signed
> modules - if the kernel is secure-booted.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
[Salvatore Bonaccorso: After fixing https://bugs.debian.org/956197 the
help text for LOCK_DOWN_IN_EFI_SECURE_BOOT was adjusted to mention that
lockdown is triggered in integrity mode (https://bugs.debian.org/1025417)]
Signed-off-by: Salvatore Bonaccorso <carnil@debian.org>
Signed-off-by: Jonathan Maple <jmaple@ciq.com>

Author: bwhacks

arch/x86/kernel/setup.c

@@ -904,6 +904,8 @@ void __init setup_arch(char **cmdline_p)
 	if (efi_enabled(EFI_BOOT))
 		efi_init();
 
+	efi_set_secure_boot(boot_params.secure_boot);
+
 	reserve_ibft_region();
 	x86_init.resources.dmi_setup();
 
@@ -1070,8 +1072,6 @@ void __init setup_arch(char **cmdline_p)
 	/* Allocate bigger log buffer */
 	setup_log_buf(1);
 
-	efi_set_secure_boot(boot_params.secure_boot);
-
 	reserve_initrd();
 
 	acpi_table_upgrade();

drivers/firmware/efi/secureboot.c

@@ -15,6 +15,7 @@
 #include <linux/efi.h>
 #include <linux/kernel.h>
 #include <linux/printk.h>
+#include <linux/security.h>
 
 /*
  * Decide what to do when UEFI secure boot mode is enabled.
@@ -28,6 +29,10 @@ void __init efi_set_secure_boot(enum efi_secureboot_mode mode)
 			break;
 		case efi_secureboot_mode_enabled:
 			set_bit(EFI_SECURE_BOOT, &efi.flags);
+#ifdef CONFIG_LOCK_DOWN_IN_EFI_SECURE_BOOT
+			lock_kernel_down("EFI Secure Boot",
+					 LOCKDOWN_INTEGRITY_MAX);
+#endif
 			pr_info("Secure boot enabled\n");
 			break;
 		default:

include/linux/security.h

@@ -522,6 +522,7 @@ int security_inode_notifysecctx(struct inode *inode, void *ctx, u32 ctxlen);
 int security_inode_setsecctx(struct dentry *dentry, void *ctx, u32 ctxlen);
 int security_inode_getsecctx(struct inode *inode, void **ctx, u32 *ctxlen);
 int security_locked_down(enum lockdown_reason what);
+int lock_kernel_down(const char *where, enum lockdown_reason level);
 int lsm_fill_user_ctx(struct lsm_ctx __user *uctx, u32 *uctx_len,
 		      void *val, size_t val_len, u64 id, u64 flags);
 int security_bdev_alloc(struct block_device *bdev);
@@ -1504,6 +1505,11 @@ static inline int security_locked_down(enum lockdown_reason what)
 {
 	return 0;
 }
+static inline int
+lock_kernel_down(const char *where, enum lockdown_reason level)
+{
+	return -EOPNOTSUPP;
+}
 static inline int lsm_fill_user_ctx(struct lsm_ctx __user *uctx,
 				    u32 *uctx_len, void *val, size_t val_len,
 				    u64 id, u64 flags)

security/lockdown/Kconfig

@@ -45,3 +45,18 @@ config LOCK_DOWN_KERNEL_FORCE_CONFIDENTIALITY
 	 disabled.
 
 endchoice
+
+config LOCK_DOWN_IN_EFI_SECURE_BOOT
+	bool "Lock down the kernel in EFI Secure Boot mode"
+	default n
+	depends on SECURITY_LOCKDOWN_LSM
+	depends on EFI
+	select SECURITY_LOCKDOWN_LSM_EARLY
+	help
+	  UEFI Secure Boot provides a mechanism for ensuring that the firmware
+	  will only load signed bootloaders and kernels.  Secure boot mode may
+	  be determined from EFI variables provided by the system firmware if
+	  not indicated by the boot parameters.
+
+	  Enabling this option results in kernel lockdown being
+	  triggered in integrity mode if EFI Secure Boot is set.

security/lockdown/lockdown.c

@@ -24,7 +24,7 @@ static const enum lockdown_reason lockdown_levels[] = {LOCKDOWN_NONE,
 /*
  * Put the kernel into lock-down mode.
  */
-static int lock_kernel_down(const char *where, enum lockdown_reason level)
+int lock_kernel_down(const char *where, enum lockdown_reason level)
 {
 	if (kernel_locked_down >= level)
 		return -EPERM;