[PATCH v6 13/30] mm: kpkeys: Introduce kpkeys_hardened_pgtables feature

[Kevin Brodsky <kevin.brodsky@arm.com>]

kpkeys_hardened_pgtables is a hardening feature based on kpkeys. It
aims to prevent the corruption of page tables by: 1. mapping all
page table pages, both kernel and user, with a privileged pkey
(KPKEYS_PKEY_PGTABLES), and 2. granting write access to that pkey
only when running at a higher kpkeys level (KPKEYS_LVL_PGTABLES).
This patch introduces basic infrastructure; the implementation of
both aspects will follow.

The feature is exposed as CONFIG_KPKEYS_HARDENED_PGTABLES; it
requires explicit architecture opt-in by selecting
ARCH_HAS_KPKEYS_HARDENED_PGTABLES, since much of the page table
handling is arch-specific.

Because this feature relies on kpkeys being available and enabled,
and modifies attributes of the linear map, it must be inactive on
boot. kpkeys_hardened_pgtables_init() enables it by toggling a
static key; this function must be called by supported architectures
in mem_init(), before any call to pagetable_alloc() is made.

Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
---
 include/asm-generic/kpkeys.h  |  4 ++++
 include/linux/kpkeys.h        | 30 +++++++++++++++++++++++++++++-
 mm/Kconfig                    |  3 +++
 mm/Makefile                   |  1 +
 mm/kpkeys_hardened_pgtables.c | 13 +++++++++++++
 security/Kconfig.hardening    | 12 ++++++++++++
 6 files changed, 62 insertions(+), 1 deletion(-)
 create mode 100644 mm/kpkeys_hardened_pgtables.c

diff --git a/include/asm-generic/kpkeys.h b/include/asm-generic/kpkeys.h
index ab819f157d6a..cec92334a9f3 100644
--- a/include/asm-generic/kpkeys.h
+++ b/include/asm-generic/kpkeys.h
@@ -2,6 +2,10 @@
 #ifndef __ASM_GENERIC_KPKEYS_H
 #define __ASM_GENERIC_KPKEYS_H
 
+#ifndef KPKEYS_PKEY_PGTABLES
+#define KPKEYS_PKEY_PGTABLES	1
+#endif
+
 #ifndef KPKEYS_PKEY_DEFAULT
 #define KPKEYS_PKEY_DEFAULT	0
 #endif
diff --git a/include/linux/kpkeys.h b/include/linux/kpkeys.h
index faa6e2615798..49af2ec76923 100644
--- a/include/linux/kpkeys.h
+++ b/include/linux/kpkeys.h
@@ -4,11 +4,13 @@
 
 #include <linux/bug.h>
 #include <linux/cleanup.h>
+#include <linux/jump_label.h>
 
 #define KPKEYS_LVL_DEFAULT	0
+#define KPKEYS_LVL_PGTABLES	1
 
 #define KPKEYS_LVL_MIN		KPKEYS_LVL_DEFAULT
-#define KPKEYS_LVL_MAX		KPKEYS_LVL_DEFAULT
+#define KPKEYS_LVL_MAX		KPKEYS_LVL_PGTABLES
 
 #define __KPKEYS_GUARD(name, set_level, restore_pkey_reg, set_arg, ...)	\
 	__DEFINE_CLASS_IS_CONDITIONAL(name, false);			\
@@ -110,4 +112,30 @@ static inline bool arch_kpkeys_enabled(void)
 
 #endif /* CONFIG_ARCH_HAS_KPKEYS */
 
+#ifdef CONFIG_KPKEYS_HARDENED_PGTABLES
+
+DECLARE_STATIC_KEY_FALSE(kpkeys_hardened_pgtables_key);
+
+static inline bool kpkeys_hardened_pgtables_enabled(void)
+{
+	return static_branch_unlikely(&kpkeys_hardened_pgtables_key);
+}
+
+/*
+ * Should be called from mem_init(): as soon as the buddy allocator becomes
+ * available and before any call to pagetable_alloc().
+ */
+void kpkeys_hardened_pgtables_init(void);
+
+#else /* CONFIG_KPKEYS_HARDENED_PGTABLES */
+
+static inline bool kpkeys_hardened_pgtables_enabled(void)
+{
+	return false;
+}
+
+static inline void kpkeys_hardened_pgtables_init(void) {}
+
+#endif /* CONFIG_KPKEYS_HARDENED_PGTABLES */
+
 #endif /* _LINUX_KPKEYS_H */
diff --git a/mm/Kconfig b/mm/Kconfig
index 2baedee59bb2..2f87ee69d16e 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -1245,6 +1245,9 @@ config ARCH_HAS_PKEYS
 	bool
 config ARCH_HAS_KPKEYS
 	bool
+# ARCH_HAS_KPKEYS must be selected when selecting this option
+config ARCH_HAS_KPKEYS_HARDENED_PGTABLES
+	bool
 
 config ARCH_USES_PG_ARCH_2
 	bool
diff --git a/mm/Makefile b/mm/Makefile
index 8ad2ab08244e..7603e6051afa 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -150,3 +150,4 @@ obj-$(CONFIG_SHRINKER_DEBUG) += shrinker_debug.o
 obj-$(CONFIG_EXECMEM) += execmem.o
 obj-$(CONFIG_TMPFS_QUOTA) += shmem_quota.o
 obj-$(CONFIG_LAZY_MMU_MODE_KUNIT_TEST) += tests/lazy_mmu_mode_kunit.o
+obj-$(CONFIG_KPKEYS_HARDENED_PGTABLES) += kpkeys_hardened_pgtables.o
diff --git a/mm/kpkeys_hardened_pgtables.c b/mm/kpkeys_hardened_pgtables.c
new file mode 100644
index 000000000000..9e4771263ad2
--- /dev/null
+++ b/mm/kpkeys_hardened_pgtables.c
@@ -0,0 +1,13 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#include <linux/kpkeys.h>
+#include <linux/mm.h>
+
+__ro_after_init DEFINE_STATIC_KEY_FALSE(kpkeys_hardened_pgtables_key);
+
+void __init kpkeys_hardened_pgtables_init(void)
+{
+	if (!arch_kpkeys_enabled())
+		return;
+
+	static_branch_enable(&kpkeys_hardened_pgtables_key);
+}
diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
index 86f8768c63d4..fdaf977d4626 100644
--- a/security/Kconfig.hardening
+++ b/security/Kconfig.hardening
@@ -275,6 +275,18 @@ config BUG_ON_DATA_CORRUPTION
 
 	  If unsure, say N.
 
+config KPKEYS_HARDENED_PGTABLES
+	bool "Harden page tables using kernel pkeys"
+	depends on ARCH_HAS_KPKEYS_HARDENED_PGTABLES
+	help
+	  This option makes all page tables mostly read-only by
+	  allocating them with a non-default protection key (pkey) and
+	  only enabling write access to that pkey in routines that are
+	  expected to write to page table entries.
+
+	  This option has no effect if the system does not support
+	  kernel pkeys.
+
 endmenu
 
 config CC_HAS_RANDSTRUCT
-- 
2.51.2