[RFC PATCH v5 01/18] mm: Introduce kpkeys

Kevin Brodsky
2025-08-15T08:55:34Z

kpkeys is a simple framework to enable the use of protection keys
(pkeys) to harden the kernel itself. This patch introduces the basic
API in <linux/kpkeys.h>: a couple of functions to set and restore
the pkey register and macros to define guard objects.

kpkeys introduces a new concept on top of pkeys: the kpkeys level.
Each level is associated to a set of permissions for the pkeys
managed by the kpkeys framework. kpkeys_set_level(lvl) sets those
permissions according to lvl, and returns the original pkey
register, to be later restored by kpkeys_restore_pkey_reg(). To
start with, only KPKEYS_LVL_DEFAULT is available, which is meant
to grant RW access to KPKEYS_PKEY_DEFAULT (i.e. all memory since
this is the only available pkey for now).

Because each architecture implementing pkeys uses a different
representation for the pkey register, and may reserve certain pkeys
for specific uses, support for kpkeys must be explicitly indicated
by selecting ARCH_HAS_KPKEYS and defining the following functions in
<asm/kpkeys.h>, in addition to the macros provided in
<asm-generic/kpkeys.h>:

- arch_kpkeys_set_level()
- arch_kpkeys_restore_pkey_reg()
- arch_kpkeys_enabled()

Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
---
include/asm-generic/kpkeys.h | 17 ++++++
include/linux/kpkeys.h | 113 +++++++++++++++++++++++++++++++++++
mm/Kconfig | 2 +
3 files changed, 132 insertions(+)
create mode 100644 include/asm-generic/kpkeys.h
create mode 100644 include/linux/kpkeys.h

diff --git a/include/asm-generic/kpkeys.h b/include/asm-generic/kpkeys.h
new file mode 100644
index 000000000000..ab819f157d6a
--- /dev/null
+++ b/include/asm-generic/kpkeys.h
@@ -0,0 +1,17 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef __ASM_GENERIC_KPKEYS_H
+#define __ASM_GENERIC_KPKEYS_H
+
+#ifndef KPKEYS_PKEY_DEFAULT
+#define KPKEYS_PKEY_DEFAULT 0
+#endif
+
+/*
+ * Represents a pkey register value that cannot be used, typically disabling
+ * access to all keys.
+ */
+#ifndef KPKEYS_PKEY_REG_INVAL
+#define KPKEYS_PKEY_REG_INVAL 0
+#endif
+
+#endif /* __ASM_GENERIC_KPKEYS_H */
diff --git a/include/linux/kpkeys.h b/include/linux/kpkeys.h
new file mode 100644
index 000000000000..faa6e2615798
--- /dev/null
+++ b/include/linux/kpkeys.h
@@ -0,0 +1,113 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef _LINUX_KPKEYS_H
+#define _LINUX_KPKEYS_H
+
+#include <linux/bug.h>
+#include <linux/cleanup.h>
+
+#define KPKEYS_LVL_DEFAULT 0
+
+#define KPKEYS_LVL_MIN KPKEYS_LVL_DEFAULT
+#define KPKEYS_LVL_MAX KPKEYS_LVL_DEFAULT
+
+#define __KPKEYS_GUARD(name, set_level, restore_pkey_reg, set_arg, ...) \
+ __DEFINE_CLASS_IS_CONDITIONAL(name, false); \
+ DEFINE_CLASS(name, u64, \
+ restore_pkey_reg, set_level, set_arg); \
+ static inline void *class_##name##_lock_ptr(u64 *_T) \
+ { return _T; }
+
+/**
+ * KPKEYS_GUARD_NOOP() - define a guard type that does nothing
+ * @name: the name of the guard type
+ * @cond_arg: an argument specification (optional)
+ *
+ * Define a guard type that does nothing, useful to match a real guard type
+ * that is defined under an #ifdef. @cond_arg may optionally be passed to match
+ * a guard defined using KPKEYS_GUARD_COND().
+ */
+#define KPKEYS_GUARD_NOOP(name, ...) \
+ __KPKEYS_GUARD(name, 0, (void)_T, ##__VA_ARGS__, void)
+
+#ifdef CONFIG_ARCH_HAS_KPKEYS
+
+#include <asm/kpkeys.h>
+
+/**
+ * KPKEYS_GUARD_COND() - define a guard type that conditionally switches to
+ * a given kpkeys level
+ * @name: the name of the guard type
+ * @level: the kpkeys level to switch to
+ * @cond: an expression that is evaluated as condition
+ * @cond_arg: an argument specification for the condition (optional)
+ *
+ * Define a guard type that switches to @level if @cond evaluates to true, and
+ * does nothing otherwise. @cond_arg may be specified to give access to a
+ * caller-defined argument to @cond.
+ */
+#define KPKEYS_GUARD_COND(name, level, cond, ...) \
+ __KPKEYS_GUARD(name, \
+ cond ? kpkeys_set_level(level) \
+ : KPKEYS_PKEY_REG_INVAL, \
+ kpkeys_restore_pkey_reg(_T), \
+ ##__VA_ARGS__, void)
+
+/**
+ * KPKEYS_GUARD() - define a guard type that switches to a given kpkeys level
+ * if kpkeys are enabled
+ * @name: the name of the guard type
+ * @level: the kpkeys level to switch to
+ *
+ * Define a guard type that switches to @level if the system supports kpkeys.
+ */
+#define KPKEYS_GUARD(name, level) \
+ KPKEYS_GUARD_COND(name, level, arch_kpkeys_enabled())
+
+/**
+ * kpkeys_set_level() - switch kpkeys level
+ * @level: the level to switch to
+ *
+ * Switches the kpkeys level to the specified value. @level must be a
+ * compile-time constant. The arch-specific pkey register will be updated
+ * accordingly, and the original value returned.
+ *
+ * Return: the original pkey register value if the register was written to, or
+ * KPKEYS_PKEY_REG_INVAL otherwise (no write to the register was
+ * required).
+ */
+static __always_inline u64 kpkeys_set_level(int level)
+{
+ BUILD_BUG_ON_MSG(!__builtin_constant_p(level),
+ "kpkeys_set_level() only takes constant levels");
+ BUILD_BUG_ON_MSG(level < KPKEYS_LVL_MIN || level > KPKEYS_LVL_MAX,
+ "Invalid level passed to kpkeys_set_level()");
+
+ return arch_kpkeys_set_level(level);
+}
+
+/**
+ * kpkeys_restore_pkey_reg() - restores a pkey register value
+ * @pkey_reg: the pkey register value to restore
+ *
+ * This function is meant to be passed the value returned by kpkeys_set_level(),
+ * in order to restore the pkey register to its original value (thus restoring
+ * the original kpkeys level).
+ */
+static __always_inline void kpkeys_restore_pkey_reg(u64 pkey_reg)
+{
+ if (pkey_reg != KPKEYS_PKEY_REG_INVAL)
+ arch_kpkeys_restore_pkey_reg(pkey_reg);
+}
+
+#else /* CONFIG_ARCH_HAS_KPKEYS */
+
+#include <asm-generic/kpkeys.h>
+
+static inline bool arch_kpkeys_enabled(void)
+{
+ return false;
+}
+
+#endif /* CONFIG_ARCH_HAS_KPKEYS */
+
+#endif /* _LINUX_KPKEYS_H */
diff --git a/mm/Kconfig b/mm/Kconfig
index e443fe8cd6cf..90f2e5c381a6 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -1173,6 +1173,8 @@ config ARCH_USES_HIGH_VMA_FLAGS
bool
config ARCH_HAS_PKEYS
bool
+config ARCH_HAS_KPKEYS
+ bool

config ARCH_USES_PG_ARCH_2
bool
--
2.47.0