[RFC PATCH 01/16] mm: Introduce kpkeys

Kevin Brodsky
2024-12-06T10:13:16Z

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 a macro 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 | 9 +++++
include/linux/kpkeys.h | 67 ++++++++++++++++++++++++++++++++++++
mm/Kconfig | 2 ++
3 files changed, 78 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..3404ce249757
--- /dev/null
+++ b/include/asm-generic/kpkeys.h
@@ -0,0 +1,9 @@
+/* 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
+
+#endif /* __ASM_GENERIC_KPKEYS_H */
diff --git a/include/linux/kpkeys.h b/include/linux/kpkeys.h
new file mode 100644
index 000000000000..bcc063425926
--- /dev/null
+++ b/include/linux/kpkeys.h
@@ -0,0 +1,67 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef _LINUX_KPKEYS_H
+#define _LINUX_KPKEYS_H
+
+#include <linux/bug.h>
+#include <linux/cleanup.h>
+#include <linux/set_memory.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) \
+ __DEFINE_CLASS_IS_CONDITIONAL(_name, false); \
+ DEFINE_CLASS(_name, u64, \
+ restore_pkey_reg, set_level, void); \
+ static inline void *class_##_name##_lock_ptr(u64 *_T) \
+ { return _T; }
+
+#ifdef CONFIG_ARCH_HAS_KPKEYS
+
+#include <asm/kpkeys.h>
+
+/**
+ * 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.
+ */
+static 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 inline void kpkeys_restore_pkey_reg(u64 pkey_reg)
+{
+ arch_kpkeys_restore_pkey_reg(pkey_reg);
+}
+
+#else /* CONFIG_ARCH_HAS_KPKEYS */
+
+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 84000b016808..f51dffca9d4e 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -1104,6 +1104,8 @@ 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