[PATCH RFC v8 01/24] mm: Introduce kpkeys

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

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 context.
Each context is associated to a set of permissions for the pkeys
managed by the kpkeys framework. kpkeys_set_context(ctx) sets those
permissions according to ctx, and returns the original pkey
register, to be later restored by kpkeys_restore_pkey_reg(). To
start with, only KPKEYS_CTX_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_context()
- arch_kpkeys_restore_pkey_reg()
- arch_supports_kpkeys()

Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
---
 include/asm-generic/kpkeys.h |  17 ++++++
 include/linux/kpkeys.h       | 122 +++++++++++++++++++++++++++++++++++++++++++
 mm/Kconfig                   |   2 +
 3 files changed, 141 insertions(+)

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..0ce0db80b4ee
--- /dev/null
+++ b/include/linux/kpkeys.h
@@ -0,0 +1,122 @@
+/* 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_CTX_DEFAULT	0
+
+#define KPKEYS_CTX_MIN		KPKEYS_CTX_DEFAULT
+#define KPKEYS_CTX_MAX		KPKEYS_CTX_DEFAULT
+
+/*
+ * ... is used to discard extra arguments - this allows users of this macro
+ * to have set_arg default to void.
+ */
+#define __KPKEYS_GUARD(name, set_context, restore_pkey_reg, set_arg, ...) \
+	__DEFINE_CLASS_IS_CONDITIONAL(name, false);			\
+	DEFINE_CLASS(name, u64,						\
+		     restore_pkey_reg, set_context, 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 context
+ * @name: the name of the guard type
+ * @ctx: the kpkeys context 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 @ctx 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, ctx, cond, ...)				\
+	__KPKEYS_GUARD(name,						\
+		       (cond) ? kpkeys_set_context(ctx)			\
+			      : KPKEYS_PKEY_REG_INVAL,			\
+		       kpkeys_restore_pkey_reg(_T),			\
+		       ##__VA_ARGS__, void)
+
+/**
+ * KPKEYS_GUARD() - define a guard type that switches to a given kpkeys context
+ *                  if kpkeys are enabled
+ * @name: the name of the guard type
+ * @ctx: the kpkeys context to switch to
+ *
+ * Define a guard type that switches to @ctx if the system supports kpkeys.
+ */
+#define KPKEYS_GUARD(name, ctx)						\
+	KPKEYS_GUARD_COND(name, ctx, kpkeys_enabled())
+
+/**
+ * kpkeys_set_context() - switch kpkeys context
+ * @ctx: the context to switch to
+ *
+ * Switches to specified kpkeys context. @ctx 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_context(int ctx)
+{
+	BUILD_BUG_ON_MSG(!__builtin_constant_p(ctx),
+			 "kpkeys_set_context() only takes constant values");
+	BUILD_BUG_ON_MSG(ctx < KPKEYS_CTX_MIN || ctx > KPKEYS_CTX_MAX,
+			 "Invalid value passed to kpkeys_set_context()");
+
+	return arch_kpkeys_set_context(ctx);
+}
+
+/**
+ * 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_context(), in order to restore the pkey register to its original
+ * value (thus restoring the original kpkeys context).
+ */
+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);
+}
+
+static inline bool kpkeys_enabled(void)
+{
+	return arch_supports_kpkeys();
+}
+
+#else /* CONFIG_ARCH_HAS_KPKEYS */
+
+#include <asm-generic/kpkeys.h>
+
+static inline bool kpkeys_enabled(void)
+{
+	return false;
+}
+
+#endif /* CONFIG_ARCH_HAS_KPKEYS */
+
+#endif /* _LINUX_KPKEYS_H */
diff --git a/mm/Kconfig b/mm/Kconfig
index e8bf1e9e6ad9..819fb0d7b7bd 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -1242,6 +1242,8 @@ config ARCH_USES_HIGH_VMA_FLAGS
 	bool
 config ARCH_HAS_PKEYS
 	bool
+config ARCH_HAS_KPKEYS
+	bool
 
 config ARCH_USES_PG_ARCH_2
 	bool

-- 
2.51.2