[PATCH RFC v9 01/25] 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 enter/leave a
kpkeys context and macros to define guard objects.

kpkeys introduces a new concept on top of pkeys: the kpkeys context.
Each context is associated with a set of permissions for the pkeys
managed by the kpkeys framework. kpkeys_enter_context(ctx) sets
those permissions according to ctx, and returns the original kpkeys
state (typically the arch-specific pkeys register) that is later
restored by calling kpkeys_leave_context(). 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).

The underlying representation of each kpkeys context is entirely
architecture-specific. Support for kpkeys must be explicitly
indicated by selecting ARCH_HAS_KPKEYS and defining the following
functions in <asm/kpkeys.h>:

* arch_kpkeys_enter_context()
* arch_kpkeys_leave_context()
* arch_supports_kpkeys()

Additionally, <asm/kpkeys_types.h> must define struct arch_kpkeys_state,
which typically provides storage for the pkeys register.

Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
---
 include/linux/kpkeys.h       | 104 +++++++++++++++++++++++++++++++++++++++++++
 include/linux/kpkeys_types.h |  25 +++++++++++
 mm/Kconfig                   |   2 +
 3 files changed, 131 insertions(+)

diff --git a/include/linux/kpkeys.h b/include/linux/kpkeys.h
new file mode 100644
index 000000000000..eac522f55214
--- /dev/null
+++ b/include/linux/kpkeys.h
@@ -0,0 +1,104 @@
+/* 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/kpkeys_types.h>
+
+/**
+ * KPKEYS_GUARD_NOOP() - define a guard type that does nothing
+ * @name: the name of the guard type
+ *
+ * Define a guard type that does nothing, useful to match a real guard type
+ * that is defined under an #ifdef.
+ */
+#define KPKEYS_GUARD_NOOP(name)						\
+	__DEFINE_CLASS_IS_CONDITIONAL(name, false);			\
+	DEFINE_CLASS(name, bool, (void)_T, false, void);		\
+	static inline void *class_##name##_lock_ptr(bool *_T)		\
+	{ return _T; }
+
+#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
+ *
+ * Define a guard type that switches to @ctx if @cond evaluates to true,
+ * and does nothing otherwise.
+ */
+#define KPKEYS_GUARD_COND(name, ctx, cond)				\
+	__DEFINE_CLASS_IS_CONDITIONAL(name, false);			\
+	DEFINE_CLASS(name, struct kpkeys_state,				\
+		     kpkeys_leave_context(&_T),				\
+		     (cond) ? kpkeys_enter_context(ctx) :		\
+			      (struct kpkeys_state) {}, void);		\
+	static inline							\
+	void *class_##name##_lock_ptr(struct kpkeys_state *_T)		\
+	{ return _T; }
+
+/**
+ * KPKEYS_GUARD() - define a guard type that switches to a given kpkeys context
+ *                  if kpkeys are supported
+ * @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_supported())
+
+/**
+ * kpkeys_enter_context() - enter a kpkeys context
+ * @ctx: the context to switch to
+ *
+ * Enters the specified kpkeys context. @ctx must be a compile-time constant.
+ *
+ * Return: state to be passed to kpkeys_leave_context().
+ */
+static __always_inline
+struct kpkeys_state kpkeys_enter_context(enum kpkeys_ctx ctx)
+{
+	BUILD_BUG_ON_MSG(!__builtin_constant_p(ctx),
+			 "kpkeys_enter_context() only takes constant values");
+	BUILD_BUG_ON_MSG(ctx < 0 || ctx >= KPKEYS_CTX_COUNT,
+			 "Invalid value passed to kpkeys_enter_context()");
+
+	return arch_kpkeys_enter_context(ctx);
+}
+
+/**
+ * kpkeys_leave_context() - leave a kpkeys context
+ * @state: state returned by kpkeys_enter_context()
+ *
+ * Restores the state saved when entering a kpkeys context. If no context was
+ * entered, this function does nothing.
+ */
+static __always_inline
+void kpkeys_leave_context(const struct kpkeys_state *state)
+{
+	if (state->entered_context)
+		arch_kpkeys_leave_context(state);
+}
+
+static inline bool kpkeys_supported(void)
+{
+	return arch_supports_kpkeys();
+}
+
+#else /* CONFIG_ARCH_HAS_KPKEYS */
+
+static inline bool kpkeys_supported(void)
+{
+	return false;
+}
+
+#endif /* CONFIG_ARCH_HAS_KPKEYS */
+
+#endif /* _LINUX_KPKEYS_H */
diff --git a/include/linux/kpkeys_types.h b/include/linux/kpkeys_types.h
new file mode 100644
index 000000000000..9de59092e9c3
--- /dev/null
+++ b/include/linux/kpkeys_types.h
@@ -0,0 +1,25 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef _LINUX_KPKEYS_TYPES_H
+#define _LINUX_KPKEYS_TYPES_H
+
+#include <linux/types.h>
+
+#ifdef CONFIG_ARCH_HAS_KPKEYS
+#include <asm/kpkeys_types.h>
+#endif
+
+#if defined(CONFIG_ARCH_HAS_KPKEYS) && !defined(__ASSEMBLY__)
+
+enum kpkeys_ctx {
+	KPKEYS_CTX_DEFAULT = 0,
+	KPKEYS_CTX_COUNT,
+};
+
+struct kpkeys_state {
+	bool entered_context;
+	struct arch_kpkeys_state arch_state;
+};
+
+#endif /* CONFIG_ARCH_HAS_KPKEYS && !__ASSEMBLY__ */
+
+#endif /* _LINUX_KPKEYS_TYPES_H */
diff --git a/mm/Kconfig b/mm/Kconfig
index 9e0ca4824905..55e6763d5792 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -1281,6 +1281,8 @@ 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