Implement the kpkeys interface if CONFIG_ARM64_POE is enabled.
The permissions for pkey 0 are set to RWX as this pkey is also used
for code mappings.

arch_kpkeys_enter_context() returns whether it modified POR_EL1, and
saves its old value when it does. arch_kpkeys_leave_context() is
only called if POR_EL1 actually needs to be restored.

To allow <asm/kpkeys.h> to be included from assembly, also add
appropriate #ifdefs to <asm/por.h>.

Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
---
 arch/arm64/include/asm/kpkeys.h       | 73 +++++++++++++++++++++++++++++++++++
 arch/arm64/include/asm/kpkeys_types.h | 15 +++++++
 arch/arm64/include/asm/por.h          |  4 ++
 3 files changed, 92 insertions(+)

diff --git a/arch/arm64/include/asm/kpkeys.h b/arch/arm64/include/asm/kpkeys.h
new file mode 100644
index 000000000000..4f9bdb369b47
--- /dev/null
+++ b/arch/arm64/include/asm/kpkeys.h
@@ -0,0 +1,73 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef __ASM_KPKEYS_H
+#define __ASM_KPKEYS_H
+
+#include <linux/kpkeys_types.h>
+
+#include <asm/barrier.h>
+#include <asm/cpufeature.h>
+#include <asm/por.h>
+
+/*
+ * Equivalent to por_set_kpkeys_context(0, KPKEYS_CTX_DEFAULT), but can also be
+ * used in assembly.
+ */
+#define POR_EL1_INIT	POR_ELx_PERM_PREP(0, POE_RWX)
+
+#ifndef __ASSEMBLY__
+
+static inline bool arch_supports_kpkeys(void)
+{
+	return system_supports_poe();
+}
+
+#ifdef CONFIG_ARM64_POE
+
+static inline u64 por_set_kpkeys_context(u64 por, enum kpkeys_ctx ctx)
+{
+	por = por_elx_set_pkey_perms(por, 0, POE_RWX);
+
+	return por;
+}
+
+static __always_inline void __kpkeys_set_pkey_reg_nosync(u64 pkey_reg)
+{
+	/*
+	 * Writing to POR_EL1 potentially affects all memory accesses, so
+	 * use barrier() to ensure load/store instructions cannot be reordered
+	 * past the write.
+	 */
+	barrier();
+	write_sysreg_s(pkey_reg, SYS_POR_EL1);
+	barrier();
+}
+
+static __always_inline
+bool arch_kpkeys_enter_context(enum kpkeys_ctx ctx,
+			       struct arch_kpkeys_state *state)
+{
+	const u64 prev_por = read_sysreg_s(SYS_POR_EL1);
+	const u64 new_por = por_set_kpkeys_context(prev_por, ctx);
+
+	if (prev_por == new_por)
+		return false;
+
+	state->por = prev_por;
+	__kpkeys_set_pkey_reg_nosync(new_por);
+	isb();
+
+	return true;
+}
+
+static __always_inline
+void arch_kpkeys_leave_context(const struct arch_kpkeys_state *state)
+{
+	__kpkeys_set_pkey_reg_nosync(state->por);
+	isb();
+}
+
+#endif /* CONFIG_ARM64_POE */
+
+#endif	/* __ASSEMBLY__ */
+
+#endif	/* __ASM_KPKEYS_H */
diff --git a/arch/arm64/include/asm/kpkeys_types.h b/arch/arm64/include/asm/kpkeys_types.h
new file mode 100644
index 000000000000..e9c59a26ee7a
--- /dev/null
+++ b/arch/arm64/include/asm/kpkeys_types.h
@@ -0,0 +1,15 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef __ASM_KPKEYS_TYPES_H
+#define __ASM_KPKEYS_TYPES_H
+
+#include <linux/types.h>
+
+#ifndef __ASSEMBLY__
+
+struct arch_kpkeys_state {
+	u64 por;
+};
+
+#endif	/* __ASSEMBLY__ */
+
+#endif	/* __ASM_KPKEYS_TYPES_H */
diff --git a/arch/arm64/include/asm/por.h b/arch/arm64/include/asm/por.h
index 66d8c9a73224..b89be3cf0a0c 100644
--- a/arch/arm64/include/asm/por.h
+++ b/arch/arm64/include/asm/por.h
@@ -10,6 +10,8 @@
 
 #define POR_EL0_INIT	POR_ELx_PERM_PREP(0, POE_RWX)
 
+#ifndef __ASSEMBLY__
+
 static inline bool por_elx_allows_read(u64 por, u8 pkey)
 {
 	u8 perm = POR_ELx_PERM_GET(pkey, por);
@@ -38,4 +40,6 @@ static inline u64 por_elx_set_pkey_perms(u64 por, u8 pkey, u8 perms)
 	return (por & ~mask) | field_prep(mask, perms);
 }
 
+#endif	/* __ASSEMBLY__ */
+
 #endif /* _ASM_ARM64_POR_H */

-- 
2.56.0