x86: kmsan: enable KMSAN builds for x86

Make KMSAN usable by adding the necessary Kconfig bits.

Also declare x86-specific functions checking address validity
in arch/x86/include/asm/kmsan.h.

Signed-off-by: Alexander Potapenko <glider@google.com>
---
v4:
 -- per Marco Elver's request, create arch/x86/include/asm/kmsan.h
    and move arch-specific inline functions there.

Link: https://linux-review.googlesource.com/id/I1d295ce8159ce15faa496d20089d953a919c125e

arch/x86/Kconfig

@@ -168,6 +168,7 @@ config X86
 	select HAVE_ARCH_KASAN			if X86_64
 	select HAVE_ARCH_KASAN_VMALLOC		if X86_64
 	select HAVE_ARCH_KFENCE
+	select HAVE_ARCH_KMSAN			if X86_64
 	select HAVE_ARCH_KGDB
 	select HAVE_ARCH_MMAP_RND_BITS		if MMU
 	select HAVE_ARCH_MMAP_RND_COMPAT_BITS	if MMU && COMPAT

arch/x86/include/asm/kmsan.h

@@ -0,0 +1,55 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * x86 KMSAN support.
+ *
+ * Copyright (C) 2022, Google LLC
+ * Author: Alexander Potapenko <glider@google.com>
+ */
+
+#ifndef _ASM_X86_KMSAN_H
+#define _ASM_X86_KMSAN_H
+
+#ifndef MODULE
+
+#include <asm/processor.h>
+#include <linux/mmzone.h>
+
+/*
+ * Taken from arch/x86/mm/physaddr.h to avoid using an instrumented version.
+ */
+static inline bool kmsan_phys_addr_valid(unsigned long addr)
+{
+	if (IS_ENABLED(CONFIG_PHYS_ADDR_T_64BIT))
+		return !(addr >> boot_cpu_data.x86_phys_bits);
+	else
+		return true;
+}
+
+/*
+ * Taken from arch/x86/mm/physaddr.c to avoid using an instrumented version.
+ */
+static inline bool kmsan_virt_addr_valid(void *addr)
+{
+	unsigned long x = (unsigned long)addr;
+	unsigned long y = x - __START_KERNEL_map;
+
+	/* use the carry flag to determine if x was < __START_KERNEL_map */
+	if (unlikely(x > y)) {
+		x = y + phys_base;
+
+		if (y >= KERNEL_IMAGE_SIZE)
+			return false;
+	} else {
+		x = y + (__START_KERNEL_map - PAGE_OFFSET);
+
+		/* carry flag will be set if starting x was >= PAGE_OFFSET */
+		if ((x > y) || !kmsan_phys_addr_valid(x))
+			return false;
+	}
+
+	return pfn_valid(x >> PAGE_SHIFT);
+}
+
+#endif /* !MODULE */
+
+#endif /* _ASM_X86_KMSAN_H */