x86_64: pmap: enable NX protection during bootstrap

arch/x86_64/include/arch/paging.h

@@ -50,5 +50,6 @@ typedef enum page_size {
 /* returns 1 if gigabyte pages are supported by the CPU, 0 otherwise.
    defined in pmap_ctrl.S */
 extern int gigabyte_pages(void);
+extern int enable_nx(void);
 
 #endif

arch/x86_64/pmap.c

@@ -226,6 +226,8 @@ void pmap_bootstrap(void)
 {
 	can_use_gbpages = gigabyte_pages();
 	printk("pmap: gigabyte pages %sabled", can_use_gbpages == 1 ? "en" : "dis");
+	enable_nx();
+	printk("pmap: NX protection enabled");
 
 	page_size_t hugepage = PS_2M;
 	if (can_use_gbpages) {

arch/x86_64/pmap_ctrl.S

@@ -29,3 +29,14 @@ gigabyte_pages:
 3:	pop %rbx
 	pop %rbp
 	ret
+
+	.global enable_nx
+	.type enable_nx, @function
+
+enable_nx:
+	mov $0xC0000080, %ecx
+	rdmsr
+	or $0x800, %eax
+	wrmsr
+
+	ret