Index: kernel/arch/amd64/include/mm/page.h
===================================================================
--- kernel/arch/amd64/include/mm/page.h	(revision b17518ef2cd636e9ee2a4593e9e3f96130bd854b)
+++ kernel/arch/amd64/include/mm/page.h	(revision 30c08260b42b92e158e687ba64c4b4cc6561218e)
@@ -119,4 +119,14 @@
 	set_pt_flags((pte_t *) (ptl3), (size_t) (i), (x))
 
+/* Set PTE present bit accessors for each level. */
+#define SET_PTL1_PRESENT_ARCH(ptl0, i) \
+	set_pt_present((pte_t *) (ptl0), (size_t) (i))
+#define SET_PTL2_PRESENT_ARCH(ptl1, i) \
+	set_pt_present((pte_t *) (ptl1), (size_t) (i))
+#define SET_PTL3_PRESENT_ARCH(ptl2, i) \
+	set_pt_present((pte_t *) (ptl2), (size_t) (i))
+#define SET_FRAME_PRESENT_ARCH(ptl3, i) \
+	set_pt_present((pte_t *) (ptl3), (size_t) (i))
+
 /* Macros for querying the last-level PTE entries. */
 #define PTE_VALID_ARCH(p) \
@@ -215,4 +225,11 @@
 }
 
+NO_TRACE static inline void set_pt_present(pte_t *pt, size_t i)
+{
+	pte_t *p = &pt[i];
+
+	p->present = 1;
+}
+
 extern void page_arch_init(void);
 extern void page_fault(unsigned int, istate_t *);
Index: kernel/arch/amd64/src/asm.S
===================================================================
--- kernel/arch/amd64/src/asm.S	(revision b17518ef2cd636e9ee2a4593e9e3f96130bd854b)
+++ kernel/arch/amd64/src/asm.S	(revision 30c08260b42b92e158e687ba64c4b4cc6561218e)
@@ -362,5 +362,13 @@
 	 */
 	call syscall_handler
-	
+
+	/*
+	 * Test if the saved return address is canonical and not-kernel.
+	 * We do this by looking at the 16 most significant bits
+	 * of the saved return address (two bytes at offset 6).
+	 */
+	testw $0xffff, ISTATE_OFFSET_RIP+6(%rsp)
+	jnz bad_rip 
+
 	cli
 	
@@ -388,4 +396,14 @@
 	sysretq
 
+bad_rip:
+	movq %rsp, %rdi
+	movabs $bad_rip_msg, %rsi
+	xorb %al, %al
+	callq fault_from_uspace
+	/* not reached */
+	
+bad_rip_msg:
+	.asciz "Invalid instruction pointer."
+
 /** Print Unicode character to EGA display.
  *
Index: kernel/arch/amd64/src/boot/multiboot.S
===================================================================
--- kernel/arch/amd64/src/boot/multiboot.S	(revision b17518ef2cd636e9ee2a4593e9e3f96130bd854b)
+++ kernel/arch/amd64/src/boot/multiboot.S	(revision 30c08260b42b92e158e687ba64c4b4cc6561218e)
@@ -76,4 +76,5 @@
 
 multiboot_image_start:
+	cli
 	cld
 	
@@ -81,6 +82,10 @@
 	movl $START_STACK, %esp
 	
-	/* Initialize Global Descriptor Table register */
+	/*
+	 * Initialize Global Descriptor Table and
+	 * Interrupt Descriptor Table registers
+	 */
 	lgdtl bootstrap_gdtr
+	lidtl bootstrap_idtr
 	
 	/* Kernel data + stack */
@@ -645,4 +650,9 @@
 .section K_DATA_START, "aw", @progbits
 
+.global bootstrap_idtr
+bootstrap_idtr:
+	.word 0
+	.long 0
+
 .global bootstrap_gdtr
 bootstrap_gdtr:
Index: kernel/arch/amd64/src/boot/multiboot2.S
===================================================================
--- kernel/arch/amd64/src/boot/multiboot2.S	(revision b17518ef2cd636e9ee2a4593e9e3f96130bd854b)
+++ kernel/arch/amd64/src/boot/multiboot2.S	(revision 30c08260b42b92e158e687ba64c4b4cc6561218e)
@@ -116,4 +116,5 @@
 
 multiboot2_image_start:
+	cli
 	cld
 	
@@ -121,6 +122,10 @@
 	movl $START_STACK, %esp
 	
-	/* Initialize Global Descriptor Table register */
+	/*
+	 * Initialize Global Descriptor Table and
+	 * Interrupt Descriptor Table registers
+	 */
 	lgdtl bootstrap_gdtr
+	lidtl bootstrap_idtr
 	
 	/* Kernel data + stack */
Index: kernel/arch/amd64/src/boot/vesa_ret.inc
===================================================================
--- kernel/arch/amd64/src/boot/vesa_ret.inc	(revision b17518ef2cd636e9ee2a4593e9e3f96130bd854b)
+++ kernel/arch/amd64/src/boot/vesa_ret.inc	(revision 30c08260b42b92e158e687ba64c4b4cc6561218e)
@@ -1,4 +1,5 @@
 .code32
 vesa_init_protected:
+	cli
 	cld
 	
Index: kernel/arch/amd64/src/mm/page.c
===================================================================
--- kernel/arch/amd64/src/mm/page.c	(revision b17518ef2cd636e9ee2a4593e9e3f96130bd854b)
+++ kernel/arch/amd64/src/mm/page.c	(revision 30c08260b42b92e158e687ba64c4b4cc6561218e)
@@ -57,5 +57,5 @@
 	uintptr_t cur;
 	unsigned int identity_flags =
-	    PAGE_CACHEABLE | PAGE_EXEC | PAGE_GLOBAL | PAGE_WRITE;
+	    PAGE_GLOBAL | PAGE_CACHEABLE | PAGE_EXEC | PAGE_WRITE | PAGE_READ;
 		
 	page_mapping_operations = &pt_mapping_operations;