Index: kernel/arch/amd64/src/asm.S
===================================================================
--- kernel/arch/amd64/src/asm.S	(revision 8f4f444d453918c0a30d77ed2e590850ef528f3c)
+++ kernel/arch/amd64/src/asm.S	(revision 4fc93d5494fe531c5527e5641a67447f42cf2587)
@@ -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 +396,13 @@
 	sysretq
 
+bad_rip:
+	movq %rsp, %rdi
+	movabs $bad_rip_msg, %rsi
+	callq fault_from_uspace
+	/* not reached */
+	
+bad_rip_msg:
+	.asciz "Invalid instruction pointer."
+
 /** Print Unicode character to EGA display.
  *