[PATCH] Add more paranoid checking in x86-64 prefetch checker

From: Andi Kleen <ak@muc.de>

Make sure we never access anything in kernel mapping while
doing the prefetch workaround checks on x86-64.

Originally suggested by Jamie Lockier.

arch/x86_64/mm/fault.c

@@ -73,6 +73,9 @@ static int is_prefetch(struct pt_regs *regs, unsigned long addr)
 	if (regs->cs & (1<<2))
 		return 0;
 
+	if ((regs->cs & 3) != 0 && regs->rip >= TASK_SIZE)
+		return 0;
+
 	while (scan_more && instr < max_instr) { 
 		unsigned char opcode;
 		unsigned char instr_hi;