x86/unwind: Disable KASAN checking in the ORC unwinder

Fengguang reported a KASAN warning:

  Kprobe smoke test: started
  ==================================================================
  BUG: KASAN: stack-out-of-bounds in deref_stack_reg+0xb5/0x11a
  Read of size 8 at addr ffff8800001c7cd8 by task swapper/1

  CPU: 0 PID: 1 Comm: swapper Not tainted 4.14.0-rc8 #26
  Call Trace:
   <#DB>
   ...
   save_trace+0xd9/0x1d3
   mark_lock+0x5f7/0xdc3
   __lock_acquire+0x6b4/0x38ef
   lock_acquire+0x1a1/0x2aa
   _raw_spin_lock_irqsave+0x46/0x55
   kretprobe_table_lock+0x1a/0x42
   pre_handler_kretprobe+0x3f5/0x521
   kprobe_int3_handler+0x19c/0x25f
   do_int3+0x61/0x142
   int3+0x30/0x60
  [...]

The ORC unwinder got confused by some kprobes changes, which isn't
surprising since the runtime code no longer matches vmlinux and the
stack was modified for kretprobes.

Until we have a way for generated code to register changes with the
unwinder, these types of warnings are inevitable.  So just disable KASAN
checks for stack accesses in the ORC unwinder.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thiago Jung Bauermann <bauerman@linux.vnet.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20171108021934.zbl6unh5hpugybc5@treble
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/unwind_orc.c

@@ -279,7 +279,7 @@ static bool deref_stack_reg(struct unwind_state *state, unsigned long addr,
 	if (!stack_access_ok(state, addr, sizeof(long)))
 		return false;
 
-	*val = READ_ONCE_TASK_STACK(state->task, *(unsigned long *)addr);
+	*val = READ_ONCE_NOCHECK(*(unsigned long *)addr);
 	return true;
 }