x86/unwind: Prevent KASAN false positive warnings in guess unwinder

The guess unwinder scans the entire stack, which can cause KASAN
"stack-out-of-bounds" false positive warnings.  Tell KASAN to ignore it.
Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Stephane Eranian <eranian@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Cc: davej@codemonkey.org.uk
Cc: dvyukov@google.com
Link: http://lkml.kernel.org/r/61939c0b2b2d63ce97ba59cba3b00fd47c2962cf.1479398226.git.jpoimboe@redhat.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/unwind_guess.c

@@ -7,11 +7,13 @@
 
 unsigned long unwind_get_return_address(struct unwind_state *state)
 {
+	unsigned long addr = READ_ONCE_NOCHECK(*state->sp);
+
 	if (unwind_done(state))
 		return 0;
 
 	return ftrace_graph_ret_addr(state->task, &state->graph_idx,
-				     *state->sp, state->sp);
+				     addr, state->sp);
 }
 EXPORT_SYMBOL_GPL(unwind_get_return_address);
 
@@ -23,8 +25,10 @@ bool unwind_next_frame(struct unwind_state *state)
 		return false;
 
 	do {
+		unsigned long addr = READ_ONCE_NOCHECK(*state->sp);
+
 		for (state->sp++; state->sp < info->end; state->sp++)
-			if (__kernel_text_address(*state->sp))
+			if (__kernel_text_address(addr))
 				return true;
 
 		state->sp = info->next_sp;