x86/dumpstack/64: Handle faults when printing the "Stack: " part of an OOPS

If we overflow the stack into a guard page, we'll recursively fault
when trying to dump the contents of the guard page.  Use
probe_kernel_address() so we can recover if this happens.
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com>
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: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/e626d47a55d7b04dcb1b4d33faa95e8505b217c8.1468527351.git.luto@kernel.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/dumpstack_64.c

@@ -272,6 +272,8 @@ show_stack_log_lvl(struct task_struct *task, struct pt_regs *regs,
 
 	stack = sp;
 	for (i = 0; i < kstack_depth_to_print; i++) {
+		unsigned long word;
+
 		if (stack >= irq_stack && stack <= irq_stack_end) {
 			if (stack == irq_stack_end) {
 				stack = (unsigned long *) (irq_stack_end[-1]);
@@ -281,12 +283,18 @@ show_stack_log_lvl(struct task_struct *task, struct pt_regs *regs,
 		if (kstack_end(stack))
 			break;
 		}
+
+		if (probe_kernel_address(stack, word))
+			break;
+
 		if ((i % STACKSLOTS_PER_LINE) == 0) {
 			if (i != 0)
 				pr_cont("\n");
-			printk("%s %016lx", log_lvl, *stack++);
+			printk("%s %016lx", log_lvl, word);
 		} else
-			pr_cont(" %016lx", *stack++);
+			pr_cont(" %016lx", word);
+
+		stack++;
 		touch_nmi_watchdog();
 	}
 	preempt_enable();