x86/dumpstack/64: Don't evaluate exception stacks before setup

Cyrill reported the following crash:

  BUG: unable to handle page fault for address: 0000000000001ff0
  #PF: supervisor read access in kernel mode
  RIP: 0010:get_stack_info+0xb3/0x148

It turns out that if the stack tracer is invoked before the exception stack
mappings are initialized in_exception_stack() can erroneously classify an
invalid address as an address inside of an exception stack:

    begin = this_cpu_read(cea_exception_stacks);  <- 0
    end = begin + sizeof(exception stacks);

i.e. any address between 0 and end will be considered as exception stack
address and the subsequent code will then try to derefence the resulting
stack frame at a non mapped address.

 end = begin + (unsigned long)ep->size;
     ==> end = 0x2000

 regs = (struct pt_regs *)end - 1;
     ==> regs = 0x2000 - sizeof(struct pt_regs *) = 0x1ff0

 info->next_sp   = (unsigned long *)regs->sp;
     ==> Crashes due to accessing 0x1ff0

Prevent this by checking the validity of the cea_exception_stack base
address and bailing out if it is zero.

Fixes: afcd21d ("x86/dumpstack/64: Use cpu_entry_area instead of orig_ist")
Reported-by: Cyrill Gorcunov <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Tested-by: Cyrill Gorcunov <[email protected]>
Acked-by: Josh Poimboeuf <[email protected]>
Cc: [email protected]
Link: https://lkml.kernel.org/r/[email protected]

Author: KAGA-KOKO

arch/x86/kernel/dumpstack_64.c

@@ -94,6 +94,13 @@ static bool in_exception_stack(unsigned long *stack, struct stack_info *info)
 	BUILD_BUG_ON(N_EXCEPTION_STACKS != 6);
 
 	begin = (unsigned long)__this_cpu_read(cea_exception_stacks);
+	/*
+	 * Handle the case where stack trace is collected _before_
+	 * cea_exception_stacks had been initialized.
+	 */
+	if (!begin)
+		return false;
+
 	end = begin + sizeof(struct cea_exception_stacks);
 	/* Bail if @stack is outside the exception stack area. */
 	if (stk < begin || stk >= end)