x86: always set IF before oopsing from page fault

Oopsing might kill the task, via rewind_stack_do_exit() at the bottom, and
that might sleep:

Aug 23 19:30:27 xpfo kernel: [   38.302714] BUG: sleeping function called from invalid context at ./include/linux/percpu-rwsem.h:33
Aug 23 19:30:27 xpfo kernel: [   38.303837] in_atomic(): 0, irqs_disabled(): 1, pid: 1970, name: lkdtm_xpfo_test
Aug 23 19:30:27 xpfo kernel: [   38.304758] CPU: 3 PID: 1970 Comm: lkdtm_xpfo_test Tainted: G      D         4.13.0-rc5+ torvalds#228
Aug 23 19:30:27 xpfo kernel: [   38.305813] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.1-1ubuntu1 04/01/2014
Aug 23 19:30:27 xpfo kernel: [   38.306926] Call Trace:
Aug 23 19:30:27 xpfo kernel: [   38.307243]  dump_stack+0x63/0x8b
Aug 23 19:30:27 xpfo kernel: [   38.307665]  ___might_sleep+0xec/0x110
Aug 23 19:30:27 xpfo kernel: [   38.308139]  __might_sleep+0x45/0x80
Aug 23 19:30:27 xpfo kernel: [   38.308593]  exit_signals+0x21/0x1c0
Aug 23 19:30:27 xpfo kernel: [   38.309046]  ? blocking_notifier_call_chain+0x11/0x20
Aug 23 19:30:27 xpfo kernel: [   38.309677]  do_exit+0x98/0xbf0
Aug 23 19:30:27 xpfo kernel: [   38.310078]  ? smp_reader+0x27/0x40 [lkdtm]
Aug 23 19:30:27 xpfo kernel: [   38.310604]  ? kthread+0x10f/0x150
Aug 23 19:30:27 xpfo kernel: [   38.311045]  ? read_user_with_flags+0x60/0x60 [lkdtm]
Aug 23 19:30:27 xpfo kernel: [   38.311680]  rewind_stack_do_exit+0x17/0x20

To be safe, let's just always enable irqs.

The particular case I'm hitting is:

Aug 23 19:30:27 xpfo kernel: [   38.278615]  __bad_area_nosemaphore+0x1a9/0x1d0
Aug 23 19:30:27 xpfo kernel: [   38.278617]  bad_area_nosemaphore+0xf/0x20
Aug 23 19:30:27 xpfo kernel: [   38.278618]  __do_page_fault+0xd1/0x540
Aug 23 19:30:27 xpfo kernel: [   38.278620]  ? irq_work_queue+0x9b/0xb0
Aug 23 19:30:27 xpfo kernel: [   38.278623]  ? wake_up_klogd+0x36/0x40
Aug 23 19:30:27 xpfo kernel: [   38.278624]  trace_do_page_fault+0x3c/0xf0
Aug 23 19:30:27 xpfo kernel: [   38.278625]  do_async_page_fault+0x14/0x60
Aug 23 19:30:27 xpfo kernel: [   38.278627]  async_page_fault+0x28/0x30

When a fault is in kernel space which has been triggered by XPFO.

Signed-off-by: Tycho Andersen <tycho@docker.com>
CC: x86@kernel.org

arch/x86/mm/fault.c

@@ -810,6 +810,12 @@ no_context(struct pt_regs *regs, unsigned long error_code,
 	/* Executive summary in case the body of the oops scrolled away */
 	printk(KERN_DEFAULT "CR2: %016lx\n", address);
 
+	/*
+	 * We're about to oops, which might kill the task. Make sure we're
+	 * allowed to sleep.
+	 */
+	flags |= X86_EFLAGS_IF;
+
 	oops_end(flags, regs, sig);
 }