[PATCH] reboot: enable IRQs before do_exit in the halt and power off fallback

[Bradley Morgan <include@grrlz.net>]

The reboot syscall calls do_exit(0) after kernel_halt() or
kernel_power_off().  Those are expected to stop the machine and not
return.  When they do return (no PM info, power off failed), the
shutdown path has already disabled interrupts: native_machine_shutdown()
calls local_irq_disable() on x86, and do_exit() then hits its
WARN_ON(irqs_disabled()) at kernel/exit.c:930.

do_exit only warns by design; make_task_dead() is the path that fixes
the IRQs disabled state (commit 001c28e57187 ("exit: Detect and fix irq
disabled state in oops")).  The reboot fallback is not an oops and wants
a clean do_exit, so enable IRQs at the two call sites instead, matching
the make_task_dead pattern.

Splat from syzbot:

  ACPI: PM: Preparing to enter system sleep state S5
  kvm: exiting hardware virtualization
  reboot: Power down
  ------------[ cut here ]------------
  irqs_disabled()
  WARNING: kernel/exit.c:930 at do_exit+0x1cf7/0x2ae0 kernel/exit.c:930, CPU#0: init/6193
  CPU: 0 UID: 0 PID: 6193 Comm: init Tainted: G             L      syzkaller #0 PREEMPT(full)
  Tainted: [L]=SOFTLOCKUP
  Call Trace:
   <TASK>
   __do_sys_reboot+0x36e/0x400 kernel/reboot.c:784
   do_syscall_64+0x115/0x840 arch/x86/entry/syscall_64.c:94
   entry_SYSCALL_64_after_hwframe+0x77/0x7f
   </TASK>

Fixes: 001c28e57187 ("exit: Detect and fix irq disabled state in oops")
Reported-by: syzbot+8fdf0d8e10bdde1c2e88@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=8fdf0d8e10bdde1c2e88
Closes: https://lore.kernel.org/all/69f5ec0c.050a0220.312cd3.0023.GAE@google.com/T/
Cc: stable@vger.kernel.org
Signed-off-by: Bradley Morgan <include@grrlz.net>
---
 kernel/reboot.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/kernel/reboot.c b/kernel/reboot.c
index bed6967bfa96..7e8ebb470721 100644
--- a/kernel/reboot.c
+++ b/kernel/reboot.c
@@ -777,10 +777,20 @@ SYSCALL_DEFINE4(reboot, int, magic1, int, magic2, unsigned int, cmd,
 
 	case LINUX_REBOOT_CMD_HALT:
 		kernel_halt();
+		/* kernel_halt() was expected to not return. */
+		if (irqs_disabled()) {
+			pr_info("reboot: halt returned with irqs disabled\n");
+			local_irq_enable();
+		}
 		do_exit(0);
 
 	case LINUX_REBOOT_CMD_POWER_OFF:
 		kernel_power_off();
+		/* kernel_power_off() was expected to not return. */
+		if (irqs_disabled()) {
+			pr_info("reboot: power off returned with irqs disabled\n");
+			local_irq_enable();
+		}
 		do_exit(0);
 		break;
 
-- 
2.53.0