[PATCH] reboot: use make_task_dead for 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, the shutdown path has already disabled
interrupts and torn down state, and do_exit() then hits its
WARN_ON(irqs_disabled()).

That is an error path, not a clean exit.  Use make_task_dead() instead
of do_exit(0): it is built for this, fixes up the irqs disabled and
preempt state, and bounds repeated failure via oops_limit.  This
matches the make_task_dead pattern that exit.c already uses for the
oops path.

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
  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
Cc: stable@vger.kernel.org
Signed-off-by: Bradley Morgan <include@grrlz.net>
---
 kernel/reboot.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

Changes since v1: v1 enabled IRQs and kept do_exit, fixing the
symptom.  Eric Biederman pointed out the real fix is to treat the
returned halt or power off as the error path it is, and call
make_task_dead instead.

diff --git a/kernel/reboot.c b/kernel/reboot.c
index bed6967bfa96..c10ac6a0200d 100644
--- a/kernel/reboot.c
+++ b/kernel/reboot.c
@@ -14,6 +14,7 @@
 #include <linux/kmod.h>
 #include <linux/kmsg_dump.h>
 #include <linux/reboot.h>
+#include <linux/sched/task.h>
 #include <linux/suspend.h>
 #include <linux/syscalls.h>
 #include <linux/syscore_ops.h>
@@ -777,11 +778,13 @@ SYSCALL_DEFINE4(reboot, int, magic1, int, magic2, unsigned int, cmd,
 
 	case LINUX_REBOOT_CMD_HALT:
 		kernel_halt();
-		do_exit(0);
+		/* machine_halt() was expected to not return. */
+		make_task_dead(SIGKILL);
 
 	case LINUX_REBOOT_CMD_POWER_OFF:
 		kernel_power_off();
-		do_exit(0);
+		/* machine_power_off() was expected to not return. */
+		make_task_dead(SIGKILL);
 		break;
 
 	case LINUX_REBOOT_CMD_RESTART2:
-- 
2.53.0