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

[Eric W. Biederman]

Bradley Morgan <include@xxxxxxxxx> writes:

> 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.

I think this is fixing symptoms not the actual cause.

How does kernel_halt or kernel_power_off manage to return?

If they aren't supposed to return changing the code to call
make_task_dead to indicate you are on an error path is probably
the better fix.

Eric

> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=8fdf0d8e10bdde1c2e88
> Closes: https://lore.kernel.org/all/69f5ec0c.050a0220.312cd3.0023.GAE@xxxxxxxxxx/T/
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Bradley Morgan <include@xxxxxxxxx>
> ---
>  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;