linux-kernel - Re: [PATCH] sched/scs: Reset the shadow stack when idle_task

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Date:   Tue, 12 Oct 2021 11:57:32 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Woody Lin <woodylin@...gle.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Ben Segall <bsegall@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/scs: Reset the shadow stack when idle_task_exit

On 12/10/21 18:35, Woody Lin wrote:
> On Tue, Oct 12, 2021 at 6:00 PM Valentin Schneider
> <valentin.schneider@....com> wrote:
>>
>> So AIUI for SCS that works just fine - one thing I'm unclear on is how the
>> following pops are going to work given the SP reset happens in the middle
>> of a call stack, but AFAICT that was already the case before I messed about
>> with init_idle(), so that must already be handled.
>
> Hi Valentin,
>
> Thanks for the question. The 'scs_task_reset' here resets only the
> '.thread_info.scs_sp' of the task, so the register (on arm64 it's x18)
> is still pointing to the same location for popping and storing call
> frames. The register will be updated to '.thread_info.scs_sp' in
> '__secondary_switched', which starts a new core and there is no popping
> after the updating, so it won't introduce an underflow.
>

I think I got it; __secondary_switched() -> init_cpu_task() -> scs_load()

Thanks!

>>
>> I'm not familiar enough with KASAN to say whether that
>> kasan_unpoison_task_stack() should rather happen upon hotplugging the CPU
>> back (rather than on hot-unplug). If that is the case, then maybe somewhere
>> around cpu_startup_entry() might work (and then you could bunch these two
>> "needs to be re-run at init for the idle task" functions into a common
>> helper).
>
> unpoison looks more like an one-time thing to me; the idle tasks will
> reuse the same stack pages until system resets, so I think we don't need
> to re-unpoison that during hotplugging as long as it's unpoisoned in
> 'init_idle'.
>

I would tend to agree, but was bitten by s390 freeing some memory on
hot-unplug and re-allocating it upon hotplug:

  6a942f578054 ("s390: preempt: Fix preempt_count initialization")

This makes me doubt whether we can assert the idle task stack pages are
perennial vs hotplug on all architectures.

>>
>> >  }
>> >
>> > --
>> > 2.33.0.882.g93a45727a2-goog