Messages in this thread | | | From | Valentin Schneider <> | Subject | Re: [PATCH] sched/scs: Reset the shadow stack when idle_task_exit | Date | Tue, 12 Oct 2021 11:57:32 +0100 |
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On 12/10/21 18:35, Woody Lin wrote: > On Tue, Oct 12, 2021 at 6:00 PM Valentin Schneider > <valentin.schneider@arm.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
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