linux-kernel - Re: [PATCH] powerpc: Save AMR/IAMR when switching tasks

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Date:   Wed, 21 Sep 2022 23:00:16 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Samuel Holland <samuel@...lland.org>,
        Nicholas Piggin <npiggin@...il.com>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>,
        Russell Currey <ruscur@...sell.cc>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH] powerpc: Save AMR/IAMR when switching tasks

Christophe Leroy <christophe.leroy@...roup.eu> writes:
> Le 19/09/2022 à 14:37, Michael Ellerman a écrit :
>> Christophe Leroy <christophe.leroy@...roup.eu> writes:
>>> Le 16/09/2022 à 07:05, Samuel Holland a écrit :
>>>> With CONFIG_PREEMPT=y (involuntary preemption enabled), it is possible
>>>> to switch away from a task inside copy_{from,to}_user. This left the CPU
>>>> with userspace access enabled until after the next IRQ or privilege
>>>> level switch, when AMR/IAMR got reset to AMR_KU[AE]P_BLOCKED. Then, when
>>>> switching back to the original task, the userspace access would fault:
>>>
>>> This is not supposed to happen. You never switch away from a task
>>> magically. Task switch will always happen in an interrupt, that means
>>> copy_{from,to}_user() get interrupted.
>> 
>> Unfortunately this isn't true when CONFIG_PREEMPT=y.
>
> Argh, yes, I wrote the above with the assumption that we properly follow 
> the main principles that no complex fonction is to be used while KUAP is 
> open ... Which is apparently not true here. x86 would have detected it 
> with objtool, but we don't have it yet in powerpc.

Yes and yes :/

>> We can switch away without an interrupt via:
>>    __copy_tofrom_user()
>>      -> __copy_tofrom_user_power7()
>>         -> exit_vmx_usercopy()
>>            -> preempt_enable()
>>               -> __preempt_schedule()
>>                  -> preempt_schedule()
>>                     -> preempt_schedule_common()
>>                        -> __schedule()
>
>
> Should we use preempt_enable_no_resched() to avoid that ?

Good point :)

...
>> 
>> Still I think it might be our best option for an easy fix.
>
> Wouldn't it be even easier and less abusive to use 
> preemt_enable_no_resched() ? Or is there definitively a good reason to 
> resched after a VMX copy while we don't with regular copies ?

I don't think it's important to reschedule there. I guess it means
another task that wants to preempt will have to wait a little longer,
but I doubt it's measurable.

One reason to do the KUAP lock is it also protects us from running
disable_kernel_altivec() with KUAP unlocked.

That in turn calls into msr_check_and_clear() and
__msr_check_and_clear(), none of which is a problem per-se, but we'd
need to make that all notrace to be 100% safe.

At the moment we're running that all with KUAP unlocked anyway, so using
preempt_enable_no_resched() would fix the actual bug and is much nicer
than my patch, so we should probably do that.

We can look at making disable_kernel_altivec() etc. notrace as a
follow-up.

cheers