linux-kernel - Re: [PATCH] x86/paravirt: Add missing noinstr to arch

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Date:   Tue, 11 Aug 2020 10:18:51 +0200
From:   Jürgen Groß <jgross@...e.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Marco Elver <elver@...gle.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        fenghua.yu@...el.com, "H. Peter Anvin" <hpa@...or.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Luck, Tony" <tony.luck@...el.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        yu-cheng.yu@...el.com, sdeep@...are.com,
        virtualization@...ts.linux-foundation.org,
        kasan-dev <kasan-dev@...glegroups.com>,
        syzbot <syzbot+8db9e1ecde74e590a657@...kaller.appspotmail.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Wei Liu <wei.liu@...nel.org>
Subject: Re: [PATCH] x86/paravirt: Add missing noinstr to arch_local*()
 helpers

On 11.08.20 10:12, Peter Zijlstra wrote:
> On Tue, Aug 11, 2020 at 09:57:55AM +0200, Jürgen Groß wrote:
>> On 11.08.20 09:41, Peter Zijlstra wrote:
>>> On Fri, Aug 07, 2020 at 05:19:03PM +0200, Marco Elver wrote:
>>>
>>>> My hypothesis here is simply that kvm_wait() may be called in a place
>>>> where we get the same case I mentioned to Peter,
>>>>
>>>> 	raw_local_irq_save(); /* or other IRQs off without tracing */
>>>> 	...
>>>> 	kvm_wait() /* IRQ state tracing gets confused */
>>>> 	...
>>>> 	raw_local_irq_restore();
>>>>
>>>> and therefore, using raw variants in kvm_wait() works. It's also safe
>>>> because it doesn't call any other libraries that would result in corrupt
>>>
>>> Yes, this is definitely an issue.
>>>
>>> Tracing, we also musn't call into tracing when using raw_local_irq_*().
>>> Because then we re-intoduce this same issue all over again.
>>>
>>> Both halt() and safe_halt() are more paravirt calls, but given we're in
>>> a KVM paravirt call already, I suppose we can directly use native_*()
>>> here.
>>>
>>> Something like so then... I suppose, but then the Xen variants need TLC
>>> too.
>>
>> Just to be sure I understand you correct:
>>
>> You mean that xen_qlock_kick() and xen_qlock_wait() and all functions
>> called by those should gain the "notrace" attribute, right?
>>
>> I am not sure why the kick variants need it, though. IMO those are
>> called only after the lock has been released, so they should be fine
>> without notrace.
> 
> The issue happens when someone uses arch_spinlock_t under
> raw_local_irq_*().

Ah, okay.

> 
>> And again: we shouldn't forget the Hyper-V variants.
> 
> Bah, my grep failed :/ Also *groan*, that's calling apic->send_IPI().

I've seen that, too. :-(


Juergen