linux-kernel - Re: KCSAN + KVM = host reset

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Message-Id: <2730C0CC-B8B5-4A65-A4ED-9DFAAE158AA6@lca.pw>
Date:   Thu, 9 Apr 2020 11:30:14 -0400
From:   Qian Cai <cai@....pw>
To:     Marco Elver <elver@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        "paul E. McKenney" <paulmck@...nel.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org
Subject: Re: KCSAN + KVM = host reset



> On Apr 9, 2020, at 11:22 AM, Marco Elver <elver@...gle.com> wrote:
> 
> On Thu, 9 Apr 2020 at 17:10, Qian Cai <cai@....pw> wrote:
>> 
>> 
>> 
>>> On Apr 9, 2020, at 3:03 AM, Marco Elver <elver@...gle.com> wrote:
>>> 
>>> On Wed, 8 Apr 2020 at 23:29, Qian Cai <cai@....pw> wrote:
>>>> 
>>>> 
>>>> 
>>>>> On Apr 8, 2020, at 5:25 PM, Paolo Bonzini <pbonzini@...hat.com> wrote:
>>>>> 
>>>>> On 08/04/20 22:59, Qian Cai wrote:
>>>>>> Running a simple thing on this AMD host would trigger a reset right away.
>>>>>> Unselect KCSAN kconfig makes everything work fine (the host would also
>>>>>> reset If only "echo off > /sys/kernel/debug/kcsan” before running qemu-kvm).
>>>>> 
>>>>> Is this a regression or something you've just started to play with?  (If
>>>>> anything, the assembly language conversion of the AMD world switch that
>>>>> is in linux-next could have reduced the likelihood of such a failure,
>>>>> not increased it).
>>>> 
>>>> I don’t remember I had tried this combination before, so don’t know if it is a
>>>> regression or not.
>>> 
>>> What happens with KASAN? My guess is that, since it also happens with
>>> "off", something that should not be instrumented is being
>>> instrumented.
>> 
>> No, KASAN + KVM works fine.
>> 
>>> 
>>> What happens if you put a 'KCSAN_SANITIZE := n' into
>>> arch/x86/kvm/Makefile? Since it's hard for me to reproduce on this
>> 
>> Yes, that works, but this below alone does not work,
>> 
>> KCSAN_SANITIZE_kvm-amd.o := n
> 
> There are some other files as well, that you could try until you hit
> the right one.
> 
> But since this is in arch, 'KCSAN_SANITIZE := n' wouldn't be too bad
> for now. If you can't narrow it down further, do you want to send a
> patch?

No, that would be pretty bad because it will disable KCSAN for Intel
KVM as well which is working perfectly fine right now. It is only AMD
is broken.

> 
> Thanks,
> -- Marco
> 
>> I have been able to reproduce this on a few AMD hosts.
>> 
>>> exact system, I'd ask you to narrow it down by placing 'KCSAN_SANITIZE
>>> := n' into suspect subsystems' Makefiles. Once you get it to work with
>>> that, we can refine the solution.
>>> 
>>> Thanks,
>>> -- Marco