linux-kernel - Re: [PATCH 1/2] KVM: x86: Emulate split-lock access as a write

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Message-Id: <777C5046-B9DE-4F8C-B04F-28A546AE4A3F@amacapital.net>
Date:   Thu, 30 Jan 2020 07:16:24 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     David Laight <David.Laight@...lab.com>
Cc:     Xiaoyao Li <xiaoyao.li@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH 1/2] KVM: x86: Emulate split-lock access as a write



> On Jan 30, 2020, at 4:31 AM, David Laight <David.Laight@...lab.com> wrote:
> 
> From: Xiaoyao Li
>> Sent: 30 January 2020 12:20
>> If split lock detect is enabled (warn/fatal), #AC handler calls die()
>> when split lock happens in kernel.
>> 
>> A sane guest should never tigger emulation on a split-lock access, but
>> it cannot prevent malicous guest from doing this. So just emulating the
>> access as a write if it's a split-lock access to avoid malicous guest
>> polluting the kernel log.
> 
> That doesn't seem right if, for example, the locked access is addx.
> ISTM it would be better to force an immediate fatal error of some
> kind than just corrupt the guest memory.
> 
>    

The existing page-spanning case is just as wrong.