linux-kernel - Re: [PATCH v2 3/6] kvm: x86: Emulate split-lock access as a write

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Message-ID: <878sl945tj.fsf@nanos.tec.linutronix.de>
Date:   Tue, 11 Feb 2020 14:22:16 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Xiaoyao Li <xiaoyao.li@...el.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...capital.net>
Cc:     x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        David Laight <David.Laight@...lab.com>
Subject: Re: [PATCH v2 3/6] kvm: x86: Emulate split-lock access as a write

Paolo Bonzini <pbonzini@...hat.com> writes:
> On 03/02/20 16:16, Xiaoyao Li wrote:
>> 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.
>
> Saying that anything doing a split lock access is malicious makes little
> sense.

Correct, but we also have to accept, that split lock access can be used
in a malicious way, aka. DoS.

> Split lock detection is essentially a debugging feature, there's a
> reason why the MSR is called "TEST_CTL".  So you don't want to make the

The fact that it ended up in MSR_TEST_CTL does not say anything. That's
where they it ended up to be as it was hastily cobbled together for
whatever reason.

Thanks,

        tglx