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

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Message-ID: <95d29a81-62d5-f5b6-0eb6-9d002c0bba23@redhat.com>
Date:   Tue, 11 Feb 2020 13:20:45 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Xiaoyao Li <xiaoyao.li@...el.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        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

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.

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
corresponding behavior wrong.  Just kill the guest with a
KVM_INTERNAL_ERROR userspace exit so people will notice quickly and
either disable the feature or see if they can fix the guest.

Paolo