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

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Date:   Tue, 11 Feb 2020 14:34:18 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        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

On 11/02/20 14:22, Thomas Gleixner wrote:
> 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.

Indeed, a more accurate emulation such as temporarily disabling
split-lock detection in the emulator would allow the guest to use split
lock access as a vehicle for DoS, but that's not what the commit message
says.  If it were only about polluting the kernel log, there's
printk_ratelimited for that.  (In fact, if we went for incorrect
emulation as in this patch, a rate-limited pr_warn would be a good idea).

It is much more convincing to say that since this is pretty much a
theoretical case, we can assume that it is only done with the purpose of
DoS-ing the host or something like that, and therefore we kill the guest.

>> 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.

Or perhaps it was there all the time in test silicon or something like
that...  That would be a very plausible reason for all the quirks behind it.

Paolo