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

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 2/27/2020 8:11 AM, Sean Christopherson wrote:
> On Tue, Feb 11, 2020 at 02:34:18PM +0100, Paolo Bonzini wrote:
>> On 11/02/20 14:22, Thomas Gleixner wrote:
>>> Paolo Bonzini <pbonzini@redhat.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.
> 
> The problem with "kill the guest", and the reason I'd prefer to emulate the
> split-lock as a write, is that killing the guest in this case is annoyingly
> difficult.
> 
> Returning X86EMUL_UNHANDLEABLE / EMULATION_FAILED gets KVM to
> handle_emulation_failure(), but handle_emulation_failure() will only "kill"
> the guest if emulation failed in L1 CPL==0.  For all other modes, it will
> inject a #UD and resume the guest.  KVM also injects a #UD for L1 CPL==0,
> but that's the least annoying thing.
> 
> Adding a new emulation type isn't an option because this code can be
> triggered through normal emulation.  A new return type could be added for
> split-lock, but that's code I'd really not add, both from an Intel
> perspective and a KVM maintenance perspective.  And, we'd still have the
> conundrum of what to do if/when split-lock #AC is exposed to L1, e.g. in
> that case, KVM should inject an #AC into L1, not kill the guest.  Again,
> totally doable, but ugly and IMO an unnecessary maintenance burden.
> 
> I completely agree that poorly emulating the instruction from the (likely)
> malicious guest is a hack, but it's a simple and easy to maintain hack.

Paolo,

What's your opinion about above?

>>>> 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
>>