Re: [PATCH v9 09/17] x86/split_lock: Handle #AC exception for split lock

Xiaoyao Li
2019-10-16T13:13:17Z

On 10/16/2019 7:26 PM, Paolo Bonzini wrote:
> On 16/10/19 13:23, Xiaoyao Li wrote:
>> KVM always traps #AC, and only advertises split-lock detection to guest
>> when the global variable split_lock_detection_enabled in host is true.
>>
>> - If guest enables #AC (CPL3 alignment check or split-lock detection
>> enabled), injecting #AC back into guest since it's supposed capable of
>> handling it.
>> - If guest doesn't enable #AC, KVM reports #AC to userspace (like other
>> unexpected exceptions), and we can print a hint in kernel, or let
>> userspace (e.g., QEMU) tell the user guest is killed because there is a
>> split-lock in guest.
>>
>> In this way, malicious guests always get killed by userspace and old
>> sane guests cannot survive as well if it causes split-lock. If we do
>> want old sane guests work we have to disable the split-lock detection
>> (through booting parameter or debugfs) in the host just the same as we
>> want to run an old and split-lock generating userspace binary.
>
> Old guests are prevalent enough that enabling split-lock detection by
> default would be a big usability issue. And even ignoring that, you
> would get the issue you describe below:

Right, whether enabling split-lock detection is made by the
administrator. The administrator is supposed to know the consequence of
enabling it. Enabling it means don't want any split-lock happens in
userspace, of course VMM softwares are under control.

>> But there is an issue that we advertise split-lock detection to guest
>> based on the value of split_lock_detection_enabled to be true in host,
>> which can be turned into false dynamically when split-lock happens in
>> host kernel.
>
> ... which means that supposedly safe guests become unsafe, and that is bad.
>
>> This causes guest's capability changes at run time and I
>> don't if there is a better way to inform guest? Maybe we need a pv
>> interface?
>
> Even a PV interface would not change the basic fact that a supposedly
> safe configuration becomes unsafe.

I don't catch you about the unsafe?

If host disables split-lock detection dynamically, then the
MST_TEST_CTL.split_lock is clear in the hardware and we can use the PV
interface to notify the guest so that guest knows it loses the
capability of split-lock detection. In this case, I think safety is
meaningless for both host and guest.

> Paolo
>