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

[Xiaoyao Li]

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