Re: [PATCH v8 12/15] kvm/vmx: Emulate MSR TEST_CTL

From: Xiaoyao Li
Date: Mon Apr 29 2019 - 01:21:23 EST

Hi, Thomas,

Base on your comments, I plan to make the design as following:

1) When host enables this feature, there is no switch between host and guest that guest running with it enabled by force. Since #AC in exception bitmap is set in current kvm, every #AC in guest will be trapped. And in handle_exception() handler in kvm, if #AC is caused by alignment check, kvm injects #AC back to guest; if #AC is caused by split lock, kvm sends a SIGBUS to userspace.

2) When host disables this feature, there needs atomic switch between host and guest if different. And in handle_exception() handler in kvm, we can just inject #AC back to guest, and let guest to handle it.

Besides, I think there might be an optimization for case #1.
When host has it enabled and guest also has it enabled, I think it's OK to inject #AC back to guest, not directly kill the guest.
Because guest kernel has it enabled means it knows what this feature is and it also want to get aware of and fault every split lock.
At this point, if guest has it enabled, we can leave it to guest. Only when guest's configuration is having it disabled, can it be regards as potentially harmful that we kill the guest once there is a #AC due to split lock.

How do you think about the design and this optimization?

Hi, Paolo,

What's your opinion about this design of split lock in KVM?


On 4/28/2019 3:09 PM, Thomas Gleixner wrote:
On Sat, 27 Apr 2019, Xiaoyao Li wrote:
On Thu, 2019-04-25 at 09:42 +0200, Thomas Gleixner wrote:
But the way more interesting question is why are you exposing the MSR and
the bit to the guest at all if the host has split lock detection enabled?

That does not make any sense as you basically allow the guest to switch it
off and then launch a slowdown attack. If the host has it enabled, then a
guest has to be treated like any other process and the #AC trap has to be
caught by the hypervisor which then kills the guest.

Only if the host has split lock detection disabled, then you can expose it
and allow the guest to turn it on and handle it on its own.

Indeed, if we use split lock detection for protection purpose, when host
has it enabled we should directly pass it to guest and forbid guest from
disabling it. And only when host disables split lock detection, we can
expose it and allow the guest to turn it on.
If it is used for protection purpose, then it should follow what you said and
this feature needs to be disabled by default. Because there are split lock
issues in old/current kernels and BIOS. That will cause the existing guest
booting failure and killed due to those split lock.

Rightfully so.

If it is only used for debug purpose, I think it might be OK to enable this
feature by default and make it indepedent between host and guest?

No. It does not make sense.

So I think how to handle this feature between host and guest depends on how we
use it? Once you give me a decision, I will follow it in next version.

As I said: The host kernel makes the decision.

If the host kernel has it enabled then the guest is not allowed to change
it. If the guest triggers an #AC it will be killed.

If the host kernel has it disabled then the guest can enable it for it's
own purposes.