Re: [PATCH v3] KVM: VMX: Enable Notify VM exit
From: Xiaoyao Li
Date: Tue Mar 01 2022 - 00:30:35 EST
On 3/1/2022 12:32 PM, Jim Mattson wrote:
On Mon, Feb 28, 2022 at 5:41 PM Xiaoyao Li <xiaoyao.li@xxxxxxxxx> wrote:
On 2/28/2022 10:30 PM, Jim Mattson wrote:
On Sun, Feb 27, 2022 at 11:10 PM Xiaoyao Li <xiaoyao.li@xxxxxxxxx> wrote:
On 2/26/2022 10:24 PM, Jim Mattson wrote:
On Fri, Feb 25, 2022 at 10:24 PM Xiaoyao Li <xiaoyao.li@xxxxxxxxx> wrote:
On 2/26/2022 12:53 PM, Jim Mattson wrote:
On Fri, Feb 25, 2022 at 8:25 PM Jim Mattson <jmattson@xxxxxxxxxx> wrote:
On Fri, Feb 25, 2022 at 8:07 PM Xiaoyao Li <xiaoyao.li@xxxxxxxxx> wrote:
On 2/25/2022 11:13 PM, Paolo Bonzini wrote:
On 2/25/22 16:12, Xiaoyao Li wrote:
I don't like the idea of making things up without notifying userspace
that this is fictional. How is my customer running nested VMs supposed
to know that L2 didn't actually shutdown, but L0 killed it because the
notify window was exceeded? If this information isn't reported to
userspace, I have no way of getting the information to the customer.
Then, maybe a dedicated software define VM exit for it instead of
reusing triple fault?
Second thought, we can even just return Notify VM exit to L1 to tell
L2 causes Notify VM exit, even thought Notify VM exit is not exposed
to L1.
That might cause NULL pointer dereferences or other nasty occurrences.
IMO, a well written VMM (in L1) should handle it correctly.
L0 KVM reports no Notify VM Exit support to L1, so L1 runs without
setting Notify VM exit. If a L2 causes notify_vm_exit with
invalid_vm_context, L0 just reflects it to L1. In L1's view, there is no
support of Notify VM Exit from VMX MSR capability. Following L1 handler
is possible:
a) if (notify_vm_exit available & notify_vm_exit enabled) {
handle in b)
} else {
report unexpected vm exit reason to userspace;
}
b) similar handler like we implement in KVM:
if (!vm_context_invalid)
re-enter guest;
else
report to userspace;
c) no Notify VM Exit related code (e.g. old KVM), it's treated as
unsupported exit reason
As long as it belongs to any case above, I think L1 can handle it
correctly. Any nasty occurrence should be caused by incorrect handler in
L1 VMM, in my opinion.
Please test some common hypervisors (e.g. ESXi and Hyper-V).
I took a look at KVM in Linux v4.9 (one of our more popular guests),
and it will not handle this case well:
if (exit_reason < kvm_vmx_max_exit_handlers
&& kvm_vmx_exit_handlers[exit_reason])
return kvm_vmx_exit_handlers[exit_reason](vcpu);
else {
WARN_ONCE(1, "vmx: unexpected exit reason 0x%x\n", exit_reason);
kvm_queue_exception(vcpu, UD_VECTOR);
return 1;
}
At least there's an L1 kernel log message for the first unexpected
NOTIFY VM-exit, but after that, there is silence. Just a completely
inexplicable #UD in L2, assuming that L2 is resumable at this point.
At least there is a message to tell L1 a notify VM exit is triggered in
L2. Yes, the inexplicable #UD won't be hit unless L2 triggers Notify VM
exit with invalid_context, which is malicious to L0 and L1.
There is only an L1 kernel log message *the first time*. That's not
good enough. And this is just one of the myriad of possible L1
hypervisors.
If we use triple_fault (i.e., shutdown), then no info to tell L1 that
it's caused by Notify VM exit with invalid context. Triple fault needs
to be extended and L1 kernel needs to be enlightened. It doesn't help
old guest kernel.
If we use Machine Check, it's somewhat same inexplicable to L2 unless
it's enlightened. But it doesn't help old guest kernel.
Anyway, for Notify VM exit with invalid context from L2, I don't see a
good solution to tell L1 VMM it's a "Notify VM exit with invalid context
from L2" and keep all kinds of L1 VMM happy, especially for those with
old kernel versions.
I agree that there is no way to make every conceivable L1 happy.
That's why the information needs to be surfaced to the L0 userspace. I
contend that any time L0 kvm violates the architectural specification
in its emulation of L1 or L2, the L0 userspace *must* be informed.
We can make the design to exit to userspace on notify vm exit
unconditionally with exit_qualification passed, then userspace can take
the same action like what this patch does in KVM that
- re-enter guest when context_invalid is false;
- stop running the guest if context_invalid is true; (userspace can
definitely re-enter the guest in this case, but it needs to take the
fall on this)
Then, for nested case, L0 needs to enable it transparently for L2 if
this feature is enabled for L1 guest (the reason as we all agreed that
cannot allow L1 to escape just by creating a L2). Then what should KVM
do when notify vm exit from L2?
- Exit to L0 userspace on L2's notify vm exit. L0 userspace takes the
same action:
- re-enter if context-invalid is false;
- kill L1 if context-invalid is true; (I don't know if there is any
interface for L0 userspace to kill L2). Then it opens the potential door
for malicious user to kill L1 by creating a L2 to trigger fatal notify
vm exit. If you guys accept it, we can implement in this way.
in conclusion, we have below solution:
1. Take this patch as is. The drawback is L1 VMM receives a triple_fault
from L2 when L2 triggers notify vm exit with invalid context. Neither of
L1 VMM, L1 userspace, nor L2 kernel know it's caused due to notify vm
exit. There is only kernel log in L0, which seems not accessible for L1
user or L2 guest.
You are correct on that last point, and I feel that I cannot stress it
enough. In a typical environment, the L0 kernel log is only available
to the administrator of the L0 host.
2. a) Inject notify vm exit back to L1 if L2 triggers notify vm exit
with invalid context. The drawback is, old L1 hypervisor is not
enlightened of it and maybe misbehave on it.
b) Inject a synthesized SHUTDOWN exit to L1, with additional info to
tell it's caused by fatal notify vm exit from L2. It has the same
drawback that old hypervisor has no idea of it and maybe misbehave on it.
3. Exit to L0 usersapce unconditionally no matter it's caused from L1 or
L2. Then it may open the door for L1 user to kill L1.
Do you have any better solution other than above? If no, we need to pick> >> one from above though it cannot make everyone happy.
Yes, I believe I have a better solution. We obviously need an API for
userspace to synthesize a SHUTDOWN event for a vCPU.
Can you elaborate on it? Do you mean userspace to inject a synthesized
SHUTDOWN to guest? If so, I have no idea how it will work.
It can probably be implemented as an extension of KVM_SET_VCPU_EVENTS
that invokes kvm_make_request(KVM_REQ_TRIPLE_FAULT).
Then, you mean
1. notify vm exit from guest;
2. exit to userspace on notify vm exit;
3. a. if context_invalid, inject SHUTDOWN to vcpu from userspace to
request KVM_REQ_TRIPLE_FAULT; goto step 4;
b. if !context_invalid, re-run vcpu; no step 4 and 5;
4. exit to userspace again with KVM_EXIT_SHUTDOWN due to triple fault;
5. userspace stop running the vcpu/VM
Then why not handle it as KVM_EXIT_SHUTDOWN directly in 3.a ? I don't
get the point of userspace to inject TRIPLE_FAULT to KVM.