Re: [RFC PATCH 1/2] KVM: x86: Add a new system attribute for dynamic XSTATE component

Chang S. Bae
2022-08-24T22:49:24Z

On 8/24/2022 2:42 PM, Sean Christopherson wrote:
> On Tue, Aug 23, 2022, Chang S. Bae wrote:
>> == Background ==
>>
>> A set of architecture-specific prctl() options offer to control dynamic
>> XSTATE components in VCPUs. Userspace VMMs may interact with the host using
>> ARCH_GET_XCOMP_GUEST_PERM and ARCH_REQ_XCOMP_GUEST_PERM.
>>
>> However, they are separated from the KVM API. KVM may select features that
>> the host supports and advertise them through the KVM_X86_XCOMP_GUEST_SUPP
>> attribute.
>>
>> == Problem ==
>>
>> QEMU [1] queries the features through the KVM API instead of using the x86
>> arch_prctl() option. But it still needs to use arch_prctl() to request the
>> permission. Then this step may become fragile because it does not guarantee
>> to comply with the KVM policy.
>
> But backdooring through KVM doesn't prevent usersepace from walking in through
> the front door (arch_prctl()), i.e. this doesn't protect the kernel in any way.

No, I don't think backdooring is established in this proposal. The body
of the arch_prctl() support is encapsulated inside of the x86 core code.
KVM is simply calling it like arch_prctl() does.

> KVM needs to ensure that _KVM_ doesn't screw up and let userspace use features
> that KVM doesn't support. The kernel's restrictions on using features goes on
> top, i.e. KVM must behave correctly irrespective of kernel restrictions.

Maybe this is a policy decision. I don't think that
ARCH_REQ_XCOMP_GUEST_PERM goes away with this. Userspace may still use
the arch_prctl() set. But then it makes more sense and consistent to use
ARCH_GET_XCOMP_SUPP in first place, instead of KVM_X86_XCOMP_GUEST_SUPP, no?

> If QEMU wants to assert that it didn't misconfigure itself, it can assert on the
> config in any number of ways, e.g. assert that ARCH_GET_XCOMP_GUEST_PERM is a
> subset of KVM_X86_XCOMP_GUEST_SUPP at the end of kvm_request_xsave_components().

Yes, but I guess the new attribute can make it simple.

Thanks,
Chang