Re: [PATCH] KVM: Don't actually set a request when evicting vCPUs for GFN cache invd

[Sean Christopherson <seanjc@google.com>]

On Sat, Feb 12, 2022, Woodhouse, David wrote:
> 
> (Apologies if this is HTML but I'm half-way to Austria and the laptop is
> buried somewhere in the car, and access to work email with sane email apps is
> difficult.)
> 
> On 12 Feb 2022 03:05, Sean Christopherson <seanjc@google.com> wrote:
> 
> Don't actually set a request bit in vcpu->requests when making a request
> purely to force a vCPU to exit the guest.  Logging the request but not
> actually consuming it causes the vCPU to get stuck in an infinite loop
> during KVM_RUN because KVM sees a pending request and bails from VM-Enter
> to service the request.
> 
> 
> Right, but there is no extant code which does this. The guest_uses_pa flag is
> unused.

Grr.  A WARN or something would have been nice to have.  Oh well.

> The series came with a proof-of-concept that attempted using it for
> fixing nesting UAFs but it was just that — a proof of concept to demonstrate
> that the new design of GPC was sufficient to address that problem.
> 
> IIRC, said proof of concept did also actually consume the req in question,

It did.  I saw that, but obviously didn't connect the dots to guest_uses_pa.

--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9826,6 +9826,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)

                if (kvm_check_request(KVM_REQ_UPDATE_CPU_DIRTY_LOGGING, vcpu))
                        static_call(kvm_x86_update_cpu_dirty_logging)(vcpu);
+               if (kvm_check_request(KVM_REQ_GPC_INVALIDATE, vcpu))
+                       ; /* Nothing to do. It just wanted to wake us */

> and one of the existing test cases did exercise it with an additional mmap
> torture added? Of course until we have kernel code that *does* this, it's
> hard to exercise it from userspace :)

Indeed.  I'll send a new version with a different changelog, that way we're not
leaving a trap for developers and each architecture doesn't need to manually handle
the request.

Thanks!