Re: [PATCH v8 9/9] KVM: VMX: enable IPI virtualization

[Sean Christopherson]

On Mon, Apr 11, 2022, Zeng Guang wrote:
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index d1a39285deab..23fbf52f7bea 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -11180,11 +11180,15 @@ static int sync_regs(struct kvm_vcpu *vcpu)
>  
>  int kvm_arch_vcpu_precreate(struct kvm *kvm, unsigned int id)
>  {
> +	int ret = 0;
> +
>  	if (kvm_check_tsc_unstable() && atomic_read(&kvm->online_vcpus) != 0)
>  		pr_warn_once("kvm: SMP vm created on host with unstable TSC; "
>  			     "guest TSC will not be reliable\n");
>  
> -	return 0;
> +	if (kvm_x86_ops.alloc_ipiv_pid_table)
> +		ret = static_call(kvm_x86_alloc_ipiv_pid_table)(kvm);

Add a generic kvm_x86_ops.vcpu_precreate, no reason to make this so specific.
And use KVM_X86_OP_RET0 instead of KVM_X86_OP_OPTIONAL, then this can simply be

	return static_call(kvm_x86_vcpu_precreate);

That said, there's a flaw in my genius plan.

  1. KVM_CREATE_VM
  2. KVM_CAP_MAX_VCPU_ID, set max_vcpu_ids=1
  3. KVM_CREATE_VCPU, create IPIv table but ultimately fails
  4. KVM decrements created_vcpus back to '0'
  5. KVM_CAP_MAX_VCPU_ID, set max_vcpu_ids=4096
  6. KVM_CREATE_VCPU w/ ID out of range

In other words, malicious userspace could trigger buffer overflow.

That could be solved by adding an arch hook to undo precreate, but that's gross
and a good indication that we're trying to solve this the wrong way.

I think it's high time we add KVM_FINALIZE_VM, though that's probably a bad name
since e.g. TDX wants to use that name for VM really, really, being finalized[*],
i.e. after all vCPUs have been created.

KVM_POST_CREATE_VM?  That's not very good either.

Paolo or anyone else, thoughts?

[*] https://lore.kernel.org/all/83768bf0f786d24f49d9b698a45ba65441ef5ef0.1646422845.git.isaku.yamahata@xxxxxxxxx