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

[Sean Christopherson <seanjc@google.com>]

On Mon, Apr 18, 2022, Chao Gao wrote:
> On Fri, Apr 15, 2022 at 03:25:06PM +0000, Sean Christopherson wrote:
> >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.
> 
> can we simply return an error (e.g., -EEXIST) on step 5 (i.e.,
> max_vcpu_ids cannot be changed after being set once)?
> 
> or
> 
> can we detect the change of max_vcpu_ids in step 6 and re-allocate PID
> table?

Returning an error is viable, but would be a rather odd ABI.  Re-allocating isn't
a good option because the PID table could be in active use by other vCPUs, e.g.
KVM would need to send a request and kick all vCPUs to have all vCPUs update their
VMCS.

And with both of those alternatives, I still don't like that every feature that
acts on max_vcpu_ids would need to handle this same edge case.

An alternative to another new ioctl() would be to to make KVM_CAP_MAX_VCPU_ID
write-once, i.e. reject attempts to change the max once set (though we could allow
re-writing the same value).  I think I like that idea better than adding an ioctl().

It can even be done without an extra flag by zero-initializing the field and instead
waiting until vCPU pre-create to lock in the value.  That would also help detect
bad usage of max_vcpu_ids, especially if we added a wrapper to get the value, e.g.
the wrapper could WARN_ON(!kvm->arch.max_vcpu_ids).

E.g.

int kvm_arch_vcpu_precreate(struct kvm *kvm, unsigned int id)
{
	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");

	if (!kvm->arch.max_vcpu_ids)
		kvm->arch.max_vcpu_ids = KVM_MAX_VCPU_IDS;

	return 0;
}


	case KVM_CAP_MAX_VCPU_ID:
		r = -EINVAL;
		if (cap->args[0] > KVM_MAX_VCPU_IDS)
			break;

		mutex_lock(&kvm->lock);
                if (kvm->arch.max_vcpu_ids == cap->args[0]) {
                        r = 0;
                } else if (!kvm->arch.max_vcpu_ids) {
			kvm->arch.max_vcpu_ids = cap->args[0];
			r = 0;
		}
		mutex_unlock(&kvm->lock);
		break;