Re: [PATCH] KVM: Fix error path in kvm_vm_ioctl_create_vcpu() on xa_store() failure

[Will Deacon]

On Tue, Jul 30, 2024 at 04:31:08PM -0700, Sean Christopherson wrote:
> On Tue, Jul 30, 2024, Michal Luczaj wrote:
> > On 7/30/24 17:56, Will Deacon wrote:
> > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > > index d0788d0a72cc..b80dd8cead8c 100644
> > > --- a/virt/kvm/kvm_main.c
> > > +++ b/virt/kvm/kvm_main.c
> > > @@ -4293,7 +4293,7 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
> > >  
> > >  	if (KVM_BUG_ON(xa_store(&kvm->vcpu_array, vcpu->vcpu_idx, vcpu, 0), kvm)) {
> > >  		r = -EINVAL;
> > > -		goto kvm_put_xa_release;
> > > +		goto err_xa_release;
> > >  	}
> > >  
> > >  	/*
> > > @@ -4310,6 +4310,7 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
> > >  
> > >  kvm_put_xa_release:
> > >  	kvm_put_kvm_no_destroy(kvm);
> > > +err_xa_release:
> > >  	xa_release(&kvm->vcpu_array, vcpu->vcpu_idx);
> > >  unlock_vcpu_destroy:
> > >  	mutex_unlock(&kvm->lock);
> > 
> > My bad for neglecting the "impossible" path. Thanks for the fix.
> > 
> > I wonder if it's complete. If we really want to consider the possibility of
> > this xa_store() failing, then keeping vCPU fd installed and calling
> > kmem_cache_free(kvm_vcpu_cache, vcpu) on the error path looks wrong.
> 
> Yeah, the vCPU is exposed to userspace, freeing its assets will just cause
> different problems.  KVM_BUG_ON() will prevent _new_ vCPU ioctl() calls (and kick
> running vCPUs out of the guest), but it doesn't interrupt other CPUs, e.g. if
> userspace is being sneaking and has already invoked a vCPU ioctl(), KVM will hit
> a use-after-free (several of them).

Damn, yes. Just because we haven't returned the fd yet, doesn't mean
userspace can't make use of it.

> As Michal alluded to, it should be impossible for xa_store() to fail since KVM
> pre-allocates/reserves memory.  Given that, deliberately leaking the vCPU seems
> like the least awful "solution".

Could we actually just move the xa_store() before the fd creation? I
can't immediately see any issues with that...

Will