Subject: Re: [PATCH v12 02/84] KVM: arm64: Disallow copying MTE to guest memory while KVM is dirty logging

From: Sean Christopherson <[email-masked]>
Date: Thu, 1 Aug 2024 11:01:30 -0700

On Thu, Aug 01, 2024, Aneesh Kumar K.V wrote:
> Sean Christopherson <[email-masked]> writes:
> 
> > Disallow copying MTE tags to guest memory while KVM is dirty logging, as
> > writing guest memory without marking the gfn as dirty in the memslot could
> > result in userspace failing to migrate the updated page.  Ideally (maybe?),
> > KVM would simply mark the gfn as dirty, but there is no vCPU to work with,
> > and presumably the only use case for copy MTE tags _to_ the guest is when
> > restoring state on the target.
> >
> > Fixes: f0376edb1ddc ("KVM: arm64: Add ioctl to fetch/store tags in a guest")
> > Signed-off-by: Sean Christopherson <[email-masked]>
> > ---
> >  arch/arm64/kvm/guest.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> >
> > diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
> > index e1f0ff08836a..962f985977c2 100644
> > --- a/arch/arm64/kvm/guest.c
> > +++ b/arch/arm64/kvm/guest.c
> > @@ -1045,6 +1045,11 @@ int kvm_vm_ioctl_mte_copy_tags(struct kvm *kvm,
> >  
> >  	mutex_lock(&kvm->slots_lock);
> >  
> > +	if (write && atomic_read(&kvm->nr_memslots_dirty_logging)) {
> > +		ret = -EBUSY;
> > +		goto out;
> > +	}
> > +
> >
> 
> is this equivalent to kvm_follow_pfn() with kfp->pin = 1 ?

No, gfn_to_pfn_prot() == FOLL_GET, kfp->pin == FOLL_PIN.  But that's not really
relevant.

> Should all those pin request fail if kvm->nr_memslots_dirty_logging != 0? 

No, the conflict with dirty logging is specifically that this code doesn't invoke
mark_page_dirty().  And it can't easily do that, because there's no loaded ("running")
vCPU, i.e. doing so would trip this WARN:

#ifdef CONFIG_HAVE_KVM_DIRTY_RING
	if (WARN_ON_ONCE(vcpu && vcpu->kvm != kvm))
		return;

	WARN_ON_ONCE(!vcpu && !kvm_arch_allow_write_without_running_vcpu(kvm)); <====
#endif