Re: [PATCH v5 11/13] KVM: Zap existing KVM mappings when pages changed in the private fd

[Sean Christopherson <seanjc@google.com>]

On Tue, Apr 05, 2022, Michael Roth wrote:
> On Thu, Mar 10, 2022 at 10:09:09PM +0800, Chao Peng wrote:
> >  static inline bool kvm_slot_is_private(const struct kvm_memory_slot *slot)
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 67349421eae3..52319f49d58a 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -841,8 +841,43 @@ static int kvm_init_mmu_notifier(struct kvm *kvm)
> >  #endif /* CONFIG_MMU_NOTIFIER && KVM_ARCH_WANT_MMU_NOTIFIER */
> >  
> >  #ifdef CONFIG_MEMFILE_NOTIFIER
> > +static void kvm_memfile_notifier_handler(struct memfile_notifier *notifier,
> > +					 pgoff_t start, pgoff_t end)
> > +{
> > +	int idx;
> > +	struct kvm_memory_slot *slot = container_of(notifier,
> > +						    struct kvm_memory_slot,
> > +						    notifier);
> > +	struct kvm_gfn_range gfn_range = {
> > +		.slot		= slot,
> > +		.start		= start - (slot->private_offset >> PAGE_SHIFT),
> > +		.end		= end - (slot->private_offset >> PAGE_SHIFT),
> > +		.may_block 	= true,
> > +	};
> > +	struct kvm *kvm = slot->kvm;
> > +
> > +	gfn_range.start = max(gfn_range.start, slot->base_gfn);
> > +	gfn_range.end = min(gfn_range.end, slot->base_gfn + slot->npages);
> > +
> > +	if (gfn_range.start >= gfn_range.end)
> > +		return;
> > +
> > +	idx = srcu_read_lock(&kvm->srcu);
> > +	KVM_MMU_LOCK(kvm);
> > +	kvm_unmap_gfn_range(kvm, &gfn_range);
> > +	kvm_flush_remote_tlbs(kvm);
> > +	KVM_MMU_UNLOCK(kvm);
> > +	srcu_read_unlock(&kvm->srcu, idx);
> 
> Should this also invalidate gfn_to_pfn_cache mappings? Otherwise it seems
> possible the kernel might end up inadvertantly writing to now-private guest
> memory via a now-stale gfn_to_pfn_cache entry.

Yes.  Ideally we'd get these flows to share common code and avoid these goofs.
I tried very briefly but they're just different enough to make it ugly.