Re: [PATCH v3 6/8] KVM: Fully serialize gfn=>pfn cache refresh via mutex

[Sean Christopherson <seanjc@google.com>]

On Fri, May 20, 2022, Paolo Bonzini wrote:
> On 4/29/22 23:00, Sean Christopherson wrote:
> > diff --git a/virt/kvm/pfncache.c b/virt/kvm/pfncache.c
> > index 05cb0bcbf662..eaef31462bbe 100644
> > --- a/virt/kvm/pfncache.c
> > +++ b/virt/kvm/pfncache.c
> > @@ -157,6 +157,13 @@ int kvm_gfn_to_pfn_cache_refresh(struct kvm *kvm, struct gfn_to_pfn_cache *gpc,
> >   	if (page_offset + len > PAGE_SIZE)
> >   		return -EINVAL;
> > +	/*
> > +	 * If another task is refreshing the cache, wait for it to complete.
> > +	 * There is no guarantee that concurrent refreshes will see the same
> > +	 * gpa, memslots generation, etc..., so they must be fully serialized.
> > +	 */
> > +	mutex_lock(&gpc->refresh_lock);
> > +
> >   	write_lock_irq(&gpc->lock);
> >   	old_pfn = gpc->pfn;
> > @@ -248,6 +255,8 @@ int kvm_gfn_to_pfn_cache_refresh(struct kvm *kvm, struct gfn_to_pfn_cache *gpc,
> >    out:
> >   	write_unlock_irq(&gpc->lock);
> > +	mutex_unlock(&gpc->refresh_lock);
> > +
> >   	gpc_release_pfn_and_khva(kvm, old_pfn, old_khva);
> >   	return ret;
> 
> Does kvm_gfn_to_pfn_cache_unmap also need to take the mutex, to avoid the
> WARN_ON(gpc->valid)?

I don't know What WARN_ON() you're referring to, but there is a double-free bug
if unmap() runs during an invalidation.  That can be solved without having to
take the mutex though, just reset valid/pfn/khva before the retry.

When searching to see how unmap() was used in the original series (there's no
other user besides destroy...), I stumbled across this likely-related syzbot bug
that unfortunately didn't Cc KVM :-(

https://lore.kernel.org/all/00000000000073f09205db439577@google.com


diff --git a/virt/kvm/pfncache.c b/virt/kvm/pfncache.c
index 72eee096a7cd..1719b0249dbc 100644
--- a/virt/kvm/pfncache.c
+++ b/virt/kvm/pfncache.c
@@ -228,6 +228,11 @@ int kvm_gfn_to_pfn_cache_refresh(struct kvm *kvm, struct gfn_to_pfn_cache *gpc,
        if (!old_valid || old_uhva != gpc->uhva) {
                void *new_khva = NULL;

+               /* blah blah blah */
+               gpc->valid = false;
+               gpc->pfn = KVM_PFN_ERR_FAULT;
+               gpc->khva = NULL;
+
                new_pfn = hva_to_pfn_retry(kvm, gpc);
                if (is_error_noslot_pfn(new_pfn)) {
                        ret = -EFAULT;
@@ -251,11 +256,7 @@ int kvm_gfn_to_pfn_cache_refresh(struct kvm *kvm, struct gfn_to_pfn_cache *gpc,
                        /* Nothing more to do, the pfn is consumed only by the guest. */
                }

-               if (ret) {
-                       gpc->valid = false;
-                       gpc->pfn = KVM_PFN_ERR_FAULT;
-                       gpc->khva = NULL;
-               } else {
+               if (!ret) {
                        gpc->valid = true;
                        gpc->pfn = new_pfn;
                        gpc->khva = new_khva;
@@ -283,6 +284,11 @@ void kvm_gfn_to_pfn_cache_unmap(struct kvm *kvm, struct gfn_to_pfn_cache *gpc)

        write_lock_irq(&gpc->lock);

+       if (!gpc->valid) {
+               write_unlock_irq(&gpc->lock);
+               return;
+       }
+
        gpc->valid = false;

        old_khva = gpc->khva - offset_in_page(gpc->khva);