KVM: avoid double put_page with gfn-to-pfn cache

If the cache's user host virtual address becomes invalid, there
is still a path from kvm_gfn_to_pfn_cache_refresh() where __release_gpc()
could release the pfn but the gpc->pfn field has not been overwritten
with an error value.  If this happens, kvm_gfn_to_pfn_cache_unmap will
call put_page again on the same page.

Cc: stable@vger.kernel.org
Fixes: 982ed0de ("KVM: Reinstate gfn_to_pfn_cache with invalidation support")
Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/pfncache.c

@@ -191,6 +191,7 @@ int kvm_gfn_to_pfn_cache_refresh(struct kvm *kvm, struct gfn_to_pfn_cache *gpc,
 		gpc->uhva = gfn_to_hva_memslot(gpc->memslot, gfn);
 
 		if (kvm_is_error_hva(gpc->uhva)) {
+			gpc->pfn = KVM_PFN_ERR_FAULT;
 			ret = -EFAULT;
 			goto out;
 		}