KVM: SVM: Issue WBINVD after deactivating an SEV guest

Currently, CLFLUSH is used to flush SEV guest memory before the guest is
terminated (or a memory hotplug region is removed). However, CLFLUSH is
not enough to ensure that SEV guest tagged data is flushed from the cache.

With 33af3a7ef9 ("KVM: SVM: Reduce WBINVD/DF_FLUSH invocations"), the
original WBINVD was removed. This then exposed crashes at random times
because of a cache flush race with a page that had both a hypervisor and
a guest tag in the cache.

Restore the WBINVD when destroying an SEV guest and add a WBINVD to the
svm_unregister_enc_region() function to ensure hotplug memory is flushed
when removed. The DF_FLUSH can still be avoided at this point.

Fixes: 33af3a7ef9 ("KVM: SVM: Reduce WBINVD/DF_FLUSH invocations")
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Message-Id: <c8bf9087ca3711c5770bdeaafa3e45b717dc5ef4.1584720426.git.thomas.lendacky@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm.c

@@ -1933,14 +1933,6 @@ static void sev_clflush_pages(struct page *pages[], unsigned long npages)
 static void __unregister_enc_region_locked(struct kvm *kvm,
 					   struct enc_region *region)
 {
-	/*
-	 * The guest may change the memory encryption attribute from C=0 -> C=1
-	 * or vice versa for this memory range. Lets make sure caches are
-	 * flushed to ensure that guest data gets written into memory with
-	 * correct C-bit.
-	 */
-	sev_clflush_pages(region->pages, region->npages);
-
 	sev_unpin_memory(kvm, region->pages, region->npages);
 	list_del(&region->list);
 	kfree(region);
@@ -1970,6 +1962,13 @@ static void sev_vm_destroy(struct kvm *kvm)
 
 	mutex_lock(&kvm->lock);
 
+	/*
+	 * Ensure that all guest tagged cache entries are flushed before
+	 * releasing the pages back to the system for use. CLFLUSH will
+	 * not do this, so issue a WBINVD.
+	 */
+	wbinvd_on_all_cpus();
+
 	/*
 	 * if userspace was terminated before unregistering the memory regions
 	 * then lets unpin all the registered memory.
@@ -7288,6 +7287,13 @@ static int svm_unregister_enc_region(struct kvm *kvm,
 		goto failed;
 	}
 
+	/*
+	 * Ensure that all guest tagged cache entries are flushed before
+	 * releasing the pages back to the system for use. CLFLUSH will
+	 * not do this, so issue a WBINVD.
+	 */
+	wbinvd_on_all_cpus();
+
 	__unregister_enc_region_locked(kvm, region);
 
 	mutex_unlock(&kvm->lock);