KVM: SEV: Allow SEV intra-host migration of VM with mirrors

For SEV-ES VMs with mirrors to be intra-host migrated they need to be
able to migrate with the mirror. This is due to that fact that all VMSAs
need to be added into the VM with LAUNCH_UPDATE_VMSA before
lAUNCH_FINISH. Allowing migration with mirrors allows users of SEV-ES to
keep the mirror VMs VMSAs during migration.

Adds a list of mirror VMs for the original VM iterate through during its
migration. During the iteration the owner pointers can be updated from
the source to the destination. This fixes the ASID leaking issue which
caused the blocking of migration of VMs with mirrors.
Signed-off-by: Peter Gonda <pgonda@google.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Marc Orr <marcorr@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: kvm@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Message-Id: <20220211193634.3183388-1-pgonda@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm/sev.c

@@ -258,6 +258,7 @@ static int sev_guest_init(struct kvm *kvm, struct kvm_sev_cmd *argp)
 		goto e_free;
 
 	INIT_LIST_HEAD(&sev->regions_list);
+	INIT_LIST_HEAD(&sev->mirror_vms);
 
 	return 0;
 
@@ -1623,9 +1624,12 @@ static void sev_unlock_vcpus_for_migration(struct kvm *kvm)
 	}
 }
 
-static void sev_migrate_from(struct kvm_sev_info *dst,
-			      struct kvm_sev_info *src)
+static void sev_migrate_from(struct kvm *dst_kvm, struct kvm *src_kvm)
 {
+	struct kvm_sev_info *dst = &to_kvm_svm(dst_kvm)->sev_info;
+	struct kvm_sev_info *src = &to_kvm_svm(src_kvm)->sev_info;
+	struct kvm_sev_info *mirror;
+
 	dst->active = true;
 	dst->asid = src->asid;
 	dst->handle = src->handle;
@@ -1639,6 +1643,30 @@ static void sev_migrate_from(struct kvm_sev_info *dst,
 	src->enc_context_owner = NULL;
 
 	list_cut_before(&dst->regions_list, &src->regions_list, &src->regions_list);
+
+	/*
+	 * If this VM has mirrors, "transfer" each mirror's refcount of the
+	 * source to the destination (this KVM).  The caller holds a reference
+	 * to the source, so there's no danger of use-after-free.
+	 */
+	list_cut_before(&dst->mirror_vms, &src->mirror_vms, &src->mirror_vms);
+	list_for_each_entry(mirror, &dst->mirror_vms, mirror_entry) {
+		kvm_get_kvm(dst_kvm);
+		kvm_put_kvm(src_kvm);
+		mirror->enc_context_owner = dst_kvm;
+	}
+
+	/*
+	 * If this VM is a mirror, remove the old mirror from the owners list
+	 * and add the new mirror to the list.
+	 */
+	if (is_mirroring_enc_context(dst_kvm)) {
+		struct kvm_sev_info *owner_sev_info =
+			&to_kvm_svm(dst->enc_context_owner)->sev_info;
+
+		list_del(&src->mirror_entry);
+		list_add_tail(&dst->mirror_entry, &owner_sev_info->mirror_vms);
+	}
 }
 
 static int sev_es_migrate_from(struct kvm *dst, struct kvm *src)
@@ -1708,15 +1736,6 @@ int sev_vm_move_enc_context_from(struct kvm *kvm, unsigned int source_fd)
 
 	src_sev = &to_kvm_svm(source_kvm)->sev_info;
 
-	/*
-	 * VMs mirroring src's encryption context rely on it to keep the
-	 * ASID allocated, but below we are clearing src_sev->asid.
-	 */
-	if (src_sev->num_mirrored_vms) {
-		ret = -EBUSY;
-		goto out_unlock;
-	}
-
 	dst_sev->misc_cg = get_current_misc_cg();
 	cg_cleanup_sev = dst_sev;
 	if (dst_sev->misc_cg != src_sev->misc_cg) {
@@ -1738,7 +1757,8 @@ int sev_vm_move_enc_context_from(struct kvm *kvm, unsigned int source_fd)
 		if (ret)
 			goto out_source_vcpu;
 	}
-	sev_migrate_from(dst_sev, src_sev);
+
+	sev_migrate_from(kvm, source_kvm);
 	kvm_vm_dead(source_kvm);
 	cg_cleanup_sev = src_sev;
 	ret = 0;
@@ -2008,10 +2028,10 @@ int sev_vm_copy_enc_context_from(struct kvm *kvm, unsigned int source_fd)
 	 */
 	source_sev = &to_kvm_svm(source_kvm)->sev_info;
 	kvm_get_kvm(source_kvm);
-	source_sev->num_mirrored_vms++;
+	mirror_sev = &to_kvm_svm(kvm)->sev_info;
+	list_add_tail(&mirror_sev->mirror_entry, &source_sev->mirror_vms);
 
 	/* Set enc_context_owner and copy its encryption context over */
-	mirror_sev = &to_kvm_svm(kvm)->sev_info;
 	mirror_sev->enc_context_owner = source_kvm;
 	mirror_sev->active = true;
 	mirror_sev->asid = source_sev->asid;
@@ -2019,6 +2039,7 @@ int sev_vm_copy_enc_context_from(struct kvm *kvm, unsigned int source_fd)
 	mirror_sev->es_active = source_sev->es_active;
 	mirror_sev->handle = source_sev->handle;
 	INIT_LIST_HEAD(&mirror_sev->regions_list);
+	INIT_LIST_HEAD(&mirror_sev->mirror_vms);
 	ret = 0;
 
 	/*
@@ -2041,19 +2062,17 @@ void sev_vm_destroy(struct kvm *kvm)
 	struct list_head *head = &sev->regions_list;
 	struct list_head *pos, *q;
 
-	WARN_ON(sev->num_mirrored_vms);
-
 	if (!sev_guest(kvm))
 		return;
 
+	WARN_ON(!list_empty(&sev->mirror_vms));
+
 	/* If this is a mirror_kvm release the enc_context_owner and skip sev cleanup */
 	if (is_mirroring_enc_context(kvm)) {
 		struct kvm *owner_kvm = sev->enc_context_owner;
-		struct kvm_sev_info *owner_sev = &to_kvm_svm(owner_kvm)->sev_info;
 
 		mutex_lock(&owner_kvm->lock);
-		if (!WARN_ON(!owner_sev->num_mirrored_vms))
-			owner_sev->num_mirrored_vms--;
+		list_del(&sev->mirror_entry);
 		mutex_unlock(&owner_kvm->lock);
 		kvm_put_kvm(owner_kvm);
 		return;

arch/x86/kvm/svm/svm.h

@@ -79,7 +79,8 @@ struct kvm_sev_info {
 	struct list_head regions_list;  /* List of registered regions */
 	u64 ap_jump_table;	/* SEV-ES AP Jump Table address */
 	struct kvm *enc_context_owner; /* Owner of copied encryption context */
-	unsigned long num_mirrored_vms; /* Number of VMs sharing this ASID */
+	struct list_head mirror_vms; /* List of VMs mirroring */
+	struct list_head mirror_entry; /* Use as a list entry of mirrors */
 	struct misc_cg *misc_cg; /* For misc cgroup accounting */
 	atomic_t migration_in_progress;
 };

tools/testing/selftests/kvm/x86_64/sev_migrate_tests.c

@@ -341,35 +341,54 @@ static void test_sev_mirror_parameters(void)
 
 static void test_sev_move_copy(void)
 {
-	struct kvm_vm *dst_vm, *sev_vm, *mirror_vm, *dst_mirror_vm;
-	int ret;
+	struct kvm_vm *dst_vm, *dst2_vm, *dst3_vm, *sev_vm, *mirror_vm,
+		      *dst_mirror_vm, *dst2_mirror_vm, *dst3_mirror_vm;
 
 	sev_vm = sev_vm_create(/* es= */ false);
 	dst_vm = aux_vm_create(true);
+	dst2_vm = aux_vm_create(true);
+	dst3_vm = aux_vm_create(true);
 	mirror_vm = aux_vm_create(false);
 	dst_mirror_vm = aux_vm_create(false);
+	dst2_mirror_vm = aux_vm_create(false);
+	dst3_mirror_vm = aux_vm_create(false);
 
 	sev_mirror_create(mirror_vm->fd, sev_vm->fd);
-	ret = __sev_migrate_from(dst_vm->fd, sev_vm->fd);
-	TEST_ASSERT(ret == -1 && errno == EBUSY,
-		    "Cannot migrate VM that has mirrors. ret %d, errno: %d\n", ret,
-		    errno);
 
-	/* The mirror itself can be migrated.  */
 	sev_migrate_from(dst_mirror_vm->fd, mirror_vm->fd);
-	ret = __sev_migrate_from(dst_vm->fd, sev_vm->fd);
-	TEST_ASSERT(ret == -1 && errno == EBUSY,
-		    "Cannot migrate VM that has mirrors. ret %d, errno: %d\n", ret,
-		    errno);
+	sev_migrate_from(dst_vm->fd, sev_vm->fd);
+
+	sev_migrate_from(dst2_vm->fd, dst_vm->fd);
+	sev_migrate_from(dst2_mirror_vm->fd, dst_mirror_vm->fd);
+
+	sev_migrate_from(dst3_mirror_vm->fd, dst2_mirror_vm->fd);
+	sev_migrate_from(dst3_vm->fd, dst2_vm->fd);
+
+	kvm_vm_free(dst_vm);
+	kvm_vm_free(sev_vm);
+	kvm_vm_free(dst2_vm);
+	kvm_vm_free(dst3_vm);
+	kvm_vm_free(mirror_vm);
+	kvm_vm_free(dst_mirror_vm);
+	kvm_vm_free(dst2_mirror_vm);
+	kvm_vm_free(dst3_mirror_vm);
 
 	/*
-	 * mirror_vm is not a mirror anymore, dst_mirror_vm is.  Thus,
-	 * the owner can be copied as soon as dst_mirror_vm is gone.
+	 * Run similar test be destroy mirrors before mirrored VMs to ensure
+	 * destruction is done safely.
 	 */
-	kvm_vm_free(dst_mirror_vm);
+	sev_vm = sev_vm_create(/* es= */ false);
+	dst_vm = aux_vm_create(true);
+	mirror_vm = aux_vm_create(false);
+	dst_mirror_vm = aux_vm_create(false);
+
+	sev_mirror_create(mirror_vm->fd, sev_vm->fd);
+
+	sev_migrate_from(dst_mirror_vm->fd, mirror_vm->fd);
 	sev_migrate_from(dst_vm->fd, sev_vm->fd);
 
 	kvm_vm_free(mirror_vm);
+	kvm_vm_free(dst_mirror_vm);
 	kvm_vm_free(dst_vm);
 	kvm_vm_free(sev_vm);
 }