KVM: SEV: Prohibit migration of a VM that has mirrors

VMs that mirror an encryption context rely on the owner to keep the
ASID allocated.  Performing a KVM_CAP_VM_MOVE_ENC_CONTEXT_FROM
would cause a dangling ASID:

1. copy context from A to B (gets ref to A)
2. move context from A to L (moves ASID from A to L)
3. close L (releases ASID from L, B still references it)

The right way to do the handoff instead is to create a fresh mirror VM
on the destination first:

1. copy context from A to B (gets ref to A)
[later] 2. close B (releases ref to A)
3. move context from A to L (moves ASID from A to L)
4. copy context from L to M

So, catch the situation by adding a count of how many VMs are
mirroring this one's encryption context.

Fixes: 0b020f5af0 ("KVM: SEV: Add support for SEV-ES intra host migration")
Message-Id: <20211123005036.2954379-11-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm/sev.c

@@ -1696,6 +1696,16 @@ int svm_vm_migrate_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) {
@@ -1987,6 +1997,7 @@ int svm_vm_copy_asid_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++;
 
 	/* Set enc_context_owner and copy its encryption context over */
 	mirror_sev = &to_kvm_svm(kvm)->sev_info;
@@ -2019,12 +2030,21 @@ 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;
 
 	/* If this is a mirror_kvm release the enc_context_owner and skip sev cleanup */
 	if (is_mirroring_enc_context(kvm)) {
-		kvm_put_kvm(sev->enc_context_owner);
+		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--;
+		mutex_unlock(&owner_kvm->lock);
+		kvm_put_kvm(owner_kvm);
 		return;
 	}
 

arch/x86/kvm/svm/svm.h

@@ -79,6 +79,7 @@ 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 misc_cg *misc_cg; /* For misc cgroup accounting */
 	atomic_t migration_in_progress;
 };

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

@@ -294,6 +294,41 @@ static void test_sev_mirror_parameters(void)
 	kvm_vm_free(vm_no_vcpu);
 }
 
+static void test_sev_move_copy(void)
+{
+	struct kvm_vm *dst_vm, *sev_vm, *mirror_vm, *dst_mirror_vm;
+	int ret;
+
+	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);
+	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);
+
+	/*
+	 * 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.
+	 */
+	kvm_vm_free(dst_mirror_vm);
+	sev_migrate_from(dst_vm->fd, sev_vm->fd);
+
+	kvm_vm_free(mirror_vm);
+	kvm_vm_free(dst_vm);
+	kvm_vm_free(sev_vm);
+}
+
 int main(int argc, char *argv[])
 {
 	if (kvm_check_cap(KVM_CAP_VM_MOVE_ENC_CONTEXT_FROM)) {
@@ -301,6 +336,8 @@ int main(int argc, char *argv[])
 		test_sev_migrate_from(/* es= */ true);
 		test_sev_migrate_locking();
 		test_sev_migrate_parameters();
+		if (kvm_check_cap(KVM_CAP_VM_COPY_ENC_CONTEXT_FROM))
+			test_sev_move_copy();
 	}
 	if (kvm_check_cap(KVM_CAP_VM_COPY_ENC_CONTEXT_FROM)) {
 		test_sev_mirror(/* es= */ false);