vfio: Add missing locking for struct vfio_group::kvm

Without locking userspace can trigger a UAF by racing
KVM_DEV_VFIO_GROUP_DEL with VFIO_GROUP_GET_DEVICE_FD:

              CPU1                               CPU2
					    ioctl(KVM_DEV_VFIO_GROUP_DEL)
 ioctl(VFIO_GROUP_GET_DEVICE_FD)
    vfio_group_get_device_fd
     open_device()
      intel_vgpu_open_device()
        vfio_register_notifier()
	 vfio_register_group_notifier()
	   blocking_notifier_call_chain(&group->notifier,
               VFIO_GROUP_NOTIFY_SET_KVM, group->kvm);

					      set_kvm()
						group->kvm = NULL
					    close()
					     kfree(kvm)

             intel_vgpu_group_notifier()
                vdev->kvm = data
    [..]
        kvm_get_kvm(vgpu->kvm);
	    // UAF!

Add a simple rwsem in the group to protect the kvm while the notifier is
using it.

Note this doesn't fix the race internal to i915 where userspace can
trigger two VFIO_GROUP_NOTIFY_SET_KVM's before we reach a consumer of
vgpu->kvm and trigger this same UAF, it just makes the notifier
self-consistent.

Fixes: ccd46db ("vfio: support notifier chain in vfio_group")
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Tested-by: Matthew Rosato <mjrosato@linux.ibm.com>
Link: https://lore.kernel.org/r/1-v2-d035a1842d81+1bf-vfio_group_locking_jgg@nvidia.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/vfio/vfio.c

@@ -76,6 +76,7 @@ struct vfio_group {
 	atomic_t			opened;
 	enum vfio_group_type		type;
 	unsigned int			dev_counter;
+	struct rw_semaphore		group_rwsem;
 	struct kvm			*kvm;
 	struct blocking_notifier_head	notifier;
 };
@@ -360,6 +361,7 @@ static struct vfio_group *vfio_group_alloc(struct iommu_group *iommu_group,
 	group->cdev.owner = THIS_MODULE;
 
 	refcount_set(&group->users, 1);
+	init_rwsem(&group->group_rwsem);
 	INIT_LIST_HEAD(&group->device_list);
 	mutex_init(&group->device_lock);
 	group->iommu_group = iommu_group;
@@ -1694,9 +1696,11 @@ void vfio_file_set_kvm(struct file *file, struct kvm *kvm)
 	if (file->f_op != &vfio_group_fops)
 		return;
 
+	down_write(&group->group_rwsem);
 	group->kvm = kvm;
 	blocking_notifier_call_chain(&group->notifier,
 				     VFIO_GROUP_NOTIFY_SET_KVM, kvm);
+	up_write(&group->group_rwsem);
 }
 EXPORT_SYMBOL_GPL(vfio_file_set_kvm);
 
@@ -2004,15 +2008,22 @@ static int vfio_register_group_notifier(struct vfio_group *group,
 		return -EINVAL;
 
 	ret = blocking_notifier_chain_register(&group->notifier, nb);
+	if (ret)
+		return ret;
 
 	/*
 	 * The attaching of kvm and vfio_group might already happen, so
 	 * here we replay once upon registration.
 	 */
-	if (!ret && set_kvm && group->kvm)
-		blocking_notifier_call_chain(&group->notifier,
-					VFIO_GROUP_NOTIFY_SET_KVM, group->kvm);
-	return ret;
+	if (set_kvm) {
+		down_read(&group->group_rwsem);
+		if (group->kvm)
+			blocking_notifier_call_chain(&group->notifier,
+						     VFIO_GROUP_NOTIFY_SET_KVM,
+						     group->kvm);
+		up_read(&group->group_rwsem);
+	}
+	return 0;
 }
 
 int vfio_register_notifier(struct vfio_device *device,