Re: [PATCH v2 1/3] vsock/virtio: use RCU to avoid use-after-free on the_virtio_vsock

From: Jason Wang <jasowang@redhat.com>
To: Stefano Garzarella <sgarzare@redhat.com>,
	Stefan Hajnoczi <stefanha@redhat.com>
Cc: netdev@vger.kernel.org, kvm@vger.kernel.org,
	virtualization@lists.linux-foundation.org,
	"Michael S. Tsirkin" <mst@redhat.com>,
	"David S. Miller" <davem@davemloft.net>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 1/3] vsock/virtio: use RCU to avoid use-after-free on the_virtio_vsock
Date: Thu, 4 Jul 2019 11:58:00 +0800	[thread overview]
Message-ID: <07e5bc00-ebde-4dac-d38c-f008fa230b5f@redhat.com> (raw)
In-Reply-To: <20190703104135.wg34dobv64k7u4jo@steredhat>

On 2019/7/3 下午6:41, Stefano Garzarella wrote:
> On Wed, Jul 03, 2019 at 05:53:58PM +0800, Jason Wang wrote:
>> On 2019/6/28 下午8:36, Stefano Garzarella wrote:
>>> Some callbacks used by the upper layers can run while we are in the
>>> .remove(). A potential use-after-free can happen, because we free
>>> the_virtio_vsock without knowing if the callbacks are over or not.
>>>
>>> To solve this issue we move the assignment of the_virtio_vsock at the
>>> end of .probe(), when we finished all the initialization, and at the
>>> beginning of .remove(), before to release resources.
>>> For the same reason, we do the same also for the vdev->priv.
>>>
>>> We use RCU to be sure that all callbacks that use the_virtio_vsock
>>> ended before freeing it. This is not required for callbacks that
>>> use vdev->priv, because after the vdev->config->del_vqs() we are sure
>>> that they are ended and will no longer be invoked.
>>>
>>> We also take the mutex during the .remove() to avoid that .probe() can
>>> run while we are resetting the device.
>>>
>>> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
>>> ---
>>>    net/vmw_vsock/virtio_transport.c | 67 +++++++++++++++++++++-----------
>>>    1 file changed, 44 insertions(+), 23 deletions(-)
>>>
>>> diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
>>> index 9c287e3e393c..7ad510ec12e0 100644
>>> --- a/net/vmw_vsock/virtio_transport.c
>>> +++ b/net/vmw_vsock/virtio_transport.c
>>> @@ -65,19 +65,22 @@ struct virtio_vsock {
>>>    	u32 guest_cid;
>>>    };
>>> -static struct virtio_vsock *virtio_vsock_get(void)
>>> -{
>>> -	return the_virtio_vsock;
>>> -}
>>> -
>>>    static u32 virtio_transport_get_local_cid(void)
>>>    {
>>> -	struct virtio_vsock *vsock = virtio_vsock_get();
>>> +	struct virtio_vsock *vsock;
>>> +	u32 ret;
>>> -	if (!vsock)
>>> -		return VMADDR_CID_ANY;
>>> +	rcu_read_lock();
>>> +	vsock = rcu_dereference(the_virtio_vsock);
>>> +	if (!vsock) {
>>> +		ret = VMADDR_CID_ANY;
>>> +		goto out_rcu;
>>> +	}
>>> -	return vsock->guest_cid;
>>> +	ret = vsock->guest_cid;
>>> +out_rcu:
>>> +	rcu_read_unlock();
>>> +	return ret;
>>>    }
>>>    static void virtio_transport_loopback_work(struct work_struct *work)
>>> @@ -197,14 +200,18 @@ virtio_transport_send_pkt(struct virtio_vsock_pkt *pkt)
>>>    	struct virtio_vsock *vsock;
>>>    	int len = pkt->len;
>>> -	vsock = virtio_vsock_get();
>>> +	rcu_read_lock();
>>> +	vsock = rcu_dereference(the_virtio_vsock);
>>>    	if (!vsock) {
>>>    		virtio_transport_free_pkt(pkt);
>>> -		return -ENODEV;
>>> +		len = -ENODEV;
>>> +		goto out_rcu;
>>>    	}
>>> -	if (le64_to_cpu(pkt->hdr.dst_cid) == vsock->guest_cid)
>>> -		return virtio_transport_send_pkt_loopback(vsock, pkt);
>>> +	if (le64_to_cpu(pkt->hdr.dst_cid) == vsock->guest_cid) {
>>> +		len = virtio_transport_send_pkt_loopback(vsock, pkt);
>>> +		goto out_rcu;
>>> +	}
>>>    	if (pkt->reply)
>>>    		atomic_inc(&vsock->queued_replies);
>>> @@ -214,6 +221,9 @@ virtio_transport_send_pkt(struct virtio_vsock_pkt *pkt)
>>>    	spin_unlock_bh(&vsock->send_pkt_list_lock);
>>>    	queue_work(virtio_vsock_workqueue, &vsock->send_pkt_work);
>>> +
>>> +out_rcu:
>>> +	rcu_read_unlock();
>>>    	return len;
>>>    }
>>> @@ -222,12 +232,14 @@ virtio_transport_cancel_pkt(struct vsock_sock *vsk)
>>>    {
>>>    	struct virtio_vsock *vsock;
>>>    	struct virtio_vsock_pkt *pkt, *n;
>>> -	int cnt = 0;
>>> +	int cnt = 0, ret;
>>>    	LIST_HEAD(freeme);
>>> -	vsock = virtio_vsock_get();
>>> +	rcu_read_lock();
>>> +	vsock = rcu_dereference(the_virtio_vsock);
>>>    	if (!vsock) {
>>> -		return -ENODEV;
>>> +		ret = -ENODEV;
>>> +		goto out_rcu;
>>>    	}
>>>    	spin_lock_bh(&vsock->send_pkt_list_lock);
>>> @@ -255,7 +267,11 @@ virtio_transport_cancel_pkt(struct vsock_sock *vsk)
>>>    			queue_work(virtio_vsock_workqueue, &vsock->rx_work);
>>>    	}
>>> -	return 0;
>>> +	ret = 0;
>>> +
>>> +out_rcu:
>>> +	rcu_read_unlock();
>>> +	return ret;
>>>    }
>>>    static void virtio_vsock_rx_fill(struct virtio_vsock *vsock)
>>> @@ -590,8 +606,6 @@ static int virtio_vsock_probe(struct virtio_device *vdev)
>>>    	vsock->rx_buf_max_nr = 0;
>>>    	atomic_set(&vsock->queued_replies, 0);
>>> -	vdev->priv = vsock;
>>> -	the_virtio_vsock = vsock;
>>>    	mutex_init(&vsock->tx_lock);
>>>    	mutex_init(&vsock->rx_lock);
>>>    	mutex_init(&vsock->event_lock);
>>> @@ -613,6 +627,9 @@ static int virtio_vsock_probe(struct virtio_device *vdev)
>>>    	virtio_vsock_event_fill(vsock);
>>>    	mutex_unlock(&vsock->event_lock);
>>> +	vdev->priv = vsock;
>>> +	rcu_assign_pointer(the_virtio_vsock, vsock);
>>
>> You probably need to use rcu_dereference_protected() to access
>> the_virtio_vsock in the function in order to survive from sparse.
>>
> Ooo, thanks!
>
> Do you mean when we check if the_virtio_vsock is not null at the beginning of
> virtio_vsock_probe()?

I mean instead of:

     /* Only one virtio-vsock device per guest is supported */
     if (the_virtio_vsock) {
         ret = -EBUSY;
         goto out;
     }

you should use:

if (rcu_dereference_protected(the_virtio_vosck, 
lock_dep_is_held(&the_virtio_vsock_mutex))

...

>
>>> +
>>>    	mutex_unlock(&the_virtio_vsock_mutex);
>>>    	return 0;
>>> @@ -627,6 +644,12 @@ static void virtio_vsock_remove(struct virtio_device *vdev)
>>>    	struct virtio_vsock *vsock = vdev->priv;
>>>    	struct virtio_vsock_pkt *pkt;
>>> +	mutex_lock(&the_virtio_vsock_mutex);
>>> +
>>> +	vdev->priv = NULL;
>>> +	rcu_assign_pointer(the_virtio_vsock, NULL);
>>
>> This is still suspicious, can we access the_virtio_vsock through vdev->priv?
>> If yes, we may still get use-after-free since it was not protected by RCU.
> We will free the object only after calling the del_vqs(), so we are sure
> that the vq_callbacks ended and will no longer be invoked.
> So, IIUC it shouldn't happen.

Yes, but any dereference that is not done in vq_callbacks will be very 
dangerous in the future.

Thanks

>
>> Another more interesting question, I believe we will do singleton for
>> virtio_vsock structure. Then what's the point of using vdev->priv to access
>> the_virtio_vsock? It looks to me we can it brings extra troubles for doing
>> synchronization.
> I thought about it when I tried to use RCU to stop the worker and I
> think make sense. Maybe can be another series after this will be merged.
>
> @Stefan, what do you think about that?
>
> Thanks,
> Stefano