Subject: Re: [PATCH net V2] vhost-vsock: fix use after free

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 27 Sep 2018 13:04:58 -0400

On Thu, Sep 27, 2018 at 08:22:04PM +0800, Jason Wang wrote:
> The access of vsock is not protected by vhost_vsock_lock. This may
> lead to use after free since vhost_vsock_dev_release() may free the
> pointer at the same time.
> 
> Fix this by holding the lock during the access.
> 
> Reported-by: [email-masked]
> Fixes: 16320f363ae1 ("vhost-vsock: add pkt cancel capability")
> Fixes: 433fc58e6bf2 ("VSOCK: Introduce vhost_vsock.ko")
> Cc: Stefan Hajnoczi <[email-masked]>
> Signed-off-by: Jason Wang <[email-masked]>

Wow is that really the best we can do? A global lock on a data path
operation? Granted use after free is nasty but Stefan said he sees
a way to fix it using a per socket refcount. He's on vacation
until Oct 4 though ...

> ---
> - V2: fix typos
> - The patch is needed for -stable.
> ---
>  drivers/vhost/vsock.c | 26 +++++++++++++++++++-------
>  1 file changed, 19 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
> index 34bc3ab40c6d..7d0b292867fd 100644
> --- a/drivers/vhost/vsock.c
> +++ b/drivers/vhost/vsock.c
> @@ -210,21 +210,27 @@ vhost_transport_send_pkt(struct virtio_vsock_pkt *pkt)
>  	struct vhost_vsock *vsock;
>  	int len = pkt->len;
>  
> +	spin_lock_bh(&vhost_vsock_lock);
> +
>  	/* Find the vhost_vsock according to guest context id  */
> -	vsock = vhost_vsock_get(le64_to_cpu(pkt->hdr.dst_cid));
> +	vsock = __vhost_vsock_get(le64_to_cpu(pkt->hdr.dst_cid));
>  	if (!vsock) {
>  		virtio_transport_free_pkt(pkt);
> +		spin_unlock_bh(&vhost_vsock_lock);
>  		return -ENODEV;
>  	}
>  
>  	if (pkt->reply)
>  		atomic_inc(&vsock->queued_replies);
>  
> -	spin_lock_bh(&vsock->send_pkt_list_lock);
> +	spin_lock(&vsock->send_pkt_list_lock);
>  	list_add_tail(&pkt->list, &vsock->send_pkt_list);
> -	spin_unlock_bh(&vsock->send_pkt_list_lock);
> +	spin_unlock(&vsock->send_pkt_list_lock);
>  
>  	vhost_work_queue(&vsock->dev, &vsock->send_pkt_work);
> +
> +	spin_unlock_bh(&vhost_vsock_lock);
> +
>  	return len;
>  }
>  
> @@ -236,18 +242,22 @@ vhost_transport_cancel_pkt(struct vsock_sock *vsk)
>  	int cnt = 0;
>  	LIST_HEAD(freeme);
>  
> +	spin_lock_bh(&vhost_vsock_lock);
> +
>  	/* Find the vhost_vsock according to guest context id  */
> -	vsock = vhost_vsock_get(vsk->remote_addr.svm_cid);
> -	if (!vsock)
> +	vsock = __vhost_vsock_get(vsk->remote_addr.svm_cid);
> +	if (!vsock) {
> +		spin_unlock_bh(&vhost_vsock_lock);
>  		return -ENODEV;
> +	}
>  
> -	spin_lock_bh(&vsock->send_pkt_list_lock);
> +	spin_lock(&vsock->send_pkt_list_lock);
>  	list_for_each_entry_safe(pkt, n, &vsock->send_pkt_list, list) {
>  		if (pkt->vsk != vsk)
>  			continue;
>  		list_move(&pkt->list, &freeme);
>  	}
> -	spin_unlock_bh(&vsock->send_pkt_list_lock);
> +	spin_unlock(&vsock->send_pkt_list_lock);
>  
>  	list_for_each_entry_safe(pkt, n, &freeme, list) {
>  		if (pkt->reply)
> @@ -265,6 +275,8 @@ vhost_transport_cancel_pkt(struct vsock_sock *vsk)
>  			vhost_poll_queue(&tx_vq->poll);
>  	}
>  
> +	spin_unlock_bh(&vhost_vsock_lock);
> +
>  	return 0;
>  }
>  
> -- 
> 2.17.1