linux-kernel - Re: [PATCH RFC 2/4] vhost/vsock: split packets to send using multiple buffers

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Date:   Fri, 5 Apr 2019 09:13:56 +0100
From:   Stefan Hajnoczi <stefanha@...il.com>
To:     Stefano Garzarella <sgarzare@...hat.com>
Cc:     netdev@...r.kernel.org, Jason Wang <jasowang@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH RFC 2/4] vhost/vsock: split packets to send using
 multiple buffers

On Thu, Apr 04, 2019 at 12:58:36PM +0200, Stefano Garzarella wrote:
> @@ -139,8 +139,18 @@ vhost_transport_do_send_pkt(struct vhost_vsock *vsock,
>  			break;
>  		}
>  
> -		len = iov_length(&vq->iov[out], in);
> -		iov_iter_init(&iov_iter, READ, &vq->iov[out], in, len);
> +		payload_len = pkt->len - pkt->off;
> +		iov_len = iov_length(&vq->iov[out], in);
> +		iov_iter_init(&iov_iter, READ, &vq->iov[out], in, iov_len);
> +
> +		/* If the packet is greater than the space available in the
> +		 * buffer, we split it using multiple buffers.
> +		 */
> +		if (payload_len > iov_len - sizeof(pkt->hdr))

Integer underflow.  iov_len is controlled by the guest and therefore
untrusted.  Please validate iov_len before assuming it's larger than
sizeof(pkt->hdr).

> -		vhost_add_used(vq, head, sizeof(pkt->hdr) + pkt->len);
> +		vhost_add_used(vq, head, sizeof(pkt->hdr) + payload_len);
>  		added = true;
>  
> +		pkt->off += payload_len;
> +
> +		/* If we didn't send all the payload we can requeue the packet
> +		 * to send it with the next available buffer.
> +		 */
> +		if (pkt->off < pkt->len) {
> +			spin_lock_bh(&vsock->send_pkt_list_lock);
> +			list_add(&pkt->list, &vsock->send_pkt_list);
> +			spin_unlock_bh(&vsock->send_pkt_list_lock);
> +			continue;

The virtio_transport_deliver_tap_pkt() call is skipped.  Packet capture
should see the exact packets that are delivered.  I think this patch
will present one large packet instead of several smaller packets that
were actually delivered.

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