vhost_net: fix possible infinite loop

BugLink: https://bugs.launchpad.net/bugs/1838824
commit e2412c07f8f3040593dfb88207865a3cd58680c0 upstream.

When the rx buffer is too small for a packet, we will discard the vq
descriptor and retry it for the next packet:

while ((sock_len = vhost_net_rx_peek_head_len(net, sock->sk,
					      &busyloop_intr))) {
...
	/* On overrun, truncate and discard */
	if (unlikely(headcount > UIO_MAXIOV)) {
		iov_iter_init(&msg.msg_iter, READ, vq->iov, 1, 1);
		err = sock->ops->recvmsg(sock, &msg,
					 1, MSG_DONTWAIT | MSG_TRUNC);
		pr_debug("Discarded rx packet: len %zd\n", sock_len);
		continue;
	}
...
}

This makes it possible to trigger a infinite while..continue loop
through the co-opreation of two VMs like:

1) Malicious VM1 allocate 1 byte rx buffer and try to slow down the
   vhost process as much as possible e.g using indirect descriptors or
   other.
2) Malicious VM2 generate packets to VM1 as fast as possible

Fixing this by checking against weight at the end of RX and TX
loop. This also eliminate other similar cases when:

- userspace is consuming the packets in the meanwhile
- theoretical TOCTOU attack if guest moving avail index back and forth
  to hit the continue after vhost find guest just add new buffers

This addresses CVE-2019-3900.

Fixes: d8316f3991d20 ("vhost: fix total length when packets are too short")
Fixes: 3a4d5c94e9593 ("vhost_net: a kernel-level virtio server")
Signed-off-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Balbir Singh <sblbir@amzn.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 7c0fcda639959ec11046f8f8c09881055794d881..a23c7e7c16521e6b22a54a63347e5a9b1cdef6fd 100644 (file)
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -487,7 +487,7 @@ static void handle_tx(struct vhost_net *net)
        hdr_size = nvq->vhost_hlen;
        zcopy = nvq->ubufs;
 
-       for (;;) {
+       do {
                bool busyloop_intr;
 
                /* Release DMAs done buffers first */
@@ -582,10 +582,7 @@ static void handle_tx(struct vhost_net *net)
                else
                        vhost_zerocopy_signal_used(net, vq);
                vhost_net_tx_packet(net);
-               if (unlikely(vhost_exceeds_weight(vq, ++sent_pkts,
-                                                 total_len)))
-                       break;
-       }
+       } while (likely(!vhost_exceeds_weight(vq, ++sent_pkts, total_len)));
 out:
        mutex_unlock(&vq->mutex);
 }
@@ -784,7 +781,11 @@ static void handle_rx(struct vhost_net *net)
                vq->log : NULL;
        mergeable = vhost_has_feature(vq, VIRTIO_NET_F_MRG_RXBUF);
 
-       while ((sock_len = vhost_net_rx_peek_head_len(net, sock->sk))) {
+       do {
+               sock_len = vhost_net_rx_peek_head_len(net, sock->sk);
+
+               if (!sock_len)
+                       break;
                sock_len += sock_hlen;
                vhost_len = sock_len + vhost_hlen;
                headcount = get_rx_bufs(vq, vq->heads, vhost_len,
@@ -865,9 +866,8 @@ static void handle_rx(struct vhost_net *net)
                        vhost_log_write(vq, vq_log, log, vhost_len,
                                        vq->iov, in);
                total_len += vhost_len;
-               if (unlikely(vhost_exceeds_weight(vq, ++recv_pkts, total_len)))
-                       goto out;
-       }
+       } while (likely(!vhost_exceeds_weight(vq, ++recv_pkts, total_len)));
+
        vhost_net_enable_vq(net, vq);
 out:
        mutex_unlock(&vq->mutex);
@@ -946,7 +946,7 @@ static int vhost_net_open(struct inode *inode, struct file *f)
                vhost_net_buf_init(&n->vqs[i].rxq);
        }
        vhost_dev_init(dev, vqs, VHOST_NET_VQ_MAX,
-                      VHOST_NET_WEIGHT, VHOST_NET_PKT_WEIGHT);
+                      VHOST_NET_PKT_WEIGHT, VHOST_NET_WEIGHT);
 
        vhost_poll_init(n->poll + VHOST_NET_VQ_TX, handle_tx_net, POLLOUT, dev);
        vhost_poll_init(n->poll + VHOST_NET_VQ_RX, handle_rx_net, POLLIN, dev);