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SubjectRe: [PATCH net] vhost_net: fix possible infinite loop
On Thu, Apr 25, 2019 at 03:33:19AM -0400, Jason Wang wrote:
> 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")

I agree this is the real issue.

> Fixes: 3a4d5c94e9593 ("vhost_net: a kernel-level virtio server")

This is just a red herring imho. We can stick this on any vhost patch :)

> Signed-off-by: Jason Wang <jasowang@redhat.com>

> ---
> drivers/vhost/net.c | 41 +++++++++++++++++++++--------------------
> 1 file changed, 21 insertions(+), 20 deletions(-)
>
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index df51a35..fb46e6b 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -778,8 +778,9 @@ static void handle_tx_copy(struct vhost_net *net, struct socket *sock)
> int err;
> int sent_pkts = 0;
> bool sock_can_batch = (sock->sk->sk_sndbuf == INT_MAX);
> + bool next_round = false;
>
> - for (;;) {
> + do {
> bool busyloop_intr = false;
>
> if (nvq->done_idx == VHOST_NET_BATCH)
> @@ -845,11 +846,10 @@ static void handle_tx_copy(struct vhost_net *net, struct socket *sock)
> vq->heads[nvq->done_idx].id = cpu_to_vhost32(vq, head);
> vq->heads[nvq->done_idx].len = 0;
> ++nvq->done_idx;
> - if (vhost_exceeds_weight(++sent_pkts, total_len)) {
> - vhost_poll_queue(&vq->poll);
> - break;
> - }
> - }
> + } while (!(next_round = vhost_exceeds_weight(++sent_pkts, total_len)));
> +
> + if (next_round)
> + vhost_poll_queue(&vq->poll);
>
> vhost_tx_batch(net, nvq, sock, &msg);
> }
> @@ -873,8 +873,9 @@ static void handle_tx_zerocopy(struct vhost_net *net, struct socket *sock)
> struct vhost_net_ubuf_ref *uninitialized_var(ubufs);
> bool zcopy_used;
> int sent_pkts = 0;
> + bool next_round = false;
>
> - for (;;) {
> + do {
> bool busyloop_intr;
>
> /* Release DMAs done buffers first */
> @@ -951,11 +952,10 @@ static void handle_tx_zerocopy(struct vhost_net *net, struct socket *sock)
> else
> vhost_zerocopy_signal_used(net, vq);
> vhost_net_tx_packet(net);
> - if (unlikely(vhost_exceeds_weight(++sent_pkts, total_len))) {
> - vhost_poll_queue(&vq->poll);
> - break;
> - }
> - }
> + } while (!(next_round = vhost_exceeds_weight(++sent_pkts, total_len)));
> +
> + if (next_round)
> + vhost_poll_queue(&vq->poll);
> }
>
> /* Expects to be always run from workqueue - which acts as
> @@ -1134,6 +1134,7 @@ static void handle_rx(struct vhost_net *net)
> struct iov_iter fixup;
> __virtio16 num_buffers;
> int recv_pkts = 0;
> + bool next_round = false;
>
> mutex_lock_nested(&vq->mutex, VHOST_NET_VQ_RX);
> sock = vq->private_data;
> @@ -1153,8 +1154,11 @@ 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,
> - &busyloop_intr))) {
> + do {
> + sock_len = vhost_net_rx_peek_head_len(net, sock->sk,
> + &busyloop_intr);
> + if (!sock_len)
> + break;
> sock_len += sock_hlen;
> vhost_len = sock_len + vhost_hlen;
> headcount = get_rx_bufs(vq, vq->heads + nvq->done_idx,
> @@ -1239,12 +1243,9 @@ 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(++recv_pkts, total_len))) {
> - vhost_poll_queue(&vq->poll);
> - goto out;
> - }
> - }
> - if (unlikely(busyloop_intr))
> + } while (!(next_round = vhost_exceeds_weight(++recv_pkts, total_len)));
> +
> + if (unlikely(busyloop_intr || next_round))
> vhost_poll_queue(&vq->poll);
> else
> vhost_net_enable_vq(net, vq);


I'm afraid with this addition the code is too much like spagetty. What
does next_round mean? Just that we are breaking out of loop? That is
what goto is for... Either let's have for(;;) with goto/break to get
outside or a while loop with a condition. Both is just unreadable.

All these checks in 3 places are exactly the same on all paths and they
are slow path. Why don't we put this in a function? E.g. like the below.
Warning: completely untested.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

---

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index df51a35cf537..a0f89a504cd9 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -761,6 +761,23 @@ static int vhost_net_build_xdp(struct vhost_net_virtqueue *nvq,
return 0;
}

+/* Returns true if caller needs to go back and re-read the ring. */
+static bool empty_ring(struct vhost_net *net, struct vhost_virtqueue *vq,
+ int pkts, size_t total_len, bool busyloop_intr)
+{
+ if (unlikely(busyloop_intr)) {
+ vhost_poll_queue(&vq->poll);
+ } else if (unlikely(vhost_enable_notify(&net->dev, vq))) {
+ /* They have slipped one in meanwhile: check again. */
+ vhost_disable_notify(&net->dev, vq);
+ if (!vhost_exceeds_weight(pkts, total_len))
+ return true;
+ vhost_poll_queue(&vq->poll);
+ }
+ /* Nothing new? Wait for eventfd to tell us they refilled. */
+ return false;
+}
+
static void handle_tx_copy(struct vhost_net *net, struct socket *sock)
{
struct vhost_net_virtqueue *nvq = &net->vqs[VHOST_NET_VQ_TX];
@@ -790,15 +807,10 @@ static void handle_tx_copy(struct vhost_net *net, struct socket *sock)
/* On error, stop handling until the next kick. */
if (unlikely(head < 0))
break;
- /* Nothing new? Wait for eventfd to tell us they refilled. */
if (head == vq->num) {
- if (unlikely(busyloop_intr)) {
- vhost_poll_queue(&vq->poll);
- } else if (unlikely(vhost_enable_notify(&net->dev,
- vq))) {
- vhost_disable_notify(&net->dev, vq);
+ if (unlikely(empty_ring(net, vq, ++sent_pkts,
+ total_len, busyloop_intr)))
continue;
- }
break;
}

@@ -886,14 +898,10 @@ static void handle_tx_zerocopy(struct vhost_net *net, struct socket *sock)
/* On error, stop handling until the next kick. */
if (unlikely(head < 0))
break;
- /* Nothing new? Wait for eventfd to tell us they refilled. */
if (head == vq->num) {
- if (unlikely(busyloop_intr)) {
- vhost_poll_queue(&vq->poll);
- } else if (unlikely(vhost_enable_notify(&net->dev, vq))) {
- vhost_disable_notify(&net->dev, vq);
+ if (unlikely(empty_ring(net, vq, ++sent_pkts,
+ total_len, busyloop_intr)))
continue;
- }
break;
}

@@ -1163,18 +1171,10 @@ static void handle_rx(struct vhost_net *net)
/* On error, stop handling until the next kick. */
if (unlikely(headcount < 0))
goto out;
- /* OK, now we need to know about added descriptors. */
if (!headcount) {
- if (unlikely(busyloop_intr)) {
- vhost_poll_queue(&vq->poll);
- } else if (unlikely(vhost_enable_notify(&net->dev, vq))) {
- /* They have slipped one in as we were
- * doing that: check again. */
- vhost_disable_notify(&net->dev, vq);
- continue;
- }
- /* Nothing new? Wait for eventfd to tell us
- * they refilled. */
+ if (unlikely(empty_ring(net, vq, ++recv_pkts,
+ total_len, busyloop_intr)))
+ continue;
goto out;
}
busyloop_intr = false;
> --
> 1.8.3.1

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