[PATCH net 5/7] virtio_net: Don't process redirected XDP frames when XDP is disabled

[Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>]

Commit 8dcc5b0ab0ec ("virtio_net: fix ndo_xdp_xmit crash towards dev not
ready for XDP") tried to avoid access to unexpected sq while XDP is
disabled, but was not complete.

There was a small window which causes out of bounds sq access in
virtnet_xdp_xmit() while disabling XDP.

An example case of
 - curr_queue_pairs = 6 (2 for SKB and 4 for XDP)
 - online_cpu_num = xdp_queue_paris = 4
when XDP is enabled:

CPU 0                         CPU 1
(Disabling XDP)               (Processing redirected XDP frames)

                              virtnet_xdp_xmit()
virtnet_xdp_set()
 _virtnet_set_queues()
  set curr_queue_pairs (2)
                               check if rq->xdp_prog is not NULL
                               virtnet_xdp_sq(vi)
                                qp = curr_queue_pairs -
                                     xdp_queue_pairs +
                                     smp_processor_id()
                                   = 2 - 4 + 1 = -1
                                sq = &vi->sq[qp] // out of bounds access
  set xdp_queue_pairs (0)
  rq->xdp_prog = NULL

Basically we should not change curr_queue_pairs and xdp_queue_pairs
while someone can read the values. Thus, when disabling XDP, assign NULL
to rq->xdp_prog first, and wait for RCU grace period, then change
xxx_queue_pairs.
Note that we need to keep the current order when enabling XDP though.

Fixes: 186b3c998c50 ("virtio-net: support XDP_REDIRECT")
Signed-off-by: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>
---
 drivers/net/virtio_net.c | 32 +++++++++++++++++++++++++-------
 1 file changed, 25 insertions(+), 7 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 204eedf..ae93f0e 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -2424,14 +2424,16 @@ static int virtnet_xdp_set(struct net_device *dev, struct bpf_prog *prog,
 		}
 	}
 
-	err = _virtnet_set_queues(vi, curr_qp + xdp_qp);
-	if (err)
-		goto err;
-	netif_set_real_num_rx_queues(dev, curr_qp + xdp_qp);
-	vi->xdp_queue_pairs = xdp_qp;
+	old_prog = rtnl_dereference(vi->rq[0].xdp_prog);
+	if (!old_prog && prog) {
+		err = _virtnet_set_queues(vi, curr_qp + xdp_qp);
+		if (err)
+			goto err_new_prog;
+		netif_set_real_num_rx_queues(dev, curr_qp + xdp_qp);
+		vi->xdp_queue_pairs = xdp_qp;
+	}
 
 	for (i = 0; i < vi->max_queue_pairs; i++) {
-		old_prog = rtnl_dereference(vi->rq[i].xdp_prog);
 		rcu_assign_pointer(vi->rq[i].xdp_prog, prog);
 		if (i == 0) {
 			if (!old_prog)
@@ -2439,6 +2441,18 @@ static int virtnet_xdp_set(struct net_device *dev, struct bpf_prog *prog,
 			if (!prog)
 				virtnet_restore_guest_offloads(vi);
 		}
+	}
+
+	if (old_prog && !prog) {
+		synchronize_net();
+		err = _virtnet_set_queues(vi, curr_qp + xdp_qp);
+		if (err)
+			goto err_old_prog;
+		netif_set_real_num_rx_queues(dev, curr_qp + xdp_qp);
+		vi->xdp_queue_pairs = xdp_qp;
+	}
+
+	for (i = 0; i < vi->max_queue_pairs; i++) {
 		if (old_prog)
 			bpf_prog_put(old_prog);
 		if (netif_running(dev)) {
@@ -2450,7 +2464,11 @@ static int virtnet_xdp_set(struct net_device *dev, struct bpf_prog *prog,
 
 	return 0;
 
-err:
+err_old_prog:
+	virtnet_clear_guest_offloads(vi);
+	for (i = 0; i < vi->max_queue_pairs; i++)
+		rcu_assign_pointer(vi->rq[i].xdp_prog, old_prog);
+err_new_prog:
 	if (netif_running(dev)) {
 		for (i = 0; i < vi->max_queue_pairs; i++) {
 			virtnet_napi_enable(vi->rq[i].vq, &vi->rq[i].napi);
-- 
1.8.3.1