Subject: Re: virtio_net: suspicious RCU usage with xdp

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 24 Apr 2019 13:37:29 -0400

On Wed, Apr 24, 2019 at 11:13:42AM -0600, David Ahern wrote:
> seeing an RCU warning testing xdp with virtio net. net-next as of commit
> b2f97f7de2f6a4df8e431330cf467576486651c5. No obvious changes so hoping
> this rings a bell with someone else.
> 
> 
> [  121.990304] =============================
> [  121.991488] WARNING: suspicious RCU usage
> [  121.992392] 5.1.0-rc5+ #60 Not tainted
> [  121.993220] -----------------------------
> [  121.994158] /home/dsa/kernel-3.git/drivers/net/virtio_net.c:516
> suspicious rcu_dereference_check() usage!
> [  121.996284]
>                other info that might help us debug this:
> 
> [  121.997988]
>                rcu_scheduler_active = 2, debug_locks = 1
> [  121.999321] no locks held by swapper/1/0.
> [  122.000328]
>                stack backtrace:
> [  122.001253] CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.1.0-rc5+ #60
> [  122.002474] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
> BIOS 1.11.1-1 04/01/2014
> [  122.004141] Call Trace:
> [  122.004651]  <IRQ>
> [  122.005082]  dump_stack+0x7e/0xbb
> [  122.005757]  lockdep_rcu_suspicious+0x102/0x10b
> [  122.006654]  virtnet_xdp_xmit+0x104/0x4fe
> [  122.007447]  ? kasan_check_read+0x11/0x13
> [  122.008267]  ? mergeable_rx_buffer_size_show+0x163/0x163
> [  122.009299]  ? __asan_loadN+0xf/0x11
> [  122.010010]  ? pvclock_clocksource_read+0xfa/0x189
> [  122.010975]  bq_xmit_all+0xdc/0x358
> [  122.011699]  __dev_map_flush+0xc2/0xef
> [  122.012472]  xdp_do_flush_map+0x5b/0x74
> [  122.013238]  virtnet_poll+0x58f/0x679

Well virtnet_xdp_xmit seems to be called from .ndo_xdp_xmit
and that isn't in an RCU read-side critical section.

Looks like we just need to add RCU read lock/unlock.
Like the below perhaps?

This issue was introduced by 8dcc5b0ab0 however I find it
inelegant that we need to do checks in each driver,
and add RCU locks just for a startup initialization issue.
Can't XDP core make sure the callback isn't invoked
at an inappropriate time instead?

--->

virtio_net: call virtnet_xdp_xmit with RCU lock

This functions uses rcu_dereference so it needs to be
called in an RCU read-side critical section.

Fixes: 8dcc5b0ab0 ("virtio_net: fix ndo_xdp_xmit crash towards dev not ready for XDP")
Signed-off-by: Michael S. Tsirkin <[email-masked]>

--

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 9010938e2d71..ccc1bdd1bb1f 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -495,8 +495,8 @@ static struct send_queue *virtnet_xdp_sq(struct virtnet_info *vi)
 	return &vi->sq[qp];
 }
 
-static int virtnet_xdp_xmit(struct net_device *dev,
-			    int n, struct xdp_frame **frames, u32 flags)
+static int __virtnet_xdp_xmit(struct net_device *dev,
+			      int n, struct xdp_frame **frames, u32 flags)
 {
 	struct virtnet_info *vi = netdev_priv(dev);
 	struct receive_queue *rq = vi->rq;
@@ -569,6 +569,17 @@ static int virtnet_xdp_xmit(struct net_device *dev,
 	return ret;
 }
 
+static int virtnet_xdp_xmit(struct net_device *dev,
+			    int n, struct xdp_frame **frames, u32 flags)
+{
+	int r;
+
+	rcu_read_lock_bh();
+	r = __virtnet_xdp_xmit(dev, n, frames, flags);
+	rcu_read_unlock_bh();
+	return r;
+}
+
 static unsigned int virtnet_get_headroom(struct virtnet_info *vi)
 {
 	return vi->xdp_queue_pairs ? VIRTIO_XDP_HEADROOM : 0;
@@ -714,7 +725,7 @@ static struct sk_buff *receive_small(struct net_device *dev,
 			xdpf = convert_to_xdp_frame(&xdp);
 			if (unlikely(!xdpf))
 				goto err_xdp;
-			err = virtnet_xdp_xmit(dev, 1, &xdpf, 0);
+			err = __virtnet_xdp_xmit(dev, 1, &xdpf, 0);
 			if (unlikely(err < 0)) {
 				trace_xdp_exception(vi->dev, xdp_prog, act);
 				goto err_xdp;
@@ -887,7 +898,7 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
 			xdpf = convert_to_xdp_frame(&xdp);
 			if (unlikely(!xdpf))
 				goto err_xdp;
-			err = virtnet_xdp_xmit(dev, 1, &xdpf, 0);
+			err = __virtnet_xdp_xmit(dev, 1, &xdpf, 0);
 			if (unlikely(err < 0)) {
 				trace_xdp_exception(vi->dev, xdp_prog, act);
 				if (unlikely(xdp_page != page))

-- 
MST