Patch "tuntap: disable preemption during XDP processing" has been added to the 4.14-stable tree

This is a note to let you know that I've just added the patch titled

tuntap: disable preemption during XDP processing

to the 4.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
tuntap-disable-preemption-during-xdp-processing.patch
and it can be found in the queue-4.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Jason Wang <[email protected]>
Date: Sat, 24 Feb 2018 11:32:25 +0800
Subject: tuntap: disable preemption during XDP processing

From: Jason Wang <[email protected]>

[ Upstream commit 23e43f07f896f8578318cfcc9466f1e8b8ab21b6 ]

Except for tuntap, all other drivers' XDP was implemented at NAPI poll() routine in a bh. This guarantees all XDP operation were done at the same CPU which is required by e.g BFP_MAP_TYPE_PERCPU_ARRAY. But for tuntap, we do it in process context and we try to protect XDP processing by RCU reader lock. This is insufficient since CONFIG_PREEMPT_RCU can preempt the RCU reader critical section which breaks the assumption that all XDP were processed in the same CPU.

Fixing this by simply disabling preemption during XDP processing.

---
 drivers/net/tun.c |    6 ++++++
 1 file changed, 6 insertions(+)

@@ -1315,6 +1315,7 @@ static struct sk_buff *tun_build_skb(str
 	else
 		*skb_xdp = 0;
 
+	preempt_disable();
 	rcu_read_lock();
 	xdp_prog = rcu_dereference(tun->xdp_prog);
 	if (xdp_prog && !*skb_xdp) {

@@ -1337,6 +1338,7 @@ static struct sk_buff *tun_build_skb(str
 			if (err)
 				goto err_redirect;
 			rcu_read_unlock();
+			preempt_enable();
 			return NULL;
 		case XDP_TX:
 			xdp_xmit = true;

@@ -1358,6 +1360,7 @@ static struct sk_buff *tun_build_skb(str
 	skb = build_skb(buf, buflen);
 	if (!skb) {
 		rcu_read_unlock();
+		preempt_enable();
 		return ERR_PTR(-ENOMEM);
 	}
 

@@ -1370,10 +1373,12 @@ static struct sk_buff *tun_build_skb(str
 		skb->dev = tun->dev;
 		generic_xdp_tx(skb, xdp_prog);
 		rcu_read_unlock();
+		preempt_enable();
 		return NULL;
 	}
 
 	rcu_read_unlock();
+	preempt_enable();
 
 	return skb;
 

@@ -1381,6 +1386,7 @@ err_redirect:
 	put_page(alloc_frag->page);
 err_xdp:
 	rcu_read_unlock();
+	preempt_enable();
 	this_cpu_inc(tun->pcpu_stats->rx_dropped);
 	return NULL;
 }

Patches currently in stable-queue which might be from [email protected] are

queue-4.14/tuntap-correctly-add-the-missing-xdp-flush.patch queue-4.14/virtio-net-disable-napi-only-when-enabled-during-xdp-set.patch queue-4.14/tuntap-disable-preemption-during-xdp-processing.patch