From: Jason Wang <jasowang@redhat.com>
Date: Sat, 24 Feb 2018 11:32:25 +0800
Subject: tuntap: disable preemption during XDP processing
Patch-mainline: v4.16-rc5
Git-commit: 23e43f07f896f8578318cfcc9466f1e8b8ab21b6
References: bsc#1109837

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.

Fixes: 761876c857cb ("tap: XDP support")
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
---
 drivers/net/tun.c |    6 ++++++
 1 file changed, 6 insertions(+)

--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1640,6 +1640,7 @@ 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) {
@@ -1663,6 +1664,7 @@ 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;
@@ -1684,6 +1686,7 @@ static struct sk_buff *tun_build_skb(str
 	skb = build_skb(buf, buflen);
 	if (!skb) {
 		rcu_read_unlock();
+		preempt_enable();
 		return ERR_PTR(-ENOMEM);
 	}
 
@@ -1696,10 +1699,12 @@ 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;
 
@@ -1707,6 +1712,7 @@ err_redirect:
 	put_page(alloc_frag->page);
 err_xdp:
 	rcu_read_unlock();
+	preempt_enable();
 	this_cpu_inc(tun->pcpu_stats->rx_dropped);
 	return NULL;
 }