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%3Ba=su...

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 stable@vger.kernel.org know about it.

From foo@baz Tue Mar 6 19:02:12 PST 2018

From: Jason Wang jasowang@redhat.com Date: Sat, 24 Feb 2018 11:32:25 +0800 Subject: tuntap: disable preemption during XDP processing

From: Jason Wang jasowang@redhat.com

[ 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.

Fixes: 761876c857cb ("tap: XDP support") Signed-off-by: Jason Wang jasowang@redhat.com Signed-off-by: David S. Miller davem@davemloft.net Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/net/tun.c | 6 ++++++ 1 file changed, 6 insertions(+)

--- a/drivers/net/tun.c +++ b/drivers/net/tun.c @@ -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 jasowang@redhat.com 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