tuntap: synchronize through tfiles array instead of tun->numqueues

When a queue(tfile) is detached through __tun_detach(), we move the
last enabled tfile to the position where detached one sit but don't
NULL out last position. We expect to synchronize the datapath through
tun->numqueues. Unfortunately, this won't work since we're lacking
sufficient mechanism to order or synchronize the access to
tun->numqueues.

To fix this, NULL out the last position during detaching and check
RCU protected tfile against NULL instead of checking tun->numqueues in
datapath.

Cc: YueHaibing <yuehaibing@huawei.com>
Cc: Cong Wang <xiyou.wangcong@gmail.com>
Cc: weiyongjun (A) <weiyongjun1@huawei.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Fixes: c8d68e6be1c3b ("tuntap: multiqueue support")
Signed-off-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index a52bd47e7aa20b63606106635335681d2bad9686..abae165dcca5aff91989dc78afe1ca33d85fc8f8 100644 (file)
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -704,6 +704,8 @@ static void __tun_detach(struct tun_file *tfile, bool clean)
                                   tun->tfiles[tun->numqueues - 1]);
                ntfile = rtnl_dereference(tun->tfiles[index]);
                ntfile->queue_index = index;
+               rcu_assign_pointer(tun->tfiles[tun->numqueues - 1],
+                                  NULL);
 
                --tun->numqueues;
                if (clean) {
@@ -1086,7 +1088,7 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
        tfile = rcu_dereference(tun->tfiles[txq]);
 
        /* Drop packet if interface is not attached */
-       if (txq >= tun->numqueues)
+       if (!tfile)
                goto drop;
 
        if (!rcu_dereference(tun->steering_prog))
@@ -1309,6 +1311,7 @@ static int tun_xdp_xmit(struct net_device *dev, int n,
 
        rcu_read_lock();
 
+resample:
        numqueues = READ_ONCE(tun->numqueues);
        if (!numqueues) {
                rcu_read_unlock();
@@ -1317,6 +1320,8 @@ static int tun_xdp_xmit(struct net_device *dev, int n,
 
        tfile = rcu_dereference(tun->tfiles[smp_processor_id() %
                                            numqueues]);
+       if (unlikely(!tfile))
+               goto resample;
 
        spin_lock(&tfile->tx_ring.producer_lock);
        for (i = 0; i < n; i++) {