ovpn: ensure packet delivery happens with BH disabled

ovpn injects decrypted packets into the netdev RX path through
ovpn_netdev_write() which invokes gro_cells_receive() and
dev_dstats_rx_add().

ovpn_netdev_write() is normally called in softirq context,
however, in case of TCP connections it may also be invoked
process context.

When this happens gro_cells_receive() will throw a warning:

  [  230.183747][   T12] WARNING: net/core/gro_cells.c:30 at gro_cells_receive+0x708/0xaa0, CPU#1: kworker/u16:0/12

and lockdep will also report a potential inconsistent lock state:

  WARNING: inconsistent lock state
  7.0.0-rc4+ #246 Tainted: G        W
  --------------------------------
  inconsistent {IN-SOFTIRQ-W} -> {SOFTIRQ-ON-W} usage.

because attempts to acquire gro_cells->bh_lock by both
contexts may lead to a deadlock.

At the same time, dev_dstats_rx_add() does not expect to race
with a softirq (which may happen when invoked in process context),
because the latter may access its per-cpu state and corrupt
it.

Fix all this by invoking local_bh_disable/enable() around
gro_cells_receive() and dev_dstats_rx_add() to ensure that
bottom halves are always disabled before calling both of
them.

Fixes: 11851cb ("ovpn: implement TCP transport")
Signed-off-by: Ralf Lici <ralf@mandelbit.com>
Signed-off-by: Antonio Quartulli <antonio@openvpn.net>

Author: ralflici

drivers/net/ovpn/io.c

@@ -91,12 +91,18 @@ static void ovpn_netdev_write(struct ovpn_peer *peer, struct sk_buff *skb)
 
 	/* cause packet to be "received" by the interface */
 	pkt_len = skb->len;
+	/* we may get here in process context in case of TCP connections,
+	 * therefore we have to disable BHs to ensure gro_cells_receive()
+	 * and dev_dstats_rx_add() do not get corrupted or enter deadlock
+	 */
+	local_bh_disable();
 	ret = gro_cells_receive(&peer->ovpn->gro_cells, skb);
 	if (likely(ret == NET_RX_SUCCESS)) {
 		/* update RX stats with the size of decrypted packet */
 		ovpn_peer_stats_increment_rx(&peer->vpn_stats, pkt_len);
 		dev_dstats_rx_add(peer->ovpn->dev, pkt_len);
 	}
+	local_bh_enable();
 }
 
 void ovpn_decrypt_post(void *data, int ret)