[PATCH net 2/2] ovpn: ensure gro_cells_receive() is invoked with BH disabled

[Antonio Quartulli <antonio@openvpn.net>]

From: Ralf Lici <ralf@mandelbit.com>

ovpn injects decrypted packets into the netdev RX path through
ovpn_netdev_write() -> gro_cells_receive().
In case of TCP connections, that injection path can be reached
from either softirq or process context.

However, gro_cells_receive() expects to be invoked in softirq
context to avoid racing its lock acquisition.

When being invoked from process context, we must ensure BH are
disabled to safeguard gro_cells_receive() assumption.

As of now this is not the case and the following WARNING is
triggered:

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

Moreover, not disabling BH was also triggering lockdep, which was
reporting a potential deadlock in case of concurrent softirq and
process context gro_cells->bh_lock acquisition (i.e in case of
concurrent UDP and TCP connection for the same ovpn interface):

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

Fix all this by invoking local_bh_disable/enable() around
gro_cells_receive().

Fixes: 11851cbd60ea ("ovpn: implement TCP transport")
Signed-off-by: Ralf Lici <ralf@mandelbit.com>
Signed-off-by: Antonio Quartulli <antonio@openvpn.net>
---
 drivers/net/ovpn/io.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/net/ovpn/io.c b/drivers/net/ovpn/io.c
index d92bb87be2b2..c0fdb9504241 100644
--- a/drivers/net/ovpn/io.c
+++ b/drivers/net/ovpn/io.c
@@ -91,7 +91,13 @@ 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()
+	 * doesn't enter deadlock
+	 */
+	local_bh_disable();
 	ret = gro_cells_receive(&peer->ovpn->gro_cells, skb);
+	local_bh_enable();
 	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);
-- 
2.53.0