rtnl: limit IFLA_NUM_TX_QUEUES and IFLA_NUM_RX_QUEUES to 4096

We have an impressive number of syzkaller bugs that are linked to the fact that syzbot was able to create a networking device with millions of TX (or RX) queues.

Let's limit the number of RX/TX queues to 4096, this really should cover all known cases.

A separate patch will add various cond_resched() in the loops handling sysfs entries at device creation and dismantle.

Tested:

lpaa6:~# ip link add gre-4097 numtxqueues 4097 numrxqueues 4097 type ip6gretap
RTNETLINK answers: Invalid argument

lpaa6:~# time ip link add gre-4096 numtxqueues 4096 numrxqueues 4096 type ip6gretap

real 0m0.180s
user 0m0.000s
sys 0m0.107s

---
 net/core/rtnetlink.c | 6 ++++++
 1 file changed, 6 insertions(+)

index 7f37fe9c65a5490c621874607f6a1c2fc32e36dd..448703312fed0f736bc7f0d58e7bda705a2344f2 100644
@@ -2835,6 +2835,12 @@ struct net_device *rtnl_create_link(struct net *net,
 	else if (ops->get_num_rx_queues)
 		num_rx_queues = ops->get_num_rx_queues();
 
+	if (num_tx_queues < 1 || num_tx_queues > 4096)
+		return ERR_PTR(-EINVAL);
+
+	if (num_rx_queues < 1 || num_rx_queues > 4096)
+		return ERR_PTR(-EINVAL);
+
 	dev = alloc_netdev_mqs(ops->priv_size, ifname, name_assign_type,
 			       ops->setup, num_tx_queues, num_rx_queues);
 	if (!dev)

2.19.0.605.g01d371f741-goog

From: Eric Dumazet <[email protected]>
Date: Tue, 2 Oct 2018 15:47:35 -0700

Applied and queued up for -stable.