net: cap tx_queue_len at S16_MAX to prevent oversized ring allocations

Several subsystems allocate ring buffers sized by dev->tx_queue_len
with no upper bound. An unprivileged user (via unshare -Urn) can set a
huge tx_queue_len and exhaust global memory with ring allocations:

- pfifo_fast: pfifo_fast_init() and pfifo_fast_change_tx_queue_len()
  allocate 3 skb_array rings of tx_queue_len entries each.
- tun: tun_queue_resize() and the queue-attach path resize ptr_rings
  to tx_queue_len on the NETDEV_CHANGE_TX_QUEUE_LEN notifier.
- tap (macvtap/ipvtap): tap_queue_resize() and tap_init() resize/init
  ptr_rings to tx_queue_len on the same notifier.

netif_change_tx_queue_len() is the single entry point for IFLA_TXQLEN,
sysfs, and the SIOCSIFTXQLEN ioctl. Cap new_len at S16_MAX (32767)
there so the oversized value is rejected at set time. This takes
effect whether the device is up or down, before dev->tx_queue_len is
written, before any notifier fires, and before any ring is allocated.
The "> S16_MAX" check also subsumes the previous unsigned-long
truncation test, and a negative ifr_qlen from the ioctl lands far
above the cap after conversion, so both old failure modes are covered
by the one comparison.

tx_queue_len is ambigious: both a per-ring sizing multiplier and a
default queue-length/limit knob for consumers that allocate
nothing at set time (pfifo/bfifo/gred/plug/sfb limits, htb
direct_qlen, qfq max_classes, teql). 32767 is chosen as the largest
value NLA_POLICY_FULL_RANGE can express for the u32 IFLA_TXQLEN
policy in patch 2/3 while staying a legitimate queue length on
high-BDP paths; the ring-memory trade-off of a shared knob is
disclosed below.

Conditions to recreate the bug:
- CONFIG_NET_SCHED=y, CONFIG_VETH=y, CONFIG_USER_NS=y, CONFIG_NET_NS=y.
- Unprivileged user in a fresh user+net namespace (unshare -Urn).
- pfifo_fast: create veth pairs, set tx_queue_len to 500000, attach
  mq+pfifo_fast. ~28 iterations OOMs a 2GB guest.
- tun: create 50 tun devices with IFF_MULTI_QUEUE, set tx_queue_len to
  500000, open 8 queues each. ~1.6GB of ptr_ring allocations OOMs a
  512MB guest.
- tap: same as tun with IFF_TAP. ~960MB OOMs a 512MB guest.
- On the fixed kernel the oversized tx_queue_len is rejected with
  -ERANGE at set time (all four paths: RTM_SETLINK, RTM_NEWLINK
  create, sysfs, ioctl - the latter two via this check, the former
  two via this check and the 2/3 parse policy respectively).

Fixes: 6a643ddb56 ("net: introduce helper dev_change_tx_queue_len()")
Reported-by: Vega <vega@nebusec.ai>
Closes: https://lore.kernel.org/netdev/20260828121902.66837-1-jhs@mojatatu.com/
Tested-by: Victor Nogueira <victor@mojatatu.com>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://patch.msgid.link/QDISC-2899.v2.20260901233641@mojatatu.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/dev.c

@@ -9982,7 +9982,7 @@ int netif_change_tx_queue_len(struct net_device *dev, unsigned long new_len)
 	unsigned int orig_len = dev->tx_queue_len;
 	int res;
 
-	if (new_len != (unsigned int)new_len)
+	if (new_len > S16_MAX)
 		return -ERANGE;
 
 	if (new_len != orig_len) {