[PATCH net 1/2] net: cap tx_queue_len at S16_MAX to prevent oversized ring allocations

[Jamal Hadi Salim <jhs@mojatatu.com>]

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 create many devices/queues to exhaust global
memory, causing a system-wide OOM:

- 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 will take 
effect whether the device is up or down before dev->tx_queue_len is
written or any notifier fires or any ring is allocated. So a good
choke spot.

S16_MAX is the virtio virtqueue size limit: the virtio specification
stores the queue size as a u16 with a maximum of 32768, so 32767 is
the largest tx_queue_len any in-tree driver can meaningfully use.
Values above that only serve to inflate ring allocations.

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.

Fixes: 6a643ddb5624 ("net: introduce helper dev_change_tx_queue_len()")
Reported-by: vega@nebusec.ai
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
---
 net/core/dev.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 38336858c168..1d3fc0a268a5 100644
--- a/net/core/dev.c
+++ b/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) {
-- 
2.43.0