[PATCH net v2 1/2] net/sched: pfifo_fast: reject oversized ring and account to memcg

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

pfifo_fast_init() and pfifo_fast_change_tx_queue_len() allocate skb
ring arrays sized by dev->tx_queue_len with GFP_KERNEL and no upper
bound. An unprivileged user (via unshare -Urn) can set a huge
tx_queue_len and attach many pfifo_fast qdiscs to exhaust global
memory, causing a system-wide OOM.

Reject tx_queue_len values exceeding S16_MAX (32767) with -ERANGE
in both pfifo_fast_init() and pfifo_fast_change_tx_queue_len().
Note: For the init path, NL_SET_ERR_MSG_FMT_MOD reports the error
via extack whereas for the resize path, the error propagates to
netif_change_tx_queue_len() which rolls back dev->tx_queue_len to
the original value. Use GFP_KERNEL_ACCOUNT so the ring allocations
are charged to the allocating process's memory cgroup.

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.

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).
- Create a veth pair, set tx_queue_len to a huge value (e.g. 500000)
  while the devices are down.
- Attach mq at root, then replace each child queue with pfifo_fast:
    tc qdisc replace dev veth0 root handle 1: mq
    tc qdisc replace dev veth0 parent 1:1 pfifo_fast
    tc qdisc replace dev veth0 parent 1:2 pfifo_fast ...
- Repeat across many veth pairs. Each pfifo_fast allocates 3 skb_array
  rings of tx_queue_len entries (~12MB per qdisc at QLEN=500000).
- On the unfixed kernel this exhausts global memory in ~28 iterations
  on a 2GB guest -> OOM panic. On the fixed kernel the oversized
  tx_queue_len is rejected with -ERANGE.

Fixes: c5ad119fb6c0 ("net: sched: pfifo_fast use skb_array")
Reported-by: vega@nebusec.ai
Tested-by: Victor Nogueira <victor@mojatatu.com>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
---
v1 -> v2:
- Replaced silent clamp + pr_warn_ratelimited with reject (-ERANGE) (Jakub)
- Changed cap from 65535 to S16_MAX (32767), matching virtio's
  virtio16 ring size limit.
- Dropped the doubled module prefix in extack (NL_SET_ERR_MSG_FMT_MOD
  already prepends KBUILD_MODNAME).
- Added resize-path tdc test case (Sashiko nipa gpt-5-6-sol-1-2).
- Fixed tdc teardown to use JSON list form for acceptable exit codes.
---
 net/sched/sch_generic.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index ef2b4bf51564..eb5c0d3f67c2 100644
--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c
@@ -910,11 +910,18 @@ static int pfifo_fast_init(struct Qdisc *qdisc, struct nlattr *opt,
 	if (!qlen)
 		return -EINVAL;
 
+	if (qlen > S16_MAX) {
+		NL_SET_ERR_MSG_FMT_MOD(extack,
+				       "ring size %u too large (max %d)",
+				       qlen, S16_MAX);
+		return -ERANGE;
+	}
+
 	for (prio = 0; prio < PFIFO_FAST_BANDS; prio++) {
 		struct skb_array *q = band2list(priv, prio);
 		int err;
 
-		err = skb_array_init(q, qlen, GFP_KERNEL);
+		err = skb_array_init(q, qlen, GFP_KERNEL_ACCOUNT);
 		if (err)
 			return -ENOMEM;
 	}
@@ -957,8 +964,11 @@ static int pfifo_fast_change_tx_queue_len(struct Qdisc *sch,
 		bands[prio] = q;
 	}
 
+	if (new_len > S16_MAX)
+		return -ERANGE;
+
 	return skb_array_resize_multiple_bh(bands, PFIFO_FAST_BANDS, new_len,
-					    GFP_KERNEL);
+					    GFP_KERNEL_ACCOUNT);
 }
 
 struct Qdisc_ops pfifo_fast_ops __read_mostly = {
-- 
2.43.0