[PATCH net-next 3/4] net/sched/sch_cake: widen buffer_used to u64

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

This is a follow-up to commit 4c660ee8c809 ("net: sched: fix 32-bit
backlog wrap in gred, bfifo and plug enqueue"), which promoted the
backlog + length admission sums in gred, bfifo and plug to u64.  CAKE's
own memory accounting has the same 32-bit wrap.

cake_sched_data.buffer_used accumulates skb->truesize and is compared
against memory_limit:

	q->buffer_used += skb->truesize;
	if (q->buffer_used <= q->buffer_limit)
		return NET_XMIT_SUCCESS;
	while (q->buffer_used > q->buffer_limit)
		drop...

buffer_used is u32, so once the resident truesize passes 2^32 it wraps to
a small value, the <= test passes and the drop loop is skipped even
though far more than the configured limit is queued.  memory_limit is a
u32 attribute (TCA_CAKE_MEMORY), so buffer_limit can be as large as
U32_MAX and never caps buffer_used below the wrap point; with a large
memory_limit the queue then grows until the machine is out of memory.

Widen buffer_used and buffer_max_used to u64, the same shape as the
gred/bfifo/plug fix.  buffer_limit stays u32; the comparison promotes it
to u64, so the bounded queue stops at the configured limit below 2^32 and
the counter can no longer wrap.  buffer_used is subtracted on dequeue and
drop, so the widened type flows through unchanged.

The ACK-filter replacement applied the net delta as one expression,
q->buffer_used += skb->truesize - ack->truesize.  Both operands are
unsigned int, so the subtraction is evaluated at u32 and a larger removed
ACK wraps the delta; the old u32 accumulator folded that back to the
correct net value, but the widened u64 accumulator would persist it as a
roughly 4 GiB over-count.  Apply the replacement as two exact operations
on the widened counter instead; the queue already accounts ack->truesize,
so the subtraction cannot underflow.

Conditions to recreate the bug: CAP_NET_ADMIN (root or a user namespace,
the qdisc attach is gated by netlink_net_capable(CAP_NET_ADMIN)).

  # hold a tun device's tx ring so the root qdisc parks packets
  ip tuntap add tun0 mode tun
  ip link set tun0 txqueuelen 32 up
  ip addr add 10.99.0.1/24 dev tun0
  tc qdisc add dev tun0 root handle 1: cake memlimit 4294967295

  # drive >4 GiB of resident truesize through the qdisc (e.g. several UDP
  # sockets with SO_SNDBUFFORCE). On the unfixed kernel buffer_used wraps,
  # the drop loop is bypassed and the guest OOMs; with the fix the counter
  # passes the limit without wrapping and drops the excess.

  # the ACK-filter over-count is reached with ack-filter enabled and a
  # queued pure ACK whose truesize is larger than the replacement ACK:
  tc qdisc add dev tun0 root handle 1: cake ack-filter

Reported-by: Sashiko (gemini) <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260818095927.15901-1-jhs@mojatatu.com
Link: https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260818095927.15901-1-jhs@mojatatu.com
Link: https://lore.kernel.org/netdev/20260818095927.15901-1-jhs@mojatatu.com/
Reviewed-by: Victor Nogueira <victor@mojatatu.com>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
---
 net/sched/sch_cake.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/net/sched/sch_cake.c b/net/sched/sch_cake.c
index 2bad0b6eb13f..4caf9718c9bd 100644
--- a/net/sched/sch_cake.c
+++ b/net/sched/sch_cake.c
@@ -234,8 +234,9 @@ struct cake_sched_data {
 	u16		tin_cnt;
 
 	/* resource tracking */
-	u32		buffer_used;
-	u32		buffer_max_used;
+
+	u64		buffer_max_used;
+	u64		buffer_used;
 	u32		buffer_limit;
 
 	/* indices for dequeue */
@@ -1850,7 +1851,8 @@ static s32 cake_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 			qdisc_qstats_drop(sch);
 			ack_pkt_len = qdisc_pkt_len(ack);
 			WRITE_ONCE(b->bytes, b->bytes + ack_pkt_len);
-			q->buffer_used += skb->truesize - ack->truesize;
+			q->buffer_used += skb->truesize;
+			q->buffer_used -= ack->truesize;
 			if (q->config->rate_flags & CAKE_FLAG_INGRESS)
 				cake_advance_shaper(q, b, ack, now, true);
 
-- 
2.43.0