CVE-2025-39766

In the Linux kernel, the following vulnerability has been resolved:

net/sched: Make cake_enqueue return NET_XMIT_CN when past buffer_limit

The following setup can trigger a WARNING in htb_activate due to

the condition: !cl->leaf.q->q.qlen

tc qdisc del dev lo root

tc qdisc add dev lo root handle 1: htb default 1

tc class add dev lo parent 1: classid 1:1 \

htb rate 64bit

tc qdisc add dev lo parent 1:1 handle f: \

cake memlimit 1b

ping -I lo -f -c1 -s64 -W0.001 127.0.0.1

This is because the low memlimit leads to a low buffer_limit, which

causes packet dropping. However, cake_enqueue still returns

NET_XMIT_SUCCESS, causing htb_enqueue to call htb_activate with an

empty child qdisc. We should return NET_XMIT_CN when packets are

dropped from the same tin and flow.

I do not believe return value of NET_XMIT_CN is necessary for packet

drops in the case of ack filtering, as that is meant to optimize

performance, not to signal congestion.