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.

Red Hat has evaluated this vulnerability as Moderate because triggering the condition requires local privileges to manipulate network traffic control configurations, limiting the threat scope to denial of service or system instability. While systems configured to panic on kernel warnings could experience an unexpected system crash, exploitation is confined locally and necessitates an atypical hierarchy combining Hierarchical Token Bucket and CAKE queuing disciplines under constrained memory settings. Under standard Red Hat Enterprise Linux deployments, the CAKE queuing discipline is not enabled by default, significantly reducing exposure.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.