CVE-2025-39766: net/sched: Make cake_enqueue return NET_XMIT_CN when past buffer_limit

Vulnerability summary

CVE-2025-39766 is a high (CVSS 7.8) vulnerability in debian debian_linux. EPSS estimates a 0.18% chance of exploitation in the next 30 days, higher than 8% of scored vulnerabilities.

Timeline

1. 11-09-2025
   PublishedVulnerability first disclosed.
2. 11-06-2026
   Last modifiedVulnerability information updated.

Description

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.

No weakness data has been published for this record.