CVE-2025-68325

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

net/sched: schcake: Fix incorrect qlen reduction in cakedrop

In cakedrop(), qdisctreereducebacklog() is used to update the qlen and backlog of the qdisc hierarchy. Its caller, cakeenqueue(), assumes that the parent qdisc will enqueue the current packet. However, this assumption breaks when cakeenqueue() returns NETXMITCN: the parent qdisc stops enqueuing current packet, leaving the tree qlen/backlog accounting inconsistent. This mismatch can lead to a NULL dereference (e.g., when the parent Qdisc is qfq_qdisc).

This patch computes the qlen/backlog delta in a more robust way by observing the difference before and after the series of cakedrop() calls, and then compensates the qdisc tree accounting if cakeenqueue() returns NETXMITCN.

To ensure correct compensation when ACK thinning is enabled, a new variable is introduced to keep qlen unchanged.