net/sched: sch_cake: Fix incorrect qlen reduction in cake_drop

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: net/sched: sch_cake: Fix incorrect qlen reduction in cake_drop In cake_drop(), qdisc_tree_reduce_backlog() is used to update the qlen and backlog of the qdisc hierarchy. Its caller, cake_enqueue(), assumes that the parent qdisc will enqueue the current packet. However, this assumption breaks when cake_enqueue() returns NET_XMIT_CN: 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 cake_drop() calls, and then compensates the qdisc tree accounting if cake_enqueue() returns NET_XMIT_CN. To ensure correct compensation when ACK thinning is enabled, a new variable is introduced to keep qlen unchanged.

References

• https://git.kernel.org/stable/c/a3f4e3de41a3f115db35276c6b186ccbc913934a
• https://git.kernel.org/stable/c/38abf6e931b169ea88d7529b49096f53a5dcf8fe
• https://git.kernel.org/stable/c/fcb91be52eb6e92e00b533ebd7c77fecada537e1
• https://git.kernel.org/stable/c/d01f0e072dadb02fe10f436b940dd957aff0d7d4
• https://git.kernel.org/stable/c/0b6216f9b3d1c33c76f74511026e5de5385ee520
• https://git.kernel.org/stable/c/529c284cc2815c8350860e9a31722050fe7117cb
• https://git.kernel.org/stable/c/3ed6c458530a547ed0c9ea0b02b19bab620be88b
• https://git.kernel.org/stable/c/9fefc78f7f02d71810776fdeb119a05a946a27cc