In the Linux kernel, the following vulnerability has been resolved: net/sched: cls_bpf: prevent...

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

net/sched: cls_bpf: prevent unbounded recursion in offload rollback

Quan Sun reported 1 a stack overflow in cls_bpf_offload_cmd().

Reproducer on netdevsim: add a skip_sw cls_bpf filter, set the bpf_tc_accept debugfs knob to 0, then tc filter replace. The replace calls tc_setup_cb_replace() which fails. cls_bpf_offload_cmd() then swaps prog/oldprog and recursively calls itself to roll back. But bpf_tc_accept=0 makes the rollback fail too, which triggers yet another rollback frame with the same arguments, and so on until the stack is exhausted.

bpf_tc_accept is just a convenient knob for the reproducer. Any driver whose tc_setup_cb_replace() fails twice in a row can hit the same loop, so this is not a netdevsim-only issue.

Two ways to fix it:

1. Have the rollback call tc_setup_cb_add() on oldprog instead of re-entering cls_bpf_offload_cmd().
2. Mark the rollback frame with a flag and skip a second-level rollback from inside it.

Go with (2). It is the smaller change and keeps the original behaviour: the rollback still goes through tc_setup_cb_replace(), so the driver gets one real chance to restore its state. If that attempt also fails, we just return the original error instead of recursing.