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

Overview

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.

Affected

• Linux >= 102740bd9436a3a6ba129af3a48271d794009fa5 < a018f208ab7512380bd4cf670064d48cba00a1b1
• Linux >= 102740bd9436a3a6ba129af3a48271d794009fa5 < 33373e1f378a501bc51aa73312f74295c84e3101
• Linux >= 102740bd9436a3a6ba129af3a48271d794009fa5 < e2d3b7bab3748c811dc5750ce9a8d62bc7f90ed7
• Linux >= 102740bd9436a3a6ba129af3a48271d794009fa5 < 4a76953c3ed043797e81529b9395e9ca6f4c7609
• Linux >= 102740bd9436a3a6ba129af3a48271d794009fa5 < 1387f252a242a51bfbb6eace29c8f8db21b457da
• Linux >= 102740bd9436a3a6ba129af3a48271d794009fa5 < 10753da2d659dd425a6e620f47f86852d604f67f
• Linux >= 102740bd9436a3a6ba129af3a48271d794009fa5 < 3fa6fb5d771c992ebedbfa7331c6bcc6f33f89b7
• Linux >= 102740bd9436a3a6ba129af3a48271d794009fa5 < 27db54b90bcc7c37867fe664107fa25ea6a116e4
• Linux 4.15

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.