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.

[1]: https://lore.kernel.org/bpf/[email protected]/T/#u

https://git.kernel.org/stable/c/10753da2d659dd425a6e620f47f86852d604f67f

https://git.kernel.org/stable/c/1387f252a242a51bfbb6eace29c8f8db21b457da

https://git.kernel.org/stable/c/27db54b90bcc7c37867fe664107fa25ea6a116e4

https://git.kernel.org/stable/c/33373e1f378a501bc51aa73312f74295c84e3101

https://git.kernel.org/stable/c/3fa6fb5d771c992ebedbfa7331c6bcc6f33f89b7

https://git.kernel.org/stable/c/4a76953c3ed043797e81529b9395e9ca6f4c7609

https://git.kernel.org/stable/c/a018f208ab7512380bd4cf670064d48cba00a1b1

https://git.kernel.org/stable/c/e2d3b7bab3748c811dc5750ce9a8d62bc7f90ed7

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