Re: sch_qfq: race conditon on qfq_aggregate (net/sched/sch_qfq.c)

[Cong Wang <xiyou.wangcong@gmail.com>]

Hi Xiang,

On Mon, Jun 30, 2025 at 11:49:02AM -0700, Xiang Mei wrote:
> Thank you very much for your time. We've re-tested the PoC and
> confirmed it works on the latest kernels (6.12.35, 6.6.95, and
> 6.16-rc4).
> 
> To help with reproduction, here are a few notes that might be useful:
> 1. The QFQ scheduler needs to be compiled into the kernel:
>     $ scripts/config --enable CONFIG_NET_SCHED
>     $ scripts/config --enable CONFIG_NET_SCH_QFQ
> 2. Since this is a race condition, the test environment should have at
> least two cores (e.g., -smp cores=2 for QEMU).
> 3. The PoC was compiled using: `gcc ./poc.c -o ./poc  -w --static`
> 4. Before running the PoC, please check that the network interface
> "lo" is in the "up" state.
> 
> Appreciate your feedback and patience.

Thanks for your detailed report and efforts on reproducing it on the
latest kernel.

I think we may have a bigger problem here, the sch_tree_lock() is to lock
the datapath, I doubt we really need to use sch_tree_lock() for
qfq->agg. _If_ it is only for control path, using RTNL lock + RCU lock
should be sufficient. We need a deeper review on the locking there.

Regards,
Cong