Re: [syzbot] [net?] INFO: rcu detected stall in unix_release

[Vladimir Oltean]

Hi Jamal,

On Fri, Aug 18, 2023 at 11:27:27AM -0400, Jamal Hadi Salim wrote:
> Can you try the attached patchlet?

Thanks for the patch. I've tried it, and it eliminates the code path
(and thus the problem) exposed by the syzbot program, by responding to
RTM_NEWQDISC messages having the NLM_F_CREATE|NLM_F_REPLACE|NLM_F_EXCL
flags with "Error: Exclusivity flag on, cannot modify.".

Actually, to be precise, the first such netlink message successfully
creates the qdisc, but then the subsequent ones leave that qdisc alone
(don't change it), by failing with this extack message.

If that's the behavior that you intended, then I guess the answer is
that it works. Thanks a lot.

What would be an appropriate Fixes: tag?

Side note: I believe that we can now also revert commit be3618d96510
("net/sched: taprio: fix slab-out-of-bounds Read in taprio_dequeue_from_txq"),
which was papering over an unknown (at the time) issue - the same as
this one - without really even completely covering it, either. Hence
this other syzbot report.
https://lore.kernel.org/netdev/3b977f76-0289-270e-8310-179315ee927d@xxxxxxxxxx/T/
https://lore.kernel.org/netdev/20230608062756.3626573-1-shaozhengchao@xxxxxxxxxx/