netdev - Re: [RFC net 1/1] net: sched: act: fix rcu race in dump

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Date:   Tue, 10 Oct 2017 09:40:12 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Alexander Aring <aring@...atatu.com>
Cc:     Jamal Hadi Salim <jhs@...atatu.com>, Jiri Pirko <jiri@...nulli.us>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        kurup.manish@...il.com, Brenda Butler <bjb@...atatu.com>
Subject: Re: [RFC net 1/1] net: sched: act: fix rcu race in dump

On Tue, Oct 10, 2017 at 5:32 AM, Alexander Aring <aring@...atatu.com> wrote:
> This patch fixes an issue with kfree_rcu which is not protected by RTNL
> lock. It could be that the current assigned rcu pointer will be freed by
> kfree_rcu while dump callback is running.

Why? kfree_rcu() respects existing readers, so why this could happen?


>
> To prevent this, we call rcu_synchronize at first. Then we are sure all
> latest rcu functions e.g. rcu_assign_pointer and kfree_rcu in init are
> done. After rcu_synchronize we dereference under RTNL lock which is also
> held in init function, which means no other rcu_assign_pointer or
> kfree_rcu will occur.

If you really want to wait for kfree_rcu(), rcu_barrier() is the one
instead of rcu_synchronize(). Just FYI.


>
> To call rcu_synchronize will also prevent weird behaviours by doing over
> netlink:
>
>  - set params A
>  - set params B
>  - dump params
>   \--> will dump params A


What's wrong with this? Existing readers could still read old data,
which is _perfectly_ fine as long as we don't free the old data before
they are gone.