netdev - Re: [Patch net] net_sched: add a temporary refcnt for struct tcindex

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Message-Id: <20200401.110710.446461527815245401.davem@davemloft.net>
Date:   Wed, 01 Apr 2020 11:07:10 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     xiyou.wangcong@...il.com
Cc:     netdev@...r.kernel.org,
        syzbot+46f513c3033d592409d2@...kaller.appspotmail.com,
        tglx@...utronix.de, paulmck@...nel.org, jhs@...atatu.com,
        jiri@...nulli.us
Subject: Re: [Patch net] net_sched: add a temporary refcnt for struct
 tcindex_data

From: Cong Wang <xiyou.wangcong@...il.com>
Date: Sat, 28 Mar 2020 12:12:59 -0700

> Although we intentionally use an ordered workqueue for all tc
> filter works, the ordering is not guaranteed by RCU work,
> given that tcf_queue_work() is esstenially a call_rcu().
> 
> This problem is demostrated by Thomas:
> 
>   CPU 0:
>     tcf_queue_work()
>       tcf_queue_work(&r->rwork, tcindex_destroy_rexts_work);
> 
>   -> Migration to CPU 1
> 
>   CPU 1:
>      tcf_queue_work(&p->rwork, tcindex_destroy_work);
> 
> so the 2nd work could be queued before the 1st one, which leads
> to a free-after-free.
> 
> Enforcing this order in RCU work is hard as it requires to change
> RCU code too. Fortunately we can workaround this problem in tcindex
> filter by taking a temporary refcnt, we only refcnt it right before
> we begin to destroy it. This simplifies the code a lot as a full
> refcnt requires much more changes in tcindex_set_parms().
> 
> Reported-by: syzbot+46f513c3033d592409d2@...kaller.appspotmail.com
> Fixes: 3d210534cc93 ("net_sched: fix a race condition in tcindex_destroy()")
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Paul E. McKenney <paulmck@...nel.org>
> Cc: Jamal Hadi Salim <jhs@...atatu.com>
> Cc: Jiri Pirko <jiri@...nulli.us>
> Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>

Applied and queued up for -stable, thanks.