From mboxrd@z Thu Jan  1 00:00:00 1970
From: Eric Dumazet <eric.dumazet@gmail.com>
Subject: Re: KASAN: use-after-free Read in tcf_block_find
Date: Thu, 27 Sep 2018 06:00:39 -0700
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Cc: Eric Dumazet <eric.dumazet@gmail.com>,
        syzbot+37b8770e6d5a8220a039@syzkaller.appspotmail.com,
        David Miller <davem@davemloft.net>,
        Jamal Hadi Salim <jhs@mojatatu.com>,
        Jiri Pirko <jiri@resnulli.us>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux Kernel Network Developers <netdev@vger.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>
To: Dmitry Vyukov <dvyukov@google.com>,
        Cong Wang <xiyou.wangcong@gmail.com>
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On 09/27/2018 01:10 AM, Dmitry Vyukov wrote:

> 
> Would a stack trace for call_rcu be helpful here? I have this idea for
> a long time, but never get around to implementing it:
> https://bugzilla.kernel.org/show_bug.cgi?id=198437
> 
> Also FWIW I recently used the following hack for another net bug. It
> made that other bug involving call_rcu way more likely to fire. Maybe
> it will be helpful here too.
> 
> diff --git a/net/core/dst.c b/net/core/dst.c
> index 81ccf20e28265..591a8d0aca545 100644
> --- a/net/core/dst.c
> +++ b/net/core/dst.c
> @@ -187,8 +187,16 @@ void dst_release(struct dst_entry *dst)
>                 if (unlikely(newrefcnt < 0))
>                         net_warn_ratelimited("%s: dst:%p refcnt:%d\n",
>                                              __func__, dst, newrefcnt);
> -               if (!newrefcnt)
> -                       call_rcu(&dst->rcu_head, dst_destroy_rcu);
> +               if (!newrefcnt) {
> +                       if (lock_is_held(&rcu_bh_lock_map) ||
> +                               lock_is_held(&rcu_lock_map) ||
> +                               lock_is_held(&rcu_sched_lock_map)) {
> +                               call_rcu(&dst->rcu_head, dst_destroy_rcu);
> +                       } else {
> +                               synchronize_rcu();

dst_release() can be called in context we hold a spinlock, this would be bad to reschedule here.

> +                               dst_destroy_rcu(&dst->rcu_head);
> +                       }
> +               }
>         }
>  }
>