linux-kernel - Re: KASAN: use-after-free Read in tcf_block

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Message-ID: <CACT4Y+ZvoVsL_BjBWr7QPnXOPGXqkqKfo-NQcox7vaAFPWVUVg@mail.gmail.com>
Date:   Thu, 27 Sep 2018 15:02:29 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Cong Wang <xiyou.wangcong@...il.com>,
        syzbot+37b8770e6d5a8220a039@...kaller.appspotmail.com,
        David Miller <davem@...emloft.net>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: KASAN: use-after-free Read in tcf_block_find

On Thu, Sep 27, 2018 at 3:00 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>
>
> 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.

I am not suggesting to commit this. This is just a hack for debugging.
It in fact lead to some warnings, but still allowed me to reproduce
the bug reliably.

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