Re: KASAN: use-after-free Read in tcf_block_find

[Dmitry Vyukov]

On Thu, Sep 27, 2018 at 3:00 PM, Eric Dumazet <eric.dumazet@xxxxxxxxx> 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);
>> +                       }
>> +               }
>>         }
>>  }