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

From: Dmitry Vyukov
Date: Thu Sep 27 2018 - 09:02:55 EST


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);
>> + }
>> + }
>> }
>> }