Re: [syzbot] KASAN: use-after-free Read in check_all_holdout_tasks_trace

["Xu, Yanfei" <yanfei.xu@windriver.com>]

On 5/25/21 11:33 AM, Paul E. McKenney wrote:
> [Please note: This e-mail is from an EXTERNAL e-mail address]
> 
> On Tue, May 25, 2021 at 10:31:55AM +0800, Xu, Yanfei wrote:
>>
>>
>> On 5/25/21 6:46 AM, Paul E. McKenney wrote:
>>> [Please note: This e-mail is from an EXTERNAL e-mail address]
>>>
>>> On Sun, May 23, 2021 at 09:13:50PM -0700, Paul E. McKenney wrote:
>>>> On Sun, May 23, 2021 at 08:51:56AM +0200, Dmitry Vyukov wrote:
>>>>> On Fri, May 21, 2021 at 7:29 PM syzbot
>>>>> <syzbot+7b2b13f4943374609532@syzkaller.appspotmail.com> wrote:
>>>>>>
>>>>>> Hello,
>>>>>>
>>>>>> syzbot found the following issue on:
>>>>>>
>>>>>> HEAD commit:    f18ba26d libbpf: Add selftests for TC-BPF management API
>>>>>> git tree:       bpf-next
>>>>>> console output: https://syzkaller.appspot.com/x/log.txt?x=17f50d1ed00000
>>>>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=8ff54addde0afb5d
>>>>>> dashboard link: https://syzkaller.appspot.com/bug?extid=7b2b13f4943374609532
>>>>>>
>>>>>> Unfortunately, I don't have any reproducer for this issue yet.
>>>>>>
>>>>>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
>>>>>> Reported-by: syzbot+7b2b13f4943374609532@syzkaller.appspotmail.com
>>>>>
>>>>> This looks rcu-related. +rcu mailing list
>>>>
>>>> I think I see a possible cause for this, and will say more after some
>>>> testing and after becoming more awake Monday morning, Pacific time.
>>>
>>> No joy.  From what I can see, within RCU Tasks Trace, the calls to
>>> get_task_struct() are properly protected (either by RCU or by an earlier
>>> get_task_struct()), and the calls to put_task_struct() are balanced by
>>> those to get_task_struct().
>>>
>>> I could of course have missed something, but at this point I am suspecting
>>> an unbalanced put_task_struct() has been added elsewhere.
>>>
>>> As always, extra eyes on this code would be a good thing.
>>>
>>> If it were reproducible, I would of course suggest bisection.  :-/
>>>
>>>                                                           Thanx, Paul
>>>
>> Hi Paul,
>>
>> Could it be?
>>
>>         CPU1                                        CPU2
>> trc_add_holdout(t, bhp)
>> //t->usage==2
>>                                        release_task
>>                                          put_task_struct_rcu_user
>>                                            delayed_put_task_struct
>>                                              ......
>>                                              put_task_struct(t)
>>                                              //t->usage==1
>>
>> check_all_holdout_tasks_trace
>>    ->trc_wait_for_one_reader
>>      ->trc_del_holdout
>>        ->put_task_struct(t)
>>        //t->usage==0 and task_struct freed
>>    READ_ONCE(t->trc_reader_checked)
>>    //ops， t had been freed.
>>
>> So, after excuting trc_wait_for_one_reader（）, task might had been removed
>> from holdout list and the corresponding task_struct was freed.
>> And we shouldn't do READ_ONCE(t->trc_reader_checked).
> 
> I was suspicious of that call to trc_del_holdout() from within
> trc_wait_for_one_reader(), but the only time it executes is in the
> context of the current running task, which means that CPU 2 had better
> not be invoking release_task() on it just yet.
> 
> Or am I missing your point?

Two times.
1. the task is current.

                trc_wait_for_one_reader
                  ->trc_del_holdout

2. task isn't current.

                trc_wait_for_one_reader
                  ->get_task_struct
                  ->try_invoke_on_locked_down_task（trc_inspect_reader）
                    ->trc_del_holdout
                  ->put_task_struct


> 
> Of course, if you can reproduce it, the following patch might be

Sorry...I can't reproduce it, just analyse syzbot's log. :(


Thanks,
Yanfei

> an interesting thing to try, my doubts notwithstanding.  But more
> important, please check the patch to make sure that we are both
> talking about the same call to trc_del_holdout()!
> 
> If we are talking about the same call to trc_del_holdout(), are you
> actually seeing that code execute except when rcu_tasks_trace_pertask()
> calls trc_wait_for_one_reader()?
> 
>> I investigate the trc_wait_for_one_reader（） and found before we excute
>> trc_del_holdout, there is always set t->trc_reader_checked=true. How about
>> we just set the checked flag and unified excute trc_del_holdout()
>> in check_all_holdout_tasks_trace with checking the flag?
> 
> The problem is that we cannot execute trc_del_holdout() except in
> the context of the RCU Tasks Trace grace-period kthread.  So it is
> necessary to manipulate ->trc_reader_checked separately from the list
> in order to safely synchronize with IPIs and with the exit code path
> for any reader tasks, see for example trc_read_check_handler() and
> exit_tasks_rcu_finish_trace().
> 
> Or are you thinking of some other approach?
> 
>                                                          Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> index efb8127f3a36..2a0d4bdd619a 100644
> --- a/kernel/rcu/tasks.h
> +++ b/kernel/rcu/tasks.h
> @@ -987,7 +987,6 @@ static void trc_wait_for_one_reader(struct task_struct *t,
>          // The current task had better be in a quiescent state.
>          if (t == current) {
>                  t->trc_reader_checked = true;
> -               trc_del_holdout(t);
>                  WARN_ON_ONCE(READ_ONCE(t->trc_reader_nesting));
>                  return;
>          }
>