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SubjectRe: [syzbot] KASAN: use-after-free Read in check_all_holdout_tasks_trace
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On 5/26/21 12:21 PM, Paul E. McKenney wrote:
> [Please note: This e-mail is from an EXTERNAL e-mail address]
>
> On Wed, May 26, 2021 at 10:22:59AM +0800, Xu, Yanfei wrote:
>> On 5/25/21 10:28 PM, Paul E. McKenney wrote:
>>> [Please note: This e-mail is from an EXTERNAL e-mail address]
>>>
>>> On Tue, May 25, 2021 at 06:24:10PM +0800, Xu, Yanfei wrote:
>>>>
>>>>
>>>> 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
>>>
>>> This one should be fine because the task cannot be freed until it
>>> actually exits, and the grace-period kthread never exits. But it
>>> could also be removed without any problem that I see. >
>>
>> Agree, current task's task_struct should be high probably safe. If you
>> think it is safe to remove, I prefer to remove it. Because it can make
>> trc_wait_for_one_reader's behavior about deleting task from holdout more
>> unified. And there should be a very small racy that the task is checked as a
>> current and then turn into a exiting task before its task_struct is accessed
>> in trc_wait_for_one_reader or check_all_holdout_tasks_trace.(or I
>> misunderstand something about rcu tasks)
>>
>>>> 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
>>>
>>> Ah, this one is more interesting, thank you!
>>>
>>> Yes, it is safe from the list's viewpoint to do the removal in the
>>> trc_inspect_reader() callback, but you are right that the grace-period
>>> kthread may touch the task structure after return, and there might not
>>> be anything else holding that task structure in place.
>>>
>>>>> Of course, if you can reproduce it, the following patch might be
>>>>
>>>> Sorry...I can't reproduce it, just analyse syzbot's log. :(
>>>
>>> Well, if it could be reproduced, that would mean that it was too easy,
>>> wouldn't it? ;-)
>>
>> Ha ;-)
>
> But it should be possible to make this happen... Is it possible to
> add lots of short-lived tasks to the test that failed?
>

Agree.

>>> How about the (untested) patch below, just to make sure that we are
>>> talking about the same thing? I have started testing, but then
>>> again, I have not yet been able to reproduce this, either.
>>>
>>> Thanx, Paul
>>
>> Yes! we are talking the same thing, Should I send a new patch?
>
> Or look at these commits that I queued this past morning (Pacific Time)
> on the "dev" branch of the -rcu tree:
>
> aac385ea2494 rcu-tasks: Don't delete holdouts within trc_inspect_reader()
> bf30dc63947c rcu-tasks: Don't delete holdouts within trc_wait_for_one_reader()

Got it, Thanks!

Regards,
Yanfei

>
> They pass initial testing, but then again, such tests passed before
> these patches were queued. :-/
>
> Thanx, Paul
>

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