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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 18 Aug 2021 19:57:32 -0700
Message-ID: <CAHk-=wgBMNC1ePTgqM6f0iBH94KE5_oHQYD2sqCbjev0KaZ6Kw@mail.gmail.com>
Subject: Re: [PATCH] kernel: make TIF_NOTIFY_SIGNAL and core dumps co-exist
To:     Jens Axboe <axboe@kernel.dk>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Olivier Langlois <olivier@trillion01.com>,
        Tony Battersby <tonyb@cybernetics.com>
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On Tue, Aug 17, 2021 at 8:06 PM Jens Axboe <axboe@kernel.dk> wrote:
>
> task_work being added with notify == TWA_SIGNAL will utilize
> TIF_NOTIFY_SIGNAL for signaling the targeted task that work is available.
> If this happens while a task is going through a core dump, it'll
> potentially disturb and truncate the dump as a signal interruption.

This patch seems (a) buggy and (b) hacky.

> --- a/kernel/task_work.c
> +++ b/kernel/task_work.c
> @@ -41,6 +41,12 @@ int task_work_add(struct task_struct *task, struct callback_head *work,
>                 head = READ_ONCE(task->task_works);
>                 if (unlikely(head == &work_exited))
>                         return -ESRCH;
> +               /*
> +                * TIF_NOTIFY_SIGNAL notifications will interfere with
> +                * a core dump in progress, reject them.
> +                */
> +               if (notify == TWA_SIGNAL && (task->flags & PF_SIGNALED))
> +                       return -ESRCH;

This basically seems to check task->flags with no serialization.

I'm sure it works 99.9% of the time in practice, since you'd be really
unlucky to hit any races, but I really don't see what the
serialization logic is.

Also, the main user that actually triggered the problem already has

        if (unlikely(tsk->flags & PF_EXITING))
                goto fail;

just above the call to task_work_add(), so this all seems very hacky indeed.

Of course, I don't see what the serialization for _that_ one is either.

Pls explain. You can't just randomly add tests for random flags that
get modified by other random code.

               Linus