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Date:   Wed, 5 Apr 2017 14:30:45 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     bsegall@...gle.com
Cc:     linux-kernel@...r.kernel.org,
        Roland McGrath <roland@...k.frob.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>
Subject: Re: [PATCHv2] ptrace: fix PTRACE_LISTEN race corrupting task->state

On 04/04, bsegall@...gle.com wrote:
>
> v2: slight clarification in comments, put the conditional around the
> whole wakeup area

Acked-by: Oleg Nesterov <oleg@...hat.com>

and I think this should go to -stable.

> Oleg mentioned a preference for making LISTEN unfreeze instead; I have
> no preference there,

Yes, but I won't insist if you prefer this more simple fix,

> just wanted to make sure that this doesn't get
> forgotten entirely.

And you are right, I forgot about this bug. Thanks!

Oleg.

>  kernel/ptrace.c | 14 ++++++++++----
>  1 file changed, 10 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/ptrace.c b/kernel/ptrace.c
> index 0af928712174..7cc49c3e73af 100644
> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -184,11 +184,17 @@ static void ptrace_unfreeze_traced(struct task_struct *task)
>
>         WARN_ON(!task->ptrace || task->parent != current);
>
> +       /*
> +        * PTRACE_LISTEN can allow ptrace_trap_notify to wake us up
> +        * remotely. Recheck state under the lock to close this race.
> +        */
>         spin_lock_irq(&task->sighand->siglock);
> -       if (__fatal_signal_pending(task))
> -               wake_up_state(task, __TASK_TRACED);
> -       else
> -               task->state = TASK_TRACED;
> +       if (task->state == __TASK_TRACED) {
> +               if (__fatal_signal_pending(task))
> +                       wake_up_state(task, __TASK_TRACED);
> +               else
> +                       task->state = TASK_TRACED;
> +       }
>         spin_unlock_irq(&task->sighand->siglock);
>  }
>
> --
> 2.12.2.715.g7642488e1d-goog
>

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