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From:   "Eric W. Biederman" <ebiederm@xmission.com>
To:     linux-kernel@vger.kernel.org
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References: <878rrrh32q.fsf_-_@email.froward.int.ebiederm.org>
        <20220426225211.308418-9-ebiederm@xmission.com>
Date:   Wed, 27 Apr 2022 18:05:47 -0500
In-Reply-To: <20220426225211.308418-9-ebiederm@xmission.com> (Eric
        W. Biederman's message of "Tue, 26 Apr 2022 17:52:11 -0500")
Message-ID: <87czh2160k.fsf@email.froward.int.ebiederm.org>
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Subject: Re: [PATCH 9/9] ptrace: Don't change __state
Precedence: bulk

"Eric W. Biederman" <ebiederm@xmission.com> writes:

> diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
> index 3c8b34876744..1947c85aa9d9 100644
> --- a/include/linux/sched/signal.h
> +++ b/include/linux/sched/signal.h
> @@ -437,7 +437,8 @@ extern void signal_wake_up_state(struct task_struct *t, unsigned int state);
>  
>  static inline void signal_wake_up(struct task_struct *t, bool resume)
>  {
> -	signal_wake_up_state(t, resume ? TASK_WAKEKILL : 0);
> +	bool wakekill = resume && !(t->jobctl & JOBCTL_DELAY_WAKEKILL);
> +	signal_wake_up_state(t, wakekill ? TASK_WAKEKILL : 0);
>  }
>  static inline void ptrace_signal_wake_up(struct task_struct *t, bool resume)
>  {

Grrr.  While looking through everything today I have realized that there
is a bug.

Suppose we have 3 processes: TRACER, TRACEE, KILLER.

Meanwhile TRACEE is in the middle of ptrace_stop, just after siglock has
been dropped.

The TRACER process has performed ptrace_attach on TRACEE and is in the
middle of a ptrace operation and has just set JOBCTL_DELAY_WAKEKILL.

Then comes in the KILLER process and sends the TRACEE a SIGKILL.
The TRACEE __state remains TASK_TRACED, as designed.

The bug appears when the TRACEE makes it to schedule().  Inside
schedule there is a call to signal_pending_state() which notices
a SIGKILL is pending and refuses to sleep.

I could avoid setting TIF_SIGPENDING in signal_wake_up but that
is insufficient as another signal may be pending.

I could avoid marking the task as __fatal_signal_pending but then
where would the information that the task needs to become
__fatal_signal_pending go.

Hmm.

This looks like I need my other pending cleanup which introduces a
helper to get this idea to work.

Eric