linux-kernel - Re: [PATCH 9/9] ptrace: Don't change _

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Date:   Thu, 28 Apr 2022 17:11:11 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     linux-kernel@...r.kernel.org, rjw@...ysocki.net, mingo@...nel.org,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, mgorman@...e.de, bigeasy@...utronix.de,
        Will Deacon <will@...nel.org>, tj@...nel.org,
        linux-pm@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Johannes Berg <johannes@...solutions.net>,
        linux-um@...ts.infradead.org, Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>,
        inux-xtensa@...ux-xtensa.org, Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH 9/9] ptrace: Don't change __state

On 04/27, Eric W. Biederman wrote:
>
> "Eric W. Biederman" <ebiederm@...ssion.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.

And I think this is fine. This doesn't really differ from the case
when the tracee was killed before it takes siglock.

The only problem (afaics) is that, once we introduce JOBCTL_TRACED,
ptrace_stop() can leak this flag. That is why I suggested to clear
it along with LISTENING/DELAY_WAKEKILL before return, exactly because
schedule() won't block if fatal_signal_pending() is true.

But may be I misunderstood you concern?

Oleg.