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

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Message-ID: <20220427171829.GJ17421@redhat.com>
Date:   Wed, 27 Apr 2022 19:18:30 +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:
>
> Oleg Nesterov <oleg@...hat.com> writes:
>
> > On 04/26, Eric W. Biederman wrote:
> >>
> >> @@ -253,7 +252,7 @@ static int ptrace_check_attach(struct task_struct *child, bool ignore_state)
> >>  	 */
> >>  	if (lock_task_sighand(child, &flags)) {
> >>  		if (child->ptrace && child->parent == current) {
> >> -			WARN_ON(READ_ONCE(child->__state) == __TASK_TRACED);
> >> +			WARN_ON(child->jobctl & JOBCTL_DELAY_WAKEKILL);
> >
> > This WARN_ON() doesn't look right.
> >
> > It is possible that this child was traced by another task and PTRACE_DETACH'ed,
> > but it didn't clear DELAY_WAKEKILL.
>
> That would be a bug.  That would mean that PTRACE_DETACHED process can
> not be SIGKILL'd.

Why? The tracee will take siglock, clear JOBCTL_DELAY_WAKEKILL and notice
SIGKILL after that.

Oleg.

> > If the new debugger attaches and calls ptrace() before the child takes siglock
> > ptrace_freeze_traced() will fail, but we can hit this WARN_ON().
> 
> Eric
>