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Date:   Wed, 27 Apr 2022 19:21:10 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     linux-kernel@vger.kernel.org, rjw@rjwysocki.net, mingo@kernel.org,
        vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
        rostedt@goodmis.org, mgorman@suse.de, bigeasy@linutronix.de,
        Will Deacon <will@kernel.org>, tj@kernel.org,
        linux-pm@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
        Richard Weinberger <richard@nod.at>,
        Anton Ivanov <anton.ivanov@cambridgegreys.com>,
        Johannes Berg <johannes@sipsolutions.net>,
        linux-um@lists.infradead.org, Chris Zankel <chris@zankel.net>,
        Max Filippov <jcmvbkbc@gmail.com>,
        inux-xtensa@linux-xtensa.org, Kees Cook <keescook@chromium.org>,
        Jann Horn <jannh@google.com>
Subject: Re: [PATCH 9/9] ptrace: Don't change __state
Message-ID: <20220427172109.GK17421@redhat.com>
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 <20220427160901.GI17421@redhat.com>
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On 04/27, Oleg Nesterov wrote:
>
> On 04/27, Eric W. Biederman wrote:
> >
> > Oleg Nesterov <oleg@redhat.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.

Not to mention that the tracee is TASK_RUNNING after PTRACE_DETACH wakes it
up, so the pending JOBCTL_DELAY_WAKEKILL simply has no effect.

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
> >