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

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Date:   Wed, 27 Apr 2022 12:31:09 -0500
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     Oleg Nesterov <oleg@...hat.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

Oleg Nesterov <oleg@...hat.com> writes:

> On 04/27, Oleg Nesterov wrote:
>>
>> 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.
>
> 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.

Oh.  You are talking about the window when between clearing the
traced state and when tracee resumes executing and clears
JOBCTL_DELAY_WAKEKILL.

I thought you were thinking about JOBCTL_DELAY_WAKEKILL being leaked.

That requires both ptrace_attach and ptrace_check_attach for the new
tracer to happen before the tracee is scheduled to run.

I agree.  I think the WARN_ON could reasonably be moved a bit later,
but I don't know that the WARN_ON is important. I simply kept it because
it seemed to make sense.

Eric