linux-kernel - Re: [PATCH 2/4] ptrace: ensure arch_ptrace/ptrace

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Date:	Mon, 21 Jan 2013 18:21:51 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Dan Carpenter <dan.carpenter@...cle.com>,
	Kernel Security <security@...nel.org>,
	Michael Davidson <md@...gle.com>,
	Suleiman Souhlal <suleiman@...gle.com>,
	Julien Tinnes <jln@...gle.com>,
	Aaron Durbin <adurbin@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Tejun Heo <tj@...nel.org>,
	Roland McGrath <roland@...k.frob.com>,
	Tony Luck <tony.luck@...el.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH 2/4] ptrace: ensure arch_ptrace/ptrace_request can
	never race with SIGKILL

On 01/20, Linus Torvalds wrote:
>
> On Sun, Jan 20, 2013 at 11:25 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> > +
> > +static void ptrace_unfreeze_traced(struct task_struct *task)
> > +{
> > +       if (task->state != __TASK_TRACED)
> > +               return;
> > +
> > +       if (WARN_ON(!task->ptrace || task->parent != current))
> > +               return;
>
> This WARN_ON() is bogus.
>
> Because you added the warning, you then need to make the callers check
> for the whole PTRACE_UNATTACH.
>
> So I think you should just remove the WARN_ON(), and then just call
> ptrace_unfreeze_traced() unconditionally after you've successfully
> done a ptrace_check_attach(). Just to keep the code simpler.

This is what initial patch did. But, assuming that ptrace_unfreeze_traced()
is called unconditionally, we need a locking or barriers, otherwise

	// another debugger attached after we did PTRACE_DETACH ?
	if (!task->ptrace || task->parent != current)
		return;

is racy. Suppose we trace the natural child, then do PTRACE_DETACH,
then another tracer comes. We can see ->ptrace and __TASK_TRACED,
but see the old task->parent == current.

Of course, this is only theoretical, and probably we can add a barrier
before this check, but I am not sure this will make the code simpler.
If nothing else, this needs a comment.

If PTRACE_DETACH doesn't do _unfreeze_, we know that the task is either
traced by us or it is exiting/exited, so we can always trust the
"state == __TASK_TRACED" check.

So I'd prefer to keep this code, but I won't insist if you still disagree.

> Also, in your corrected version, you had
>
> +               if (!wait_task_inactive(child, __TASK_TRACED)) {
> +                       /* This can only happen if may_ptrace_stop() fails */
> +                       WARN_ON(child->state == __TASK_TRACED);
> +                       ret = -ESRCH;
>
> where I actually think that the comment is not really helpful. It
> doesn't really explain what he child can do to get to ptrace_stop() in
> the first place, and what that does to the child state...

OK. Agreed. This comment reflects the fact that the first version removed
may_ptrace_stop() to ensure wait_task_inactive() can't fail.

I'll update the comment and resend.

Oleg.

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