linux-kernel - Re: [PATCH v3 00/13] Virtually mapped stacks with guard pages (x86, core)

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Message-ID: <20160623170352.GA17372@redhat.com>
Date:	Thu, 23 Jun 2016 19:03:52 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Andy Lutomirski <luto@...nel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Nadav Amit <nadav.amit@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Brian Gerst <brgerst@...il.com>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Jann Horn <jann@...jh.net>,
	Heiko Carstens <heiko.carstens@...ibm.com>
Subject: Re: [PATCH v3 00/13] Virtually mapped stacks with guard pages (x86,
 core)

On 06/23, Linus Torvalds wrote:
>
> But that does bring up another possibility: do it at wait() time, when
> we do release_thread(). That's when we *used* to synchronously free
> it, before we did the lockless RCU walks.

Let me quote my previous email ;)

	And we can't free/nullify it when the parent/debuger reaps a zombie,
	say, mark_oom_victim() expects that get_task_struct() protects
	thread_info as well.

probably we can fix all such users though...

> At that point, it has been removed from all the thread lists. So the
> only way to find it is through the RCU walks. Do any of *those* touch
> ti->flags? I'm not seeing it,

Neither me, although I didn't try to grep too much.

> and it sounds fixable if any do

probably yes, but this would mean that tasklist_lock protects task->stack,
doesn't look really nice...

Oleg.