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

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Message-ID: <20160623191123.GE30935@twins.programming.kicks-ass.net>
Date:	Thu, 23 Jun 2016 21:11:23 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	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 Thu, Jun 23, 2016 at 04:31:26PM +0200, Oleg Nesterov wrote:
> On 06/22, Linus Torvalds wrote:
> >
> > Oleg, what do you think? Would it be reasonable to free the stack and
> > thread_info synchronously at exit time, clear the pointer (to catch
> > any odd use), and only RCU-delay the task_struct itself?
> 
> I didn't see the patches yet, quite possibly I misunderstood... But no,
> I don't this we can do this (if we are not going to move ti->flags to
> task_struct at least).

Didn't we talk about using SLAB_DESTROY_BY_RCU for task_struct before?
If that is possible, a reuse in per-cpu cache is equally possible.

All we really want to guarantee is that the memory remains a
task_struct, it need not remain the same task, right?