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

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Message-ID: <CAGXu5jLd=cUa-8Fdp+6hypvn=cVVVkv-MWcZBnEy956fA96aMg@mail.gmail.com>
Date:	Thu, 23 Jun 2016 11:00:08 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.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>,
	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 10:52 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Thu, Jun 23, 2016 at 10:44 AM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>>
>> The thread_info->tsk pointer, that was one of the most critical issues
>> and the main raison d'être of the thread_info, has been replaced on
>> x86 by just using the per-cpu "current_task". Yes,.there are probably
>> more than a few "ti->task" users left for legacy reasons, harking back
>> to when the thread-info was cheaper to access, but it shouldn't be a
>> big deal.
>
> Ugh. Looking around at this, it turns out that a great example of this
> kind of legacy issue is the debug_mutex stuff.
>
> It uses "struct thread_info *" as the owner pointer, and there is _no_
> existing reason for it. In fact, in every single place it actually
> wants the task_struct, and it does task_thread_info(task) just to
> convert it to the thread-info, and then converts it back with
> "ti->task".

Heh, yeah, that looks like a nice clean-up.

> So the attached patch seems to be the right thing to do regardless of
> this whole discussion.

Why does __mutex_lock_common() have "task" as a stack variable? It's
only assigned at the start, and is always "current". (I only noticed
from the patch changing "current_thread_info()" and
"task_thread_info(task)" both to "task".)

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security