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Date: Tue, 21 Jun 2016 10:16:21 -0700
From: Kees Cook <keescook@...omium.org>
To: Arnd Bergmann <arnd@...db.de>
Cc: Andy Lutomirski <luto@...nel.org>, "x86@...nel.org" <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>, 
	linux-arch <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>, 
	Linus Torvalds <torvalds@...ux-foundation.org>, 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 Tue, Jun 21, 2016 at 2:24 AM, Arnd Bergmann <arnd@...db.de> wrote:
> On Monday, June 20, 2016 4:43:30 PM CEST Andy Lutomirski wrote:
>>
>> On my laptop, this adds about 1.5┬Ás of overhead to task creation,
>> which seems to be mainly caused by vmalloc inefficiently allocating
>> individual pages even when a higher-order page is available on the
>> freelist.
>
> Would it help to have a fixed virtual address for the stack instead
> and map the current stack to that during a task switch, similar to
> how we handle fixmap pages?
>
> That would of course trade the allocation overhead for a task switch
> overhead, which may be better or worse. It would also give "current"
> a constant address, which may give a small performance advantage
> but may also introduce a new attack vector unless we randomize it
> again.

Right: we don't want a fixed address. That makes attacks WAY easier.

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security

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