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Date: Tue, 21 Jun 2016 11:24:18 +0200
From: Arnd Bergmann <arnd@...db.de>
To: Andy Lutomirski <luto@...nel.org>
Cc: x86@...nel.org, linux-kernel@...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>, 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 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.

	Arnd

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