linux-kernel - Re: [PATCH 0/3] MIPS executable stack protection

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Message-ID: <CA+55aFynSp90n=jdUdmY7nq-9t4pHS82Tj-WfZOBfot7ip0hBw@mail.gmail.com>
Date:	Sat, 4 Oct 2014 09:03:53 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Leonid Yegoshin <Leonid.Yegoshin@...tec.com>,
	linux-mips <linux-mips@...ux-mips.org>,
	Zubair.Kakakhel@...tec.com, David Daney <david.daney@...ium.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Davidlohr Bueso <davidlohr@...com>,
	"Maciej W. Rozycki" <macro@...ux-mips.org>, chenhc@...ote.com,
	Rafał Miłecki <zajec5@...il.com>,
	James Hogan <james.hogan@...tec.com>,
	Kees Cook <keescook@...omium.org>, alex@...x-smith.me.uk,
	Thomas Gleixner <tglx@...utronix.de>,
	John Crispin <blogic@...nwrt.org>, jchandra@...adcom.com,
	paul.burton@...tec.com, qais.yousef@...tec.com,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ralf Baechle <ralf@...ux-mips.org>, markos.chandras@...tec.com,
	Manuel Lauss <manuel.lauss@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	lars.persson@...s.com
Subject: Re: [PATCH 0/3] MIPS executable stack protection

On Sat, Oct 4, 2014 at 1:23 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Fri, Oct 03, 2014 at 08:17:14PM -0700, Leonid Yegoshin wrote:
>> The following series implements an executable stack protection in MIPS.
>>
>> It sets up a per-thread 'VDSO' page and appropriate TLB support.
>
> So traditionally we've always avoided per-thread pages like that. What
> makes it worth it on MIPS?

Nothing makes it worth it on MIPS.

It may be easy to implement when you have all software-fill of TLB's,
but it's a mistake even then - because it means that you'll never be
able to do hardware TLB walkers.

And MIPS *does* have hardware TLB walkers, even if they are not
necessarily available everywhere.

So this is a horrible idea. Don't do it. Page tables need to be
per-process, not per thread, so that page tables can be shared.

              Linus
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