linux-kernel - Re: [PATCH 2/2] arm: apply more __ro_after

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Message-ID: <20160810230645.GP1041@n2100.armlinux.org.uk>
Date:	Thu, 11 Aug 2016 00:06:45 +0100
From:	Russell King - ARM Linux <linux@...linux.org.uk>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	linux-arm-kernel@...ts.infradead.org,
	Kees Cook <keescook@...omium.org>,
	linux-arch <linux-arch@...r.kernel.org>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	"x86@...nel.org" <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mathias Krause <minipli@...glemail.com>
Subject: Re: [PATCH 2/2] arm: apply more __ro_after_init

On Wed, Aug 10, 2016 at 09:41:23PM +0200, Arnd Bergmann wrote:
> It might be better to start by making the fixed mapping readonly,
> as KASLR doesn't protect that one at all, and change the TLS
> code accordingly.

I think that's impossible, because we gave userspace permission to
read 0xffff0ff0 directly without using __kuser_get_tls.  You're
talking about potentially breaking userspace.

If you disable kuser helpers, then the page becomes read-only and
invisible to userspace anyway.  So, everything is being done there
which can be done - if you have kuser helpers enabled, then you
lose some opportunities for these security improvements.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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