linux-kernel - Re: [PATCH 00/18] arm64: Unmap the kernel whilst running in userspace (KAISER)

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Message-ID: <20171122161913.GB12684@amd>
Date:   Wed, 22 Nov 2017 17:19:14 +0100
From:   Pavel Machek <pavel@....cz>
To:     Will Deacon <will.deacon@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        catalin.marinas@....com, mark.rutland@....com,
        ard.biesheuvel@...aro.org, sboyd@...eaurora.org,
        dave.hansen@...ux.intel.com, keescook@...omium.org
Subject: Re: [PATCH 00/18] arm64: Unmap the kernel whilst running in
 userspace (KAISER)

Hi!

> This patch series implements something along the lines of KAISER for arm64:
> 
>   https://gruss.cc/files/kaiser.pdf
> 
> although I wrote this from scratch because the paper has some funny
> assumptions about how the architecture works. There is a patch series
> in review for x86, which follows a similar approach:
> 
>   http://lkml.kernel.org/r/<20171110193058.BECA7D88@...go.jf.intel.com>
> 
> and the topic was recently covered by LWN (currently subscriber-only):
> 
>   https://lwn.net/Articles/738975/
> 
> The basic idea is that transitions to and from userspace are proxied
> through a trampoline page which is mapped into a separate page table and
> can switch the full kernel mapping in and out on exception entry and
> exit respectively. This is a valuable defence against various KASLR and
> timing attacks, particularly as the trampoline page is at a fixed virtual
> address and therefore the kernel text can be randomized
> independently.

If I'm willing to do timing attacks to defeat KASLR... what prevents
me from using CPU caches to do that?

There was blackhat talk about exactly that IIRC...
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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