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

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Message-ID: <20171122193713.GL22648@arm.com>
Date:   Wed, 22 Nov 2017 19:37:14 +0000
From:   Will Deacon <will.deacon@....com>
To:     Pavel Machek <pavel@....cz>
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)

On Wed, Nov 22, 2017 at 05:19:14PM +0100, Pavel Machek wrote:
> > 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?

Is that a rhetorical question? If not, then I'm probably not the best person
to answer it. All I'm doing here is protecting against a class of attacks on
kaslr that make use of the TLB/page-table walker to determine where the
kernel is mapped.

> There was blackhat talk about exactly that IIRC...

Got a link? I'd be interested to see how the idea works in case there's an
orthogonal defence against it.

Will