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

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Date:   Wed, 22 Nov 2017 23:19:09 +0000
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Pavel Machek <pavel@....cz>
Cc:     Will Deacon <will.deacon@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Stephen Boyd <sboyd@...eaurora.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH 00/18] arm64: Unmap the kernel whilst running in userspace (KAISER)


> On 22 Nov 2017, at 22:33, Pavel Machek <pavel@....cz> wrote:
> 
>> On Wed 2017-11-22 21:19:28, Ard Biesheuvel wrote:
>>> On 22 November 2017 at 16:19, Pavel Machek <pavel@....cz> wrote:
>>> 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?
>>> 
>> 
>> Because it is impossible to get a cache hit on an access to an
>> unmapped address?
> 
> Um, no, I don't need to be able to directly access kernel addresses. I
> just put some data in _same place in cache where kernel data would
> go_, then do syscall and look if my data are still cached. Caches
> don't have infinite associativity.
> 

Ah ok. Interesting.

But how does that leak address bits that are covered by the tag?