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

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Date:   Thu, 23 Nov 2017 10:07:47 +0100
From:   Pavel Machek <pavel@....cz>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
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)

Hi!

> > On 22 Nov 2017, at 23:37, Pavel Machek <pavel@....cz> wrote:
> > 
> > Hi!
> > 
> >>>>> 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?
> > 
> > Same as leaking any other address bits? Caches are "virtually
> > indexed",
> 
> Not on arm64, although I don’t see how that is relevant if you are trying to defeat kaslr.
> 
> > and tag does not come into play...
> > 
> 
> Well, I must be missing something then, because I don’t see how knowledge about which userland address shares a cache way with a kernel address can leak anything beyond the bits that make up the index (i.e., which cache way is being shared)
> 

Well, KASLR is about keeping bits of kernel virtual address secret
from userland. Leaking them through cache sidechannel means KASLR is
defeated.


> > Maybe this explains it?
> > 
> 
> No not really. It explains how cache timing can be used as a side channel, not how it defeats kaslr.

Ok, look at this one:

https://www.blackhat.com/docs/us-16/materials/us-16-Jang-Breaking-Kernel-Address-Space-Layout-Randomization-KASLR-With-Intel-TSX-wp.pdf

You can use timing instead of TSX, right?
     	 	    	     	       	      	     	       	    Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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