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

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Date:   Thu, 23 Nov 2017 06:51:09 +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 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)

> Maybe this explains it?
> 

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

Thanks,
Ard.