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

From: Ard Biesheuvel
Date: Thu Nov 23 2017 - 01:51:27 EST

> On 22 Nov 2017, at 23:37, Pavel Machek <pavel@xxxxxx> 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 donât 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 donât 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.