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

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