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

[Pavel Machek]

On Wed 2017-11-22 21:19:28, Ard Biesheuvel wrote:
> On 22 November 2017 at 16:19, Pavel Machek <pavel@xxxxxx> 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@xxxxxxxxxxxxxxxxxx>
> >>
> >> 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.

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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