On Wed 2017-11-22 21:19:28, Ard Biesheuvel wrote: > On 22 November 2017 at 16:19, Pavel Machek <pa...@ucw.cz> 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.beca7...@viggo.jf.intel.com> > >> > >> 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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