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

[Will Deacon]

On Wed, Nov 22, 2017 at 05:19:14PM +0100, Pavel Machek wrote:
> > 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?

Is that a rhetorical question? If not, then I'm probably not the best person
to answer it. All I'm doing here is protecting against a class of attacks on
kaslr that make use of the TLB/page-table walker to determine where the
kernel is mapped.

> There was blackhat talk about exactly that IIRC...

Got a link? I'd be interested to see how the idea works in case there's an
orthogonal defence against it.

Will