From mboxrd@z Thu Jan 1 00:00:00 1970 From: pavel@ucw.cz (Pavel Machek) Date: Wed, 22 Nov 2017 23:36:50 +0100 Subject: [PATCH 00/18] arm64: Unmap the kernel whilst running in userspace (KAISER) In-Reply-To: <20171122193713.GL22648@arm.com> References: <1510942921-12564-1-git-send-email-will.deacon@arm.com> <20171122161913.GB12684@amd> <20171122193713.GL22648@arm.com> Message-ID: <20171122223650.GA6130@amd> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Wed 2017-11-22 19:37:14, Will Deacon wrote: > 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@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? > > 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. Yeah. What I'm saying is that I can use cache effects to probe where kernel is mapped (and what it is doing). > > 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. https://www.youtube.com/watch?v=9KsnFWejpQg (Tell me if it is not the right one). As of defenses... yes. "maxcpus=1" and flush caches on switch to usermode will do the trick :-). Ok, so that was sarcastic. I'm not sure if good defense exists. ARM is better than i386 because reading time and cache flushing is priviledged, but... Pavel -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html -------------- next part -------------- A non-text attachment was scrubbed... Name: signature.asc Type: application/pgp-signature Size: 181 bytes Desc: Digital signature URL: