From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753420AbdKVWeA (ORCPT ); Wed, 22 Nov 2017 17:34:00 -0500 Received: from atrey.karlin.mff.cuni.cz ([195.113.26.193]:54247 "EHLO atrey.karlin.mff.cuni.cz" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751910AbdKVWd5 (ORCPT ); Wed, 22 Nov 2017 17:33:57 -0500 Date: Wed, 22 Nov 2017 23:33:55 +0100 From: Pavel Machek To: Ard Biesheuvel Cc: Will Deacon , "linux-arm-kernel@lists.infradead.org" , "linux-kernel@vger.kernel.org" , Catalin Marinas , Mark Rutland , Stephen Boyd , Dave Hansen , Kees Cook Subject: Re: [PATCH 00/18] arm64: Unmap the kernel whilst running in userspace (KAISER) Message-ID: <20171122223355.GA5877@amd> References: <1510942921-12564-1-git-send-email-will.deacon@arm.com> <20171122161913.GB12684@amd> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="AhhlLboLdkugWU4S" Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --AhhlLboLdkugWU4S Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Wed 2017-11-22 21:19:28, Ard Biesheuvel wrote: > On 22 November 2017 at 16:19, Pavel Machek wrote: > > Hi! > > > >> This patch series implements something along the lines of KAISER for a= rm64: > >> > >> 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 a= nd > >> 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 virt= ual > >> 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? > > >=20 > 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 --=20 (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blo= g.html --AhhlLboLdkugWU4S Content-Type: application/pgp-signature; name="signature.asc" Content-Description: Digital signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 iEYEARECAAYFAloV+1IACgkQMOfwapXb+vIc4QCfSkaM2/eKj6v4zBoYH06EaMS9 SPoAoL4QlpZ/ra9876qiW0zU2fpaNBYT =iajI -----END PGP SIGNATURE----- --AhhlLboLdkugWU4S--