From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751599AbdKVThR (ORCPT ); Wed, 22 Nov 2017 14:37:17 -0500 Received: from foss.arm.com ([217.140.101.70]:55504 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751368AbdKVThO (ORCPT ); Wed, 22 Nov 2017 14:37:14 -0500 Date: Wed, 22 Nov 2017 19:37:14 +0000 From: Will Deacon To: Pavel Machek Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, catalin.marinas@arm.com, mark.rutland@arm.com, ard.biesheuvel@linaro.org, sboyd@codeaurora.org, dave.hansen@linux.intel.com, keescook@chromium.org Subject: Re: [PATCH 00/18] arm64: Unmap the kernel whilst running in userspace (KAISER) Message-ID: <20171122193713.GL22648@arm.com> References: <1510942921-12564-1-git-send-email-will.deacon@arm.com> <20171122161913.GB12684@amd> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171122161913.GB12684@amd> 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 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. > 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