From mboxrd@z Thu Jan 1 00:00:00 1970 From: Will Deacon Subject: Re: [PATCH 00/17] ARMv8.3 pointer authentication support Date: Fri, 19 Oct 2018 13:36:47 +0100 Message-ID: <20181019123646.GG14246@arm.com> References: <20181005084754.20950-1-kristina.martsenko@arm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20181005084754.20950-1-kristina.martsenko@arm.com> Sender: linux-kernel-owner@vger.kernel.org To: Kristina Martsenko Cc: linux-arm-kernel@lists.infradead.org, Adam Wallis , Amit Kachhap , Andrew Jones , Ard Biesheuvel , Arnd Bergmann , Catalin Marinas , Christoffer Dall , Dave P Martin , Jacob Bramley , Kees Cook , Marc Zyngier , Mark Rutland , Ramana Radhakrishnan , "Suzuki K . Poulose" , kvmarm@lists.cs.columbia.edu, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org List-Id: linux-arch.vger.kernel.org On Fri, Oct 05, 2018 at 09:47:37AM +0100, Kristina Martsenko wrote: > 1) Key support > > This series enables the use of instructions using APIAKey, which is > initialised and maintained per-process (shared by all threads). GCC > currently only makes use of APIAKey. > > This series does not add support for APIBKey, APDAKey, APDBKey, nor > APGAKey. HINT-space instructions using these keys will currently execute > as NOPs. Support for these keys can be added as users appear. > > Note that while we expose the cpuid register (ID_AA64ISAR1_EL1) to > userspace, it only contains one feature for address authentication > (API/APA), so it cannot be used by userspace to tell which keys the > kernel supports. For this the kernel exposes HWCAP bits, one per key > (currently only APIAKey), which must be checked instead. Given that the architecture doesn't provide an identification mechanism for the case where only one of the keys is available, I would much prefer that we expose both of the keys to userspace. Is the only downside of that a possible exception entry overhead if the kernel wants to use pointer authentication as well? Having an initial implementation where the B key operations act as NOPs isn't ideal if we want to support future users -- chances are they'll be put off because deployed kernels don't give them whatever security guarantees they require. It's a bit of a chicken-and-egg problem, so unless we have good reasons to keep the B key hidden, I think we should be exposing it from the start. Will From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:51820 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727011AbeJSUmk (ORCPT ); Fri, 19 Oct 2018 16:42:40 -0400 Date: Fri, 19 Oct 2018 13:36:47 +0100 From: Will Deacon Subject: Re: [PATCH 00/17] ARMv8.3 pointer authentication support Message-ID: <20181019123646.GG14246@arm.com> References: <20181005084754.20950-1-kristina.martsenko@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181005084754.20950-1-kristina.martsenko@arm.com> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Kristina Martsenko Cc: linux-arm-kernel@lists.infradead.org, Adam Wallis , Amit Kachhap , Andrew Jones , Ard Biesheuvel , Arnd Bergmann , Catalin Marinas , Christoffer Dall , Dave P Martin , Jacob Bramley , Kees Cook , Marc Zyngier , Mark Rutland , Ramana Radhakrishnan , "Suzuki K . Poulose" , kvmarm@lists.cs.columbia.edu, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org Message-ID: <20181019123647.a5Rxa1qw3x2Tl1KLSAGw3sVuMrLnd8Bdp0fSoPEGIT4@z> On Fri, Oct 05, 2018 at 09:47:37AM +0100, Kristina Martsenko wrote: > 1) Key support > > This series enables the use of instructions using APIAKey, which is > initialised and maintained per-process (shared by all threads). GCC > currently only makes use of APIAKey. > > This series does not add support for APIBKey, APDAKey, APDBKey, nor > APGAKey. HINT-space instructions using these keys will currently execute > as NOPs. Support for these keys can be added as users appear. > > Note that while we expose the cpuid register (ID_AA64ISAR1_EL1) to > userspace, it only contains one feature for address authentication > (API/APA), so it cannot be used by userspace to tell which keys the > kernel supports. For this the kernel exposes HWCAP bits, one per key > (currently only APIAKey), which must be checked instead. Given that the architecture doesn't provide an identification mechanism for the case where only one of the keys is available, I would much prefer that we expose both of the keys to userspace. Is the only downside of that a possible exception entry overhead if the kernel wants to use pointer authentication as well? Having an initial implementation where the B key operations act as NOPs isn't ideal if we want to support future users -- chances are they'll be put off because deployed kernels don't give them whatever security guarantees they require. It's a bit of a chicken-and-egg problem, so unless we have good reasons to keep the B key hidden, I think we should be exposing it from the start. Will