From patchwork Fri Jul 17 07:20:46 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Ira Weiny X-Patchwork-Id: 11669299 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 962671751 for ; Fri, 17 Jul 2020 07:22:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 84FEB207E8 for ; Fri, 17 Jul 2020 07:22:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728233AbgGQHWB (ORCPT ); Fri, 17 Jul 2020 03:22:01 -0400 Received: from mga05.intel.com ([192.55.52.43]:11404 "EHLO mga05.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728028AbgGQHVG (ORCPT ); Fri, 17 Jul 2020 03:21:06 -0400 IronPort-SDR: y8M7ecUu7mQKvQVt7RWQ/5RwvkQVYTTfp9Zaey2QbBOpnQtpGf/0F6Rvpq56zyj2Cb4RBrtvZZ lT/P/Bf5Rpew== X-IronPort-AV: E=McAfee;i="6000,8403,9684"; a="234401106" X-IronPort-AV: E=Sophos;i="5.75,362,1589266800"; d="scan'208";a="234401106" X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from fmsmga001.fm.intel.com ([10.253.24.23]) by fmsmga105.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 17 Jul 2020 00:21:05 -0700 IronPort-SDR: eL/eYlbNkfkhXTx6gIGCgsYd7ci/pLT9RgTZZEmFfQU8rSbbhOjANaABpVhvsoM0DmjWGusrNx +AjAMeXEQdcg== X-IronPort-AV: E=Sophos;i="5.75,362,1589266800"; d="scan'208";a="391321142" Received: from iweiny-desk2.sc.intel.com (HELO localhost) ([10.3.52.147]) by fmsmga001-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 17 Jul 2020 00:21:04 -0700 From: ira.weiny@intel.com To: Thomas Gleixner , Ingo Molnar , Borislav Petkov , Andy Lutomirski , Peter Zijlstra Cc: Ira Weiny , x86@kernel.org, Dave Hansen , Dan Williams , Vishal Verma , Andrew Morton , Fenghua Yu , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-nvdimm@lists.01.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org Subject: [PATCH RFC V2 07/17] Documentation/pkeys: Update documentation for kernel pkeys Date: Fri, 17 Jul 2020 00:20:46 -0700 Message-Id: <20200717072056.73134-8-ira.weiny@intel.com> X-Mailer: git-send-email 2.28.0.rc0.12.gb6a658bd00c9 In-Reply-To: <20200717072056.73134-1-ira.weiny@intel.com> References: <20200717072056.73134-1-ira.weiny@intel.com> MIME-Version: 1.0 Sender: linux-kselftest-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kselftest@vger.kernel.org From: Ira Weiny Future Intel CPUS will support Protection Key Supervisor (PKS). Update the protection key documentation to cover pkeys on supervisor pages. Signed-off-by: Ira Weiny --- Documentation/core-api/protection-keys.rst | 81 +++++++++++++++++----- 1 file changed, 63 insertions(+), 18 deletions(-) diff --git a/Documentation/core-api/protection-keys.rst b/Documentation/core-api/protection-keys.rst index ec575e72d0b2..5ac400a5a306 100644 --- a/Documentation/core-api/protection-keys.rst +++ b/Documentation/core-api/protection-keys.rst @@ -4,25 +4,33 @@ Memory Protection Keys ====================== -Memory Protection Keys for Userspace (PKU aka PKEYs) is a feature -which is found on Intel's Skylake (and later) "Scalable Processor" -Server CPUs. It will be available in future non-server Intel parts -and future AMD processors. - -For anyone wishing to test or use this feature, it is available in -Amazon's EC2 C5 instances and is known to work there using an Ubuntu -17.04 image. - Memory Protection Keys provides a mechanism for enforcing page-based protections, but without requiring modification of the page tables -when an application changes protection domains. It works by -dedicating 4 previously ignored bits in each page table entry to a -"protection key", giving 16 possible keys. +when an application changes protection domains. + +PKeys Userspace (PKU) is a feature which is found on Intel's Skylake "Scalable +Processor" Server CPUs and later. And It will be available in future +non-server Intel parts and future AMD processors. + +Future Intel processors will support Protection Keys for Supervisor pages +(PKS). + +For anyone wishing to test or use user space pkeys, it is available in Amazon's +EC2 C5 instances and is known to work there using an Ubuntu 17.04 image. + +pkes work by dedicating 4 previously Reserved bits in each page table entry to +a "protection key", giving 16 possible keys. User and Supervisor pages are +treated separately. -There is also a new user-accessible register (PKRU) with two separate -bits (Access Disable and Write Disable) for each key. Being a CPU -register, PKRU is inherently thread-local, potentially giving each -thread a different set of protections from every other thread. +Protections for each page are controlled with per CPU registers for each type +of page User and Supervisor. Each of these 32 bit register stores two separate +bits (Access Disable and Write Disable) for each key. + +For Userspace the register is user-accessible (rdpkru/wrpkru). For +Supervisor, the register (MSR_IA32_PKRS) is accessible only to the kernel. + +Being a CPU register, pkes are inherently thread-local, potentially giving +each thread an independent set of protections from every other thread. There are two new instructions (RDPKRU/WRPKRU) for reading and writing to the new register. The feature is only available in 64-bit mode, @@ -30,8 +38,11 @@ even though there is theoretically space in the PAE PTEs. These permissions are enforced on data access only and have no effect on instruction fetches. -Syscalls -======== +For kernel space rdmsr/wrmsr are used to access the kernel MSRs. + + +Syscalls for user space keys +============================ There are 3 system calls which directly interact with pkeys:: @@ -98,3 +109,37 @@ with a read():: The kernel will send a SIGSEGV in both cases, but si_code will be set to SEGV_PKERR when violating protection keys versus SEGV_ACCERR when the plain mprotect() permissions are violated. + + +Kernel API for PKS support +========================== + +PKS is intended to harden against unwanted access to kernel pages. But it does +not completely restrict access under all conditions. For example the MSR +setting is not saved/restored during irqs. Thus the use of PKS is a mitigation +strategy rather than a form of strict security. + +The following calls are used to allocate, use, and deallocate a pkey which +defines a 'protection domain' within the kernel. Setting a pkey value in a +supervisor mapping adds that mapping to the protection domain. Then calls can be +used to enable/disable read and/or write access to all of the pages mapped with +that key: + + int pks_key_alloc(const char * const pkey_user); + #define PAGE_KERNEL_PKEY(pkey) + #define _PAGE_KEY(pkey) + int pks_update_protection(int pkey, unsigned long protection); + void pks_key_free(int pkey); + +In-kernel users must be prepared to set PAGE_KERNEL_PKEY() permission in the +page table entries for the mappings they want to ptorect. + +WARNING: It is imperative that callers check for errors from pks_key_alloc() +because pkeys are a limited resource and so callers should be prepared to work +without PKS support. + +For admins a debugfs interface provides a list of the current keys in use at: + + /sys/kernel/debug/x86/pks_keys_allocated + +Some example code can be found in lib/pks/pks_test.c