From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753389AbbKQDhr (ORCPT ); Mon, 16 Nov 2015 22:37:47 -0500 Received: from mga11.intel.com ([192.55.52.93]:11573 "EHLO mga11.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751929AbbKQDgD (ORCPT ); Mon, 16 Nov 2015 22:36:03 -0500 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.20,305,1444719600"; d="scan'208";a="852570502" Subject: [PATCH 37/37] x86, pkeys: Documentation To: linux-kernel@vger.kernel.org Cc: x86@kernel.org, Dave Hansen , dave.hansen@linux.intel.com From: Dave Hansen Date: Mon, 16 Nov 2015 19:36:02 -0800 References: <20151117033511.BFFA1440@viggo.jf.intel.com> In-Reply-To: <20151117033511.BFFA1440@viggo.jf.intel.com> Message-Id: <20151117033602.9AFB88DD@viggo.jf.intel.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Dave Hansen Signed-off-by: Dave Hansen --- b/Documentation/x86/protection-keys.txt | 53 ++++++++++++++++++++++++++++++++ 1 file changed, 53 insertions(+) diff -puN /dev/null Documentation/x86/protection-keys.txt --- /dev/null 2015-07-13 14:24:11.435656502 -0700 +++ b/Documentation/x86/protection-keys.txt 2015-11-16 19:14:16.943465016 -0800 @@ -0,0 +1,53 @@ +Memory Protection Keys for Userspace (PKU aka PKEYs) is a CPU feature +which will be found on future Intel CPUs. + +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. + +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. + +There are two new instructions (RDPKRU/WRPKRU) for reading and writing +to the new register. The feature is only available in 64-bit mode, +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. + +The kernel attempts to make protection keys consistent with the +behavior of a plain mprotect(). For instance if you do this: + + mprotect(ptr, size, PROT_NONE); + something(ptr); + +you can expect the same effects with protection keys when doing this: + + sys_pkey_alloc(no_flag, PKEY_DISABLE_WRITE | PKEY_DISABLE_READ); + sys_mprotect_pkey(ptr, size, PROT_READ|PROT_WRITE); + something(ptr); + +That should be true whether something() is a direct access to 'ptr' +like: + + *ptr = foo; + +or when the kernel does the access on the application's behalf like +with a read(): + + read(fd, ptr, 1); + +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. + +=========================== Config Option =========================== + +This config option adds approximately 1.5kb of text. and 50 bytes of +data to the executable. A workload which does large O_DIRECT reads +of holes in XFS files was run to exercise get_user_pages_fast(). No +performance delta was observed with the config option +enabled or disabled. _