From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754515AbcIILNd (ORCPT ); Fri, 9 Sep 2016 07:13:33 -0400 Received: from terminus.zytor.com ([198.137.202.10]:44068 "EHLO terminus.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754079AbcIILN2 (ORCPT ); Fri, 9 Sep 2016 07:13:28 -0400 Date: Fri, 9 Sep 2016 04:13:17 -0700 From: tip-bot for Dave Hansen Message-ID: Cc: dave@sr71.net, linux-kernel@vger.kernel.org, tglx@linutronix.de, hpa@zytor.com, dave.hansen@linux.intel.com, mingo@kernel.org Reply-To: linux-kernel@vger.kernel.org, tglx@linutronix.de, mingo@kernel.org, dave.hansen@linux.intel.com, hpa@zytor.com, dave@sr71.net In-Reply-To: <20160729163020.59350E33@viggo.jf.intel.com> References: <20160729163020.59350E33@viggo.jf.intel.com> To: linux-tip-commits@vger.kernel.org Subject: [tip:mm/pkeys] pkeys: Add details of system call use to Documentation/ Git-Commit-ID: c74fe3940848c6afea83bfbda64a9baf9da547c8 X-Mailer: tip-git-log-daemon Robot-ID: Robot-Unsubscribe: Contact to get blacklisted from these emails MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset=UTF-8 Content-Disposition: inline Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Commit-ID: c74fe3940848c6afea83bfbda64a9baf9da547c8 Gitweb: http://git.kernel.org/tip/c74fe3940848c6afea83bfbda64a9baf9da547c8 Author: Dave Hansen AuthorDate: Fri, 29 Jul 2016 09:30:20 -0700 Committer: Thomas Gleixner CommitDate: Fri, 9 Sep 2016 13:02:28 +0200 pkeys: Add details of system call use to Documentation/ This spells out all of the pkey-related system calls that we have and provides some example code fragments to demonstrate how we expect them to be used. Signed-off-by: Dave Hansen Cc: linux-arch@vger.kernel.org Cc: Dave Hansen Cc: mgorman@techsingularity.net Cc: arnd@arndb.de Cc: linux-api@vger.kernel.org Cc: linux-mm@kvack.org Cc: luto@kernel.org Cc: akpm@linux-foundation.org Cc: torvalds@linux-foundation.org Link: http://lkml.kernel.org/r/20160729163020.59350E33@viggo.jf.intel.com Signed-off-by: Thomas Gleixner --- Documentation/x86/protection-keys.txt | 62 +++++++++++++++++++++++++++++++++++ 1 file changed, 62 insertions(+) diff --git a/Documentation/x86/protection-keys.txt b/Documentation/x86/protection-keys.txt index c281ded..6da7689 100644 --- a/Documentation/x86/protection-keys.txt +++ b/Documentation/x86/protection-keys.txt @@ -18,6 +18,68 @@ 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 =========================== + +There are 2 system calls which directly interact with pkeys: + + int pkey_alloc(unsigned long flags, unsigned long init_access_rights) + int pkey_free(int pkey); + int pkey_mprotect(unsigned long start, size_t len, + unsigned long prot, int pkey); + +Before a pkey can be used, it must first be allocated with +pkey_alloc(). An application calls the WRPKRU instruction +directly in order to change access permissions to memory covered +with a key. In this example WRPKRU is wrapped by a C function +called pkey_set(). + + int real_prot = PROT_READ|PROT_WRITE; + pkey = pkey_alloc(0, PKEY_DENY_WRITE); + ptr = mmap(NULL, PAGE_SIZE, PROT_NONE, MAP_ANONYMOUS|MAP_PRIVATE, -1, 0); + ret = pkey_mprotect(ptr, PAGE_SIZE, real_prot, pkey); + ... application runs here + +Now, if the application needs to update the data at 'ptr', it can +gain access, do the update, then remove its write access: + + pkey_set(pkey, 0); // clear PKEY_DENY_WRITE + *ptr = foo; // assign something + pkey_set(pkey, PKEY_DENY_WRITE); // set PKEY_DENY_WRITE again + +Now when it frees the memory, it will also free the pkey since it +is no longer in use: + + munmap(ptr, PAGE_SIZE); + pkey_free(pkey); + +=========================== Behavior =========================== + +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: + + pkey = pkey_alloc(0, PKEY_DISABLE_WRITE | PKEY_DISABLE_READ); + pkey_mprotect(ptr, size, PROT_READ|PROT_WRITE, pkey); + 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