From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933597AbcFIABq (ORCPT ); Wed, 8 Jun 2016 20:01:46 -0400 Received: from mga02.intel.com ([134.134.136.20]:32695 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933550AbcFIABj (ORCPT ); Wed, 8 Jun 2016 20:01:39 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.26,441,1459839600"; d="scan'208";a="971856438" Subject: [PATCH 8/9] pkeys: add details of system call use to Documentation/ To: linux-kernel@vger.kernel.org Cc: x86@kernel.org, linux-api@vger.kernel.org, linux-arch@vger.kernel.org, linux-mm@kvack.org, torvalds@linux-foundation.org, akpm@linux-foundation.org, Dave Hansen , dave.hansen@linux.intel.com From: Dave Hansen Date: Wed, 08 Jun 2016 17:01:34 -0700 References: <20160609000117.71AC7623@viggo.jf.intel.com> In-Reply-To: <20160609000117.71AC7623@viggo.jf.intel.com> Message-Id: <20160609000134.4EF51C0B@viggo.jf.intel.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Dave Hansen 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-api@vger.kernel.org Cc: linux-mm@kvack.org Cc: x86@kernel.org Cc: torvalds@linux-foundation.org Cc: akpm@linux-foundation.org --- b/Documentation/x86/protection-keys.txt | 63 ++++++++++++++++++++++++++++++++ 1 file changed, 63 insertions(+) diff -puN Documentation/x86/protection-keys.txt~pkeys-120-syscall-docs Documentation/x86/protection-keys.txt --- a/Documentation/x86/protection-keys.txt~pkeys-120-syscall-docs 2016-06-08 16:26:36.871014363 -0700 +++ b/Documentation/x86/protection-keys.txt 2016-06-08 16:26:36.874014499 -0700 @@ -18,6 +18,69 @@ even though there is theoretically space permissions are enforced on data access only and have no effect on instruction fetches. +=========================== Syscalls =========================== + +There are 5 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); + unsigned long pkey_get(int pkey); + int pkey_set(int pkey, unsigned long access_rights); + +Before a pkey can be used, it must first be allocated with +pkey_alloc(). An application may either call pkey_set() or the +WRPKRU instruction directly in order to change access permissions +to memory covered with a key. + + 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 _