From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755451AbbI2Gjv (ORCPT ); Tue, 29 Sep 2015 02:39:51 -0400 Received: from ozlabs.org ([103.22.144.67]:59429 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755301AbbI2Gjo (ORCPT ); Tue, 29 Sep 2015 02:39:44 -0400 Message-ID: <1443508783.29119.2.camel@ellerman.id.au> Subject: Re: [PATCH 21/25] mm: implement new mprotect_key() system call From: Michael Ellerman To: Dave Hansen Cc: borntraeger@de.ibm.com, x86@kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, dave.hansen@linux.intel.com, linux-api@vger.kernel.org Date: Tue, 29 Sep 2015 16:39:43 +1000 In-Reply-To: <20150928191826.F1CD5256@viggo.jf.intel.com> References: <20150928191817.035A64E2@viggo.jf.intel.com> <20150928191826.F1CD5256@viggo.jf.intel.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.12.11-0ubuntu3 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 2015-09-28 at 12:18 -0700, Dave Hansen wrote: > From: Dave Hansen > > mprotect_key() is just like mprotect, except it also takes a > protection key as an argument. On systems that do not support > protection keys, it still works, but requires that key=0. I'm not sure how userspace is going to use the key=0 feature? ie. userspace will still have to detect that keys are not supported and use key 0 everywhere. At that point it could just as well skip the mprotect_key() syscalls entirely couldn't it? > I expect it to get used like this, if you want to guarantee that > any mapping you create can *never* be accessed without the right > protection keys set up. > > pkey_deny_access(11); // random pkey > int real_prot = PROT_READ|PROT_WRITE; > ptr = mmap(NULL, PAGE_SIZE, PROT_NONE, MAP_ANONYMOUS|MAP_PRIVATE, -1, 0); > ret = mprotect_key(ptr, PAGE_SIZE, real_prot, 11); > > This way, there is *no* window where the mapping is accessible > since it was always either PROT_NONE or had a protection key set. > > We settled on 'unsigned long' for the type of the key here. We > only need 4 bits on x86 today, but I figured that other > architectures might need some more space. If the existing mprotect() syscall had a flags argument you could have just used that. So is it worth just adding mprotect2() now and using it for this? ie: int mprotect2(unsigned long start, size_t len, unsigned long prot, unsigned long flags) .. And then you define bit zero of flags to say you're passing a pkey, and it's in bits 1-63? That way if other arches need to do something different you at least have the flags available? cheers