From mboxrd@z Thu Jan 1 00:00:00 1970 From: Florian Weimer Subject: Re: [PATCH] x86, powerpc : pkey-mprotect must allow pkey-0 Date: Wed, 14 Mar 2018 09:05:30 +0100 Message-ID: <0a6981b3-dcd2-4dce-3209-7f8055d8548f@redhat.com> References: <1520583161-11741-1-git-send-email-linuxram@us.ibm.com> <20180309200017.GR1060@ram.oc3035372033.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Return-path: In-Reply-To: Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org To: Ram Pai Cc: mpe@ellerman.id.au, mingo@redhat.com, akpm@linux-foundation.org, linuxppc-dev@lists.ozlabs.org, linux-mm@kvack.org, x86@kernel.org, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, dave.hansen@intel.com, benh@kernel.crashing.org, paulus@samba.org, khandual@linux.vnet.ibm.com, aneesh.kumar@linux.vnet.ibm.com, bsingharora@gmail.com, hbabu@us.ibm.com, mhocko@kernel.org, bauerman@linux.vnet.ibm.com, ebiederm@xmission.com, corbet@lwn.net, arnd@arndb.de, msuchanek@suse.com, Ulrich.Weigand@de.ibm.com List-Id: linux-arch.vger.kernel.org On 03/14/2018 09:00 AM, Florian Weimer wrote: > On 03/09/2018 09:00 PM, Ram Pai wrote: >> On Fri, Mar 09, 2018 at 12:04:49PM +0100, Florian Weimer wrote: >>> On 03/09/2018 09:12 AM, Ram Pai wrote: >>>> Once an address range is associated with an allocated pkey, it >>>> cannot be >>>> reverted back to key-0. There is no valid reason for the above >>>> behavior. >>> >>> mprotect without a key does not necessarily use key 0, e.g. if >>> protection keys are used to emulate page protection flag combination >>> which is not directly supported by the hardware. >>> >>> Therefore, it seems to me that filtering out non-allocated keys is >>> the right thing to do. >> >> I am not sure, what you mean. Do you agree with the patch or otherwise? > > I think it's inconsistent to make key 0 allocated, but not the key which > is used for PROT_EXEC emulation, which is still reserved.  Even if you > change the key 0 behavior, it is still not possible to emulate mprotect > behavior faithfully with an allocated key. Ugh. Should have read the code first before replying: /* Do we need to assign a pkey for mm's execute-only maps? */ if (execute_only_pkey == -1) { /* Go allocate one to use, which might fail */ execute_only_pkey = mm_pkey_alloc(mm); if (execute_only_pkey < 0) return -1; need_to_set_mm_pkey = true; } So we do allocate the PROT_EXEC-only key, and I assume it means that the key can be restored using pkey_mprotect. So the key 0 behavior is a true exception after all, and it makes sense to realign the behavior with the other keys. Thanks, Florian From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx3-rdu2.redhat.com ([66.187.233.73]:47674 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1753221AbeCNIFf (ORCPT ); Wed, 14 Mar 2018 04:05:35 -0400 Subject: Re: [PATCH] x86, powerpc : pkey-mprotect must allow pkey-0 From: Florian Weimer References: <1520583161-11741-1-git-send-email-linuxram@us.ibm.com> <20180309200017.GR1060@ram.oc3035372033.ibm.com> Message-ID: <0a6981b3-dcd2-4dce-3209-7f8055d8548f@redhat.com> Date: Wed, 14 Mar 2018 09:05:30 +0100 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-arch-owner@vger.kernel.org List-ID: To: Ram Pai Cc: mpe@ellerman.id.au, mingo@redhat.com, akpm@linux-foundation.org, linuxppc-dev@lists.ozlabs.org, linux-mm@kvack.org, x86@kernel.org, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, dave.hansen@intel.com, benh@kernel.crashing.org, paulus@samba.org, khandual@linux.vnet.ibm.com, aneesh.kumar@linux.vnet.ibm.com, bsingharora@gmail.com, hbabu@us.ibm.com, mhocko@kernel.org, bauerman@linux.vnet.ibm.com, ebiederm@xmission.com, corbet@lwn.net, arnd@arndb.de, msuchanek@suse.com, Ulrich.Weigand@de.ibm.com Message-ID: <20180314080530.sEkro1x8892BVw5091v1pFntlSsW1jZy1NKsiIFYB6Q@z> On 03/14/2018 09:00 AM, Florian Weimer wrote: > On 03/09/2018 09:00 PM, Ram Pai wrote: >> On Fri, Mar 09, 2018 at 12:04:49PM +0100, Florian Weimer wrote: >>> On 03/09/2018 09:12 AM, Ram Pai wrote: >>>> Once an address range is associated with an allocated pkey, it >>>> cannot be >>>> reverted back to key-0. There is no valid reason for the above >>>> behavior. >>> >>> mprotect without a key does not necessarily use key 0, e.g. if >>> protection keys are used to emulate page protection flag combination >>> which is not directly supported by the hardware. >>> >>> Therefore, it seems to me that filtering out non-allocated keys is >>> the right thing to do. >> >> I am not sure, what you mean. Do you agree with the patch or otherwise? > > I think it's inconsistent to make key 0 allocated, but not the key which > is used for PROT_EXEC emulation, which is still reserved.  Even if you > change the key 0 behavior, it is still not possible to emulate mprotect > behavior faithfully with an allocated key. Ugh. Should have read the code first before replying: /* Do we need to assign a pkey for mm's execute-only maps? */ if (execute_only_pkey == -1) { /* Go allocate one to use, which might fail */ execute_only_pkey = mm_pkey_alloc(mm); if (execute_only_pkey < 0) return -1; need_to_set_mm_pkey = true; } So we do allocate the PROT_EXEC-only key, and I assume it means that the key can be restored using pkey_mprotect. So the key 0 behavior is a true exception after all, and it makes sense to realign the behavior with the other keys. Thanks, Florian