From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751431AbeCITyy (ORCPT ); Fri, 9 Mar 2018 14:54:54 -0500 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:46682 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751227AbeCITyv (ORCPT ); Fri, 9 Mar 2018 14:54:51 -0500 Date: Fri, 9 Mar 2018 11:54:35 -0800 From: Ram Pai To: Balbir Singh Cc: Michael Ellerman , Ingo Molnar , "akpm@linux-foundation.org" , "open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)" , linux-mm , "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" , linux-arch , "linux-kernel@vger.kernel.org" , Dave Hansen , Benjamin Herrenschmidt , Paul Mackerras , Anshuman Khandual , Aneesh Kumar KV , Haren Myneni/Beaverton/IBM , Michal Hocko , Thiago Jung Bauermann , "Eric W. Biederman" , Jonathan Corbet , Arnd Bergmann , fweimer@redhat.com, msuchanek@suse.com, Ulrich.Weigand@de.ibm.com Subject: Re: [PATCH] x86, powerpc : pkey-mprotect must allow pkey-0 Reply-To: Ram Pai References: <1520583161-11741-1-git-send-email-linuxram@us.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.20 (2009-12-10) X-TM-AS-GCONF: 00 x-cbid: 18030919-0012-0000-0000-000005BAF5E6 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18030919-0013-0000-0000-000019371BC0 Message-Id: <20180309195435.GQ1060@ram.oc3035372033.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2018-03-09_10:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1011 lowpriorityscore=0 impostorscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1709140000 definitions=main-1803090239 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Mar 09, 2018 at 07:37:04PM +1100, Balbir Singh wrote: > On Fri, Mar 9, 2018 at 7:12 PM, 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. On > > the contrary applications need the ability to do so. > > > > The patch relaxes the restriction. > > I looked at the code and my observation was going to be that we need > to change mm_pkey_is_allocated. I still fail to understand what > happens if pkey 0 is reserved? What is the default key is it the first > available key? Assuming 0 is the default key may work and seems to > work, but I am sure its mostly by accident. It would be nice, if we > could have a notion of the default key. I don't like the special > meaning given to key 0 here. Remember on powerpc if 0 is reserved and > UAMOR/AMOR does not allow modification because it's reserved, setting > 0 will still fail The linux pkey API, assumes pkey-0 is the default key. If no key is explicitly associated with a page, the default key gets associated. When a default key gets associated with a page, the permissions on the page are not dictated by the permissions of the default key, but by the permission of other bits in the pte; i.e _PAGE_RWX. On powerpc, and AFAICT on x86, neither the hardware nor the hypervisor reserves key-0. Hence the OS is free to use the key value, the way it chooses. On Linux we choose to associate key-0 the special status called default-key. However I see your point. If some cpu architecture takes away key-0 from Linux, than implementing the special status for key-0 on that architecture can become challenging, though not impossible. That architecture implementation can internally map key-0 value to some other available key, and associate that key to the page. And offcourse make sure that the hardware/MMU uses the pte's RWX bits to enforce permissions, for that key. -- Ram Pai