Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751464AbdFFNaB (ORCPT ); Tue, 6 Jun 2017 09:30:01 -0400 Received: from pegase1.c-s.fr ([93.17.236.30]:57647 "EHLO pegase1.c-s.fr" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751384AbdFFNaA (ORCPT ); Tue, 6 Jun 2017 09:30:00 -0400 Subject: Re: [PATCH 2/5] powerpc/mm: split store_updates_sp() in two parts in do_page_fault() To: Michael Ellerman , Benjamin Herrenschmidt , Paul Mackerras , Scott Wood Cc: linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org References: <58f17a04cee5726467ef4e283dfbd7da68fa6ab4.1492606298.git.christophe.leroy@c-s.fr> <871sr23flh.fsf@concordia.ellerman.id.au> <6daf8f4e-9b39-d585-2c64-9b0348fef123@c-s.fr> <87shjer9vx.fsf@concordia.ellerman.id.au> <97200860-c6da-9d4d-fb53-2aa9c9ca655f@c-s.fr> <87r2yxpeic.fsf@concordia.ellerman.id.au> From: Christophe LEROY Message-ID: <7f84df86-37b0-a7a5-9294-92adbacf8f49@c-s.fr> Date: Tue, 6 Jun 2017 15:29:57 +0200 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.1.1 MIME-Version: 1.0 In-Reply-To: <87r2yxpeic.fsf@concordia.ellerman.id.au> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: fr Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3370 Lines: 99 Le 06/06/2017 à 13:00, Michael Ellerman a écrit : > christophe leroy writes: > >> Le 05/06/2017 à 12:45, Michael Ellerman a écrit : >>> Christophe LEROY writes: >>> >>>> Le 02/06/2017 à 11:26, Michael Ellerman a écrit : >>>>> Christophe Leroy writes: >>>>> >>>>>> Only the get_user() in store_updates_sp() has to be done outside >>>>>> the mm semaphore. All the comparison can be done within the semaphore, >>>>>> so only when really needed. >>>>>> >>>>>> As we got a DSI exception, the address pointed by regs->nip is >>>>>> obviously valid, otherwise we would have had a instruction exception. >>>>>> So __get_user() can be used instead of get_user() >>>>> >>>>> I don't think that part is true. >>>>> >>>>> You took a DSI so there *was* an instruction at NIP, but since then it >>>>> may have been unmapped by another thread. >>>>> >>>>> So I don't think you can assume the get_user() will succeed. >>>> >>>> The difference between get_user() and __get_user() is that get_user() >>>> performs an access_ok() in addition. >>>> >>>> Doesn't access_ok() only checks whether addr is below TASK_SIZE to >>>> ensure it is a valid user address ? >>> >>> Yeah more or less, via some gross macros. >>> >>> I was actually not that worried about the switch from get_user() to >>> __get_user(), but rather that you removed the check of the return value. >>> ie. >>> >>> - if (get_user(inst, (unsigned int __user *)regs->nip)) >>> - return 0; >>> >>> Became: >>> >>> if (is_write && user_mode(regs)) >>> - store_update_sp = store_updates_sp(regs); >>> + __get_user(inst, (unsigned int __user *)regs->nip); >>> >>> >>> I think dropping the access_ok() probably is alright, because the NIP >>> must (should!) have been in userspace, though as Ben says it's always >>> good to be paranoid. >>> >>> But ignoring that the address can fault at all is wrong AFAICS. >> >> I see what you mean now. >> >> Indeed, >> >> - unsigned int inst; >> >> Became >> >> + unsigned int inst = 0; >> >> Since __get_user() doesn't modify 'inst' in case of error, 'inst' >> remains 0, and store_updates_sp(0) return false. That was the idea behind. > > Ugh. OK, my bad. Though it is a little subtle. > > How about: > > @@ -286,10 +290,13 @@ int do_page_fault(struct pt_regs *regs, unsigned long address, > /* > * We want to do this outside mmap_sem, because reading code around nip > * can result in fault, which will cause a deadlock when called with > - * mmap_sem held > + * mmap_sem held. We don't need to check if get_user() fails, if it does > + * it won't modify inst, and an inst of 0 will return false from > + * store_updates_sp(). > */ > + inst = 0; > if (is_write && is_user) > - store_update_sp = store_updates_sp(regs); > + get_user(inst, (unsigned int __user *)regs->nip); > > if (is_user) > flags |= FAULT_FLAG_USER; > > > Then this one can go in. > I just submitted v4 version of the patch "powerpc/mm: Only read faulting instruction when necessary in do_page_fault()", skipping this step and going directly to the final solution. The new approach has been to keep everything inside store_updates_sp() function and just move the call. Christophe