From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id DFE92C3A5A2 for ; Wed, 4 Sep 2019 02:18:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id BE75D206BB for ; Wed, 4 Sep 2019 02:18:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727701AbfIDCSW (ORCPT ); Tue, 3 Sep 2019 22:18:22 -0400 Received: from szxga08-in.huawei.com ([45.249.212.255]:34538 "EHLO huawei.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727065AbfIDCSV (ORCPT ); Tue, 3 Sep 2019 22:18:21 -0400 Received: from DGGEMM403-HUB.china.huawei.com (unknown [172.30.72.56]) by Forcepoint Email with ESMTP id BC21340E4231C3AB44ED; Wed, 4 Sep 2019 10:18:19 +0800 (CST) Received: from dggeme764-chm.china.huawei.com (10.3.19.110) by DGGEMM403-HUB.china.huawei.com (10.3.20.211) with Microsoft SMTP Server (TLS) id 14.3.439.0; Wed, 4 Sep 2019 10:18:11 +0800 Received: from [127.0.0.1] (10.184.39.28) by dggeme764-chm.china.huawei.com (10.3.19.110) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256_P256) id 15.1.1591.10; Wed, 4 Sep 2019 10:18:11 +0800 Subject: Re: [PATCH] arm: fix page faults in do_alignment To: "Eric W. Biederman" , "Russell King - ARM Linux admin" References: <1567171877-101949-1-git-send-email-jingxiangfeng@huawei.com> <20190830133522.GZ13294@shell.armlinux.org.uk> <87d0gmwi73.fsf@x220.int.ebiederm.org> <20190830203052.GG13294@shell.armlinux.org.uk> <87y2zav01z.fsf@x220.int.ebiederm.org> <20190830222906.GH13294@shell.armlinux.org.uk> <87mufmioqv.fsf@x220.int.ebiederm.org> CC: , , , , , , , , From: Jing Xiangfeng Message-ID: <5D6F1EC9.5070909@huawei.com> Date: Wed, 4 Sep 2019 10:17:45 +0800 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:38.0) Gecko/20100101 Thunderbird/38.1.0 MIME-Version: 1.0 In-Reply-To: <87mufmioqv.fsf@x220.int.ebiederm.org> Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit X-Originating-IP: [10.184.39.28] X-ClientProxiedBy: dggeme718-chm.china.huawei.com (10.1.199.114) To dggeme764-chm.china.huawei.com (10.3.19.110) X-CFilter-Loop: Reflected Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2019/9/3 1:36, Eric W. Biederman wrote: > Russell King - ARM Linux admin writes: > >> On Fri, Aug 30, 2019 at 04:02:48PM -0500, Eric W. Biederman wrote: >>> Russell King - ARM Linux admin writes: >>> >>>> On Fri, Aug 30, 2019 at 02:45:36PM -0500, Eric W. Biederman wrote: >>>>> Russell King - ARM Linux admin writes: >>>>> >>>>>> On Fri, Aug 30, 2019 at 09:31:17PM +0800, Jing Xiangfeng wrote: >>>>>>> The function do_alignment can handle misaligned address for user and >>>>>>> kernel space. If it is a userspace access, do_alignment may fail on >>>>>>> a low-memory situation, because page faults are disabled in >>>>>>> probe_kernel_address. >>>>>>> >>>>>>> Fix this by using __copy_from_user stead of probe_kernel_address. >>>>>>> >>>>>>> Fixes: b255188 ("ARM: fix scheduling while atomic warning in alignment handling code") >>>>>>> Signed-off-by: Jing Xiangfeng >>>>>> >>>>>> NAK. >>>>>> >>>>>> The "scheduling while atomic warning in alignment handling code" is >>>>>> caused by fixing up the page fault while trying to handle the >>>>>> mis-alignment fault generated from an instruction in atomic context. >>>>>> >>>>>> Your patch re-introduces that bug. >>>>> >>>>> And the patch that fixed scheduling while atomic apparently introduced a >>>>> regression. Admittedly a regression that took 6 years to track down but >>>>> still. >>>> >>>> Right, and given the number of years, we are trading one regression for >>>> a different regression. If we revert to the original code where we >>>> fix up, we will end up with people complaining about a "new" regression >>>> caused by reverting the previous fix. Follow this policy and we just >>>> end up constantly reverting the previous revert. >>>> >>>> The window is very small - the page in question will have had to have >>>> instructions read from it immediately prior to the handler being entered, >>>> and would have had to be made "old" before subsequently being unmapped. >>> >>>> Rather than excessively complicating the code and making it even more >>>> inefficient (as in your patch), we could instead retry executing the >>>> instruction when we discover that the page is unavailable, which should >>>> cause the page to be paged back in. >>> >>> My patch does not introduce any inefficiencies. It onlys moves the >>> check for user_mode up a bit. My patch did duplicate the code. >>> >>>> If the page really is unavailable, the prefetch abort should cause a >>>> SEGV to be raised, otherwise the re-execution should replace the page. >>>> >>>> The danger to that approach is we page it back in, and it gets paged >>>> back out before we're able to read the instruction indefinitely. >>> >>> I would think either a little code duplication or a function that looks >>> at user_mode(regs) and picks the appropriate kind of copy to do would be >>> the best way to go. Because what needs to happen in the two cases for >>> reading the instruction are almost completely different. >> >> That is what I mean. I'd prefer to avoid that with the large chunk of >> code. How about instead adding a local replacement for >> probe_kernel_address() that just sorts out the reading, rather than >> duplicating all the code to deal with thumb fixup. > > So something like this should be fine? > > Jing Xiangfeng can you test this please? I think this fixes your issue > but I don't currently have an arm development box where I could test this. > Yes, I have tested and it can fix my issue in kernel 4.19. > diff --git a/arch/arm/mm/alignment.c b/arch/arm/mm/alignment.c > index 04b36436cbc0..b07d17ca0ae5 100644 > --- a/arch/arm/mm/alignment.c > +++ b/arch/arm/mm/alignment.c > @@ -767,6 +767,23 @@ do_alignment_t32_to_handler(unsigned long *pinstr, struct pt_regs *regs, > return NULL; > } > > +static inline unsigned long > +copy_instr(bool umode, void *dst, unsigned long instrptr, size_t size) > +{ > + unsigned long result; > + if (umode) { > + void __user *src = (void *)instrptr; > + result = copy_from_user(dst, src, size); > + } else { > + void *src = (void *)instrptr; > + result = probe_kernel_read(dst, src, size); > + } > + /* Convert short reads into -EFAULT */ > + if ((result >= 0) && (result < size)) > + result = -EFAULT; > + return result; > +} > + > static int > do_alignment(unsigned long addr, unsigned int fsr, struct pt_regs *regs) > { > @@ -778,22 +795,24 @@ do_alignment(unsigned long addr, unsigned int fsr, struct pt_regs *regs) > u16 tinstr = 0; > int isize = 4; > int thumb2_32b = 0; > + bool umode; > > if (interrupts_enabled(regs)) > local_irq_enable(); > > instrptr = instruction_pointer(regs); > + umode = user_mode(regs); > > if (thumb_mode(regs)) { > - u16 *ptr = (u16 *)(instrptr & ~1); > - fault = probe_kernel_address(ptr, tinstr); > + unsigned long tinstrptr = instrptr & ~1; > + fault = copy_instr(umode, &tinstr, tinstrptr, 2); > tinstr = __mem_to_opcode_thumb16(tinstr); > if (!fault) { > if (cpu_architecture() >= CPU_ARCH_ARMv7 && > IS_T32(tinstr)) { > /* Thumb-2 32-bit */ > u16 tinst2 = 0; > - fault = probe_kernel_address(ptr + 1, tinst2); > + fault = copy_instr(umode, &tinst2, tinstrptr + 2, 2); > tinst2 = __mem_to_opcode_thumb16(tinst2); > instr = __opcode_thumb32_compose(tinstr, tinst2); > thumb2_32b = 1; > @@ -803,7 +822,7 @@ do_alignment(unsigned long addr, unsigned int fsr, struct pt_regs *regs) > } > } > } else { > - fault = probe_kernel_address((void *)instrptr, instr); > + fault = copy_instr(umode, &instr, instrptr, 4); > instr = __mem_to_opcode_arm(instr); > } > > @@ -812,7 +831,7 @@ do_alignment(unsigned long addr, unsigned int fsr, struct pt_regs *regs) > goto bad_or_fault; > } > > - if (user_mode(regs)) > + if (umode) > goto user; > > ai_sys += 1; > > . >