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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED 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 EBE82C282C2 for ; Wed, 13 Feb 2019 14:24:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C436A222BA for ; Wed, 13 Feb 2019 14:24:41 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2392095AbfBMOYk (ORCPT ); Wed, 13 Feb 2019 09:24:40 -0500 Received: from foss.arm.com ([217.140.101.70]:55008 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731144AbfBMOYk (ORCPT ); Wed, 13 Feb 2019 09:24:40 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 7B6B280D; Wed, 13 Feb 2019 06:24:39 -0800 (PST) Received: from [10.1.197.45] (e112298-lin.cambridge.arm.com [10.1.197.45]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 5D0193F557; Wed, 13 Feb 2019 06:24:36 -0800 (PST) Subject: Re: [PATCH v3 3/4] uaccess: Check no rescheduling function is called in unsafe region To: Peter Zijlstra , Will Deacon Cc: Ingo Molnar , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, mingo@redhat.com, catalin.marinas@arm.com, james.morse@arm.com, hpa@zytor.com, valentin.schneider@arm.com References: <1547560709-56207-1-git-send-email-julien.thierry@arm.com> <1547560709-56207-4-git-send-email-julien.thierry@arm.com> <20190211134527.GA121589@gmail.com> <20190211135159.GC32511@hirez.programming.kicks-ass.net> <20190213103553.GO32494@hirez.programming.kicks-ass.net> <1c2429a4-9df9-40a3-98e0-51577de4bd6a@arm.com> <20190213131720.GU32494@hirez.programming.kicks-ass.net> <20190213140025.GB6346@brain-police> <20190213141712.GV32494@hirez.programming.kicks-ass.net> From: Julien Thierry Message-ID: Date: Wed, 13 Feb 2019 14:24:34 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1 MIME-Version: 1.0 In-Reply-To: <20190213141712.GV32494@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 13/02/2019 14:17, Peter Zijlstra wrote: > On Wed, Feb 13, 2019 at 02:00:26PM +0000, Will Deacon wrote: >>> This; how is getting preempted fundamentally different from scheduling >>> ourselves? >> >> The difference is because getting preempted in the sequence above is >> triggered off the back of an interrupt. On arm64, and I think also on x86, >> the user access state (SMAP or PAN) is saved and restored across exceptions >> but not across context switch. Consequently, taking an irq in a >> user_access_{begin,end} section and then scheduling is fine, but calling >> schedule directly within such a section is not. > > So how's this then: > > if (user_access_begin()) { > > preempt_disable(); > > > set_need_resched(); > > > preempt_enable() > __schedule(); > > user_access_end(); > } > > That _should_ work just fine but explodes with the proposed nonsense. AFAICT, This does not work properly because when you schedule out this task, you won't be saving the EFLAGS.AF/PSTATE.PAN bit on the stack, and next time you schedule the task back in, it might no longer have the correct flag value (so an unsafe_get/put_user() will fail even though you haven't reached user_access_end()). One solution is to deal with this in task switching code, but so far I've been told that calling schedule() in such a context is not expected to be supported. Cheers, -- Julien Thierry