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 95726C43610 for ; Fri, 23 Nov 2018 09:27:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 51D6B20861 for ; Fri, 23 Nov 2018 09:27:09 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 51D6B20861 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2409025AbeKWUKd (ORCPT ); Fri, 23 Nov 2018 15:10:33 -0500 Received: from foss.arm.com ([217.140.101.70]:38976 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2409000AbeKWUKc (ORCPT ); Fri, 23 Nov 2018 15:10:32 -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 4477535F5; Fri, 23 Nov 2018 01:27:06 -0800 (PST) Received: from [10.1.197.36] (e112298-lin.cambridge.arm.com [10.1.197.36]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id D1AA03F5CF; Fri, 23 Nov 2018 01:27:04 -0800 (PST) To: Peter Anvin , Ingo Molnar , LKML , "linux-arm-kernel@lists.infradead.org" Cc: Catalin Marinas , Will Deacon , James Morse From: Julien Thierry Subject: Sleeping in user_access section Message-ID: <3e881ee6-0ff6-b8b6-0633-3d4a7743411d@arm.com> Date: Fri, 23 Nov 2018 09:27:02 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.8.0 MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed 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 Hi, I made an attempt at implementing the user_access_begin()/user_access_end() macros along with the get/put_user_unsafe() for arm64 by toggling the status of PAN (more or less similar to x86's STAC/CTAC). With a small mistake in my patch, we realized that directly calling function that could reschedule while in a user_access section could lead to: - scheduling another task keeping the user_access status enabled despite the task never calling user_access_begin() - when re-scheduling the task that was mid user_access section, user_access would be disabled and the task would fault on the next get/put_user_unsafe. This is because __switch_to does not alter the user_access status when switching from next to prev (at least on arm64 we currently don't, and by looking at the x86 code I don't think this is done either). From my understanding, this is not an issue when the task in user_access mode gets scheduled out/in as a result of an interrupt as PAN and EFLAGS.AC get saved/restore on exception entry/exit (at least I know it is the case for PAN, I am less sure for the x86 side). So, the question is, should __switch_to take care of the user_access status when scheduling new tasks? Or should there be a restriction about scheduling out a task with user_access mode enabled and maybe add a warning if we can detect this? (Or did we miss something and this is not an issue on x86?) Thanks, -- Julien Thierry