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=-0.9 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,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 BB7DFC6778C for ; Tue, 3 Jul 2018 17:26:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 74B0D22565 for ; Tue, 3 Jul 2018 17:26:49 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=efficios.com header.i=@efficios.com header.b="Omo621aI" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 74B0D22565 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=efficios.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 S934378AbeGCR0o (ORCPT ); Tue, 3 Jul 2018 13:26:44 -0400 Received: from mail.efficios.com ([167.114.142.138]:56414 "EHLO mail.efficios.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934074AbeGCR0l (ORCPT ); Tue, 3 Jul 2018 13:26:41 -0400 Received: from localhost (ip6-localhost [IPv6:::1]) by mail.efficios.com (Postfix) with ESMTP id 68007230195; Tue, 3 Jul 2018 13:26:40 -0400 (EDT) Received: from mail.efficios.com ([IPv6:::1]) by localhost (mail02.efficios.com [IPv6:::1]) (amavisd-new, port 10032) with ESMTP id eSPUoTI7tUqO; Tue, 3 Jul 2018 13:26:39 -0400 (EDT) Received: from localhost (ip6-localhost [IPv6:::1]) by mail.efficios.com (Postfix) with ESMTP id C1774230192; Tue, 3 Jul 2018 13:26:39 -0400 (EDT) DKIM-Filter: OpenDKIM Filter v2.10.3 mail.efficios.com C1774230192 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=efficios.com; s=default; t=1530638799; bh=zgZSJZBAZSDaPGbe9Vl06QB2i7y5d9PRZOyzqbTqMQQ=; h=Date:From:To:Message-ID:MIME-Version; b=Omo621aIh+jWjrkRsdHweQXhQQC4gUpqadLICJqNok54yyVtzAEGmdUNTgYtfVZTh mTwRkyX1+xJYtBOMUt2XCfpYZqZCLkaT6PTjlcInQ4x+KL0hamG0FwO+dw8LuJO+kP Pv9tLyfhOgy/Gd+8vtJD/ihviQFnyPcIDnz7orLx6jWMZQvnh8kXc84rlXDNVu8HdT 1c0klrWCeDonp7sBIfv2KWnrBezU3onGqyM7w9LFGM5jDXX/vPS+dj3PSzLuvSMbfu Bk6fwfhBD3nxg7oqcSwr+hSKAGU5/njUkFqyrBuUWh+rK9Y4r3/0URfVek2ZXGf/yw FAVESUnXmBnXg== X-Virus-Scanned: amavisd-new at efficios.com Received: from mail.efficios.com ([IPv6:::1]) by localhost (mail02.efficios.com [IPv6:::1]) (amavisd-new, port 10026) with ESMTP id eBh6q0wSKmR9; Tue, 3 Jul 2018 13:26:39 -0400 (EDT) Received: from mail02.efficios.com (mail02.efficios.com [167.114.142.138]) by mail.efficios.com (Postfix) with ESMTP id A600023018B; Tue, 3 Jul 2018 13:26:39 -0400 (EDT) Date: Tue, 3 Jul 2018 13:26:39 -0400 (EDT) From: Mathieu Desnoyers To: Linus Torvalds Cc: Andi Kleen , Peter Zijlstra , heiko carstens , Andy Lutomirski , Thomas Gleixner , linux-kernel , linux-api , "Paul E. McKenney" , Boqun Feng , Dave Watson , Paul Turner , Andrew Morton , Russell King , Ingo Molnar , "H. Peter Anvin" , Chris Lameter , Ben Maurer , rostedt , Josh Triplett , Catalin Marinas , Will Deacon , Michael Kerrisk , Joel Fernandes , Michal Simek , schwidefsky , gor@linux.ibm.com Message-ID: <1047204530.11737.1530638799580.JavaMail.zimbra@efficios.com> In-Reply-To: References: <8B2E4CEB-3080-4602-8B62-774E400892EB@amacapital.net> <20180703081449.GT2494@hirez.programming.kicks-ass.net> <20180703082955.GH3704@osiris> <20180703084312.GU2494@hirez.programming.kicks-ass.net> <20180703085546.GJ3704@osiris> <20180703092113.GV2494@hirez.programming.kicks-ass.net> <20180703164048.i2te5gjemcafqzwf@two.firstfloor.org> Subject: Re: [RFC PATCH for 4.18] rseq: use __u64 for rseq_cs fields, validate user inputs MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-Originating-IP: [167.114.142.138] X-Mailer: Zimbra 8.8.8_GA_2096 (ZimbraWebClient - FF52 (Linux)/8.8.8_GA_1703) Thread-Topic: rseq: use __u64 for rseq_cs fields, validate user inputs Thread-Index: j66OqF9zbUhN0xBsQ1otYplWTFq6fg== Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org ----- On Jul 3, 2018, at 1:10 PM, Linus Torvalds torvalds@linux-foundation.org wrote: > On Tue, Jul 3, 2018 at 9:40 AM Andi Kleen wrote: >> >> So it sounds like architectures that don't have an instruction atomic u64 >> *_user need to disable interrupts during the access, and somehow handle that >> case when a page fault happens? > > No. It's actually the store by *user* space that is the critical one. > Not the whole 64-bit value, just the low pointer part. > > The kernel could do it as a byte-by-byte load, really. It's > per-thread, and once the kernel is running, it's not going to change. > The kernel never changes the value, it just loads it from user space. > > So all the atomicity worries for the kernel are a red herring. They'd > arguably be nice to have - but only for an insane case that makes > absolutely no sense (a different thread trying to change the value). > > Can we please stop the idiocy already? The kernel could read the rseq > pointer one bit at a time, and do a little dance with "yield()" in > between, and take interrupts and page faults, and it wouldn't matter > AT ALL. > > It's not even that we read the value from an interrupt context, it's > that as we return to user space (which can be the result of an > interrupt) we can read the value. > > This whole thread has been filled with crazy "what if" things that don't matter. Sorry to come back in the thread late, looks like I've missed all the fun. I agree with Linus: we can simply document that updates to rseq->rseq_cs should be thread-local in the rseq uapi and be done with it. This would allow using get_user(u64) even on 32-bit architectures, because we cannot care less if an architecture chooses to read the u64 byte-wise while standing on its feet. With this added requirement, Andy's idea of using a union between __u64 and upper/lower __u32 would fit very nicely. If everyone is OK with that approach, I can prepare an updated patch. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com