From mboxrd@z Thu Jan 1 00:00:00 1970 From: will.deacon@arm.com (Will Deacon) Date: Thu, 9 Aug 2018 13:38:12 +0100 Subject: [PATCH] arm64: Trap WFI executed in userspace In-Reply-To: <20180809123457.GN9097@e103592.cambridge.arm.com> References: <20180807093326.5090-1-marc.zyngier@arm.com> <20180807100437.GA9097@e103592.cambridge.arm.com> <9af8bb9a-7c6c-2560-5965-118dfadf8141@arm.com> <20180808123408.GC24736@iMac.local> <20180809123457.GN9097@e103592.cambridge.arm.com> Message-ID: <20180809123812.GB29785@arm.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Thu, Aug 09, 2018 at 01:34:57PM +0100, Dave Martin wrote: > On Wed, Aug 08, 2018 at 01:34:09PM +0100, Catalin Marinas wrote: > > On Tue, Aug 07, 2018 at 11:24:34AM +0100, Marc Zyngier wrote: > > > On 07/08/18 11:05, Dave Martin wrote: > > > > On Tue, Aug 07, 2018 at 10:33:26AM +0100, Marc Zyngier wrote: > > > >> It recently came to light that userspace can execute WFI, and that > > > >> the arm64 kernel doesn trap this event. This sounds rather benign, > > > > Nitpick: "doesn't". > > > > > >> but the kernel should decide when it wants to wait for an interrupt, > > > >> and not userspace. > > > >> > > > >> Let's trap WFI and treat it as a way to yield the CPU to another > > > >> process. > > [...] > > > > I can't think of a legitimate reason for userspace to execute WFI > > > > however. Userspace doesn't have interrupts under Linux, so it makes > > > > no sense to wait for one. > > > > > > > > Have we seen anybody using WFI in userspace? It may be cleaner to > > > > map this to SIGILL rather than be permissive and regret it later. > > > > > > I couldn't find any user, and I'm happy to just send userspace to hell > > > in that case. But it could also been said that since it was never > > > prevented, it is a de-facto ABI. > > > > I wouldn't really go as far as SIGILL on WFI. I think the patch is fine > > as it is. In case Will plans to merge it: > > For practical purposes I agree, because we can't control the binary > blobs out there: I just wanted to bang the drum because we are creating > semantics here and there is not an obvious correct answer to what they > should be. > > I'd still like to see rationale for why this should map to schedule() > (which userspace currently has no direct way to trigger) as opposed to > sched_yield() or something like that. A better idea might just be to do pc +=4 and return. If there's work pending, we'll hit it on the return path (just like any other ret_to_user call). I initially thought about sched_yield(), but it's not clear whether that creates a problem if, e.g. seccomp has been used to restrict that syscall. Will 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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT 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 C0720C46460 for ; Thu, 9 Aug 2018 12:38:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7FA92208A5 for ; Thu, 9 Aug 2018 12:38:10 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 7FA92208A5 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 S1732324AbeHIPCv (ORCPT ); Thu, 9 Aug 2018 11:02:51 -0400 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:53410 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730506AbeHIPCu (ORCPT ); Thu, 9 Aug 2018 11:02:50 -0400 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 159A480D; Thu, 9 Aug 2018 05:38:08 -0700 (PDT) Received: from edgewater-inn.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.72.51.249]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPA id DACFB3F5D4; Thu, 9 Aug 2018 05:38:07 -0700 (PDT) Received: by edgewater-inn.cambridge.arm.com (Postfix, from userid 1000) id 98D941AE180A; Thu, 9 Aug 2018 13:38:12 +0100 (BST) Date: Thu, 9 Aug 2018 13:38:12 +0100 From: Will Deacon To: Dave Martin Cc: Catalin Marinas , Marc Zyngier , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] arm64: Trap WFI executed in userspace Message-ID: <20180809123812.GB29785@arm.com> References: <20180807093326.5090-1-marc.zyngier@arm.com> <20180807100437.GA9097@e103592.cambridge.arm.com> <9af8bb9a-7c6c-2560-5965-118dfadf8141@arm.com> <20180808123408.GC24736@iMac.local> <20180809123457.GN9097@e103592.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180809123457.GN9097@e103592.cambridge.arm.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Aug 09, 2018 at 01:34:57PM +0100, Dave Martin wrote: > On Wed, Aug 08, 2018 at 01:34:09PM +0100, Catalin Marinas wrote: > > On Tue, Aug 07, 2018 at 11:24:34AM +0100, Marc Zyngier wrote: > > > On 07/08/18 11:05, Dave Martin wrote: > > > > On Tue, Aug 07, 2018 at 10:33:26AM +0100, Marc Zyngier wrote: > > > >> It recently came to light that userspace can execute WFI, and that > > > >> the arm64 kernel doesn trap this event. This sounds rather benign, > > > > Nitpick: "doesn't". > > > > > >> but the kernel should decide when it wants to wait for an interrupt, > > > >> and not userspace. > > > >> > > > >> Let's trap WFI and treat it as a way to yield the CPU to another > > > >> process. > > [...] > > > > I can't think of a legitimate reason for userspace to execute WFI > > > > however. Userspace doesn't have interrupts under Linux, so it makes > > > > no sense to wait for one. > > > > > > > > Have we seen anybody using WFI in userspace? It may be cleaner to > > > > map this to SIGILL rather than be permissive and regret it later. > > > > > > I couldn't find any user, and I'm happy to just send userspace to hell > > > in that case. But it could also been said that since it was never > > > prevented, it is a de-facto ABI. > > > > I wouldn't really go as far as SIGILL on WFI. I think the patch is fine > > as it is. In case Will plans to merge it: > > For practical purposes I agree, because we can't control the binary > blobs out there: I just wanted to bang the drum because we are creating > semantics here and there is not an obvious correct answer to what they > should be. > > I'd still like to see rationale for why this should map to schedule() > (which userspace currently has no direct way to trigger) as opposed to > sched_yield() or something like that. A better idea might just be to do pc +=4 and return. If there's work pending, we'll hit it on the return path (just like any other ret_to_user call). I initially thought about sched_yield(), but it's not clear whether that creates a problem if, e.g. seccomp has been used to restrict that syscall. Will