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=-2.3 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED, 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 03FDAC282C2 for ; Wed, 13 Feb 2019 14:40:07 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C5BD8222B5 for ; Wed, 13 Feb 2019 14:40:06 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="rRo5bPbA" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2392367AbfBMOkF (ORCPT ); Wed, 13 Feb 2019 09:40:05 -0500 Received: from bombadil.infradead.org ([198.137.202.133]:53630 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2390684AbfBMOkF (ORCPT ); Wed, 13 Feb 2019 09:40:05 -0500 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=VR4hOcRIJE5nvd9WG8ErdD7kw0njcUqaZiFFYD0Af1c=; b=rRo5bPbAoityizAzOoc4He1/2 g878QIXS3PwYXX6/Y8nrL+quskiCadBPbFLcx/oNLwsSBw44T5cjO2+mEy6ouIs7208sxk9erG+OH tZ68ZUoXu97JtwM5CtkSKpFmFvCo5U6zz2zyVXCrGiI11K/FH5szq74861f/gBkBkBVjEsLJ0Jilx J/N3IAzLU10pCTCNga6GL/93OZWV91W1Z8tXWK432qn2ebNHBbAEBsQpsdwp38pjUnIBdrMYWv48B LazQkUFpBQgSB+kdqwBmxwBfx4mvwVqh1XTDPE5APMuZIc54ADL2lvG0VRb98Uvb2alddRsOCSni4 fxo7gayWQ==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by bombadil.infradead.org with esmtpsa (Exim 4.90_1 #2 (Red Hat Linux)) id 1gtvhi-00018v-Go; Wed, 13 Feb 2019 14:40:02 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id CCA2422BBEBE6; Wed, 13 Feb 2019 15:40:00 +0100 (CET) Date: Wed, 13 Feb 2019 15:40:00 +0100 From: Peter Zijlstra To: Julien Thierry Cc: Will Deacon , 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 Subject: Re: [PATCH v3 3/4] uaccess: Check no rescheduling function is called in unsafe region Message-ID: <20190213144000.GX32494@hirez.programming.kicks-ass.net> References: <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Feb 13, 2019 at 02:24:34PM +0000, Julien Thierry wrote: > 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 EFLAGS.AC, but yes. > 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()). /me looks at __switch_to_asm() and there is indeed a distinct lack of pushing and popping EFLAGS :/ > 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. Well, per the above it breaks the preemption model. And I hates that. And the added WARN doesn't even begin to cover it, since you'd have to actually hit the preempt_enable() reschedule for it trigger. So far, all 6 in-tree users are indeed free of dodgy code, but *groan*.