From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <SRS0=QTfK=ED=lists.ozlabs.org=linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@kernel.org>
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=-5.3 required=3.0 tests=BAYES_00,DKIM_INVALID,
	DKIM_SIGNED,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1
	autolearn=no 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 486E3C4363A
	for <linuxppc-dev@archiver.kernel.org>; Wed, 28 Oct 2020 11:23:25 +0000 (UTC)
Received: from lists.ozlabs.org (lists.ozlabs.org [203.11.71.2])
	(using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits))
	(No client certificate requested)
	by mail.kernel.org (Postfix) with ESMTPS id 452F6246BF
	for <linuxppc-dev@archiver.kernel.org>; Wed, 28 Oct 2020 11:23:24 +0000 (UTC)
Authentication-Results: mail.kernel.org;
	dkim=fail reason="signature verification failed" (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="nZtyj9b5"
DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 452F6246BF
Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org
Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org
Received: from bilbo.ozlabs.org (lists.ozlabs.org [IPv6:2401:3900:2:1::3])
	by lists.ozlabs.org (Postfix) with ESMTP id 4CLmQx6tj9zDqTb
	for <linuxppc-dev@archiver.kernel.org>; Wed, 28 Oct 2020 22:23:21 +1100 (AEDT)
Authentication-Results: lists.ozlabs.org; spf=pass (sender SPF authorized)
 smtp.mailfrom=kernel.org (client-ip=198.145.29.99; helo=mail.kernel.org;
 envelope-from=will@kernel.org; receiver=<UNKNOWN>)
Authentication-Results: lists.ozlabs.org;
 dmarc=pass (p=none dis=none) header.from=kernel.org
Authentication-Results: lists.ozlabs.org; dkim=pass (1024-bit key;
 unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256
 header.s=default header.b=nZtyj9b5; dkim-atps=neutral
Received: from mail.kernel.org (mail.kernel.org [198.145.29.99])
 (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits))
 (No client certificate requested)
 by lists.ozlabs.org (Postfix) with ESMTPS id 4CLmMZ2dRHzDqTh
 for <linuxppc-dev@lists.ozlabs.org>; Wed, 28 Oct 2020 22:20:26 +1100 (AEDT)
Received: from willie-the-truck (236.31.169.217.in-addr.arpa [217.169.31.236])
 (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256
 bits)) (No client certificate requested)
 by mail.kernel.org (Postfix) with ESMTPSA id 673E0246B9;
 Wed, 28 Oct 2020 11:20:16 +0000 (UTC)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org;
 s=default; t=1603884023;
 bh=mTg9fm0maDJRILTxhkQjp8X0Lq65oDuCI72pj3yFYDE=;
 h=Date:From:To:Cc:Subject:References:In-Reply-To:From;
 b=nZtyj9b5x6hecvlSEKot7wmBAZql8U1dePTmmXQiFrrzoaGBN9VqsVNPuzebn4sFU
 1HxFke7nJaXnQn6G39t0fFfqokJ+mMYm+26TSIP2esL1hnqarlb48UzMIoRkRNzXTB
 QjioeXNRaTtnP6jlK5TH2GSPSYibVZ9M4SKhISmY=
Date: Wed, 28 Oct 2020 11:20:12 +0000
From: Will Deacon <will@kernel.org>
To: Mike Rapoport <rppt@kernel.org>
Subject: Re: [PATCH 0/4] arch, mm: improve robustness of direct map
 manipulation
Message-ID: <20201028112011.GB27927@willie-the-truck>
References: <20201025101555.3057-1-rppt@kernel.org>
 <ae82f905a0092adb7e0f0ac206335c1883b3170f.camel@intel.com>
 <20201026090526.GA1154158@kernel.org>
 <a0212b073b3b2f62c3dbf1bf398f03fa402997be.camel@intel.com>
 <20201027083816.GG1154158@kernel.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20201027083816.GG1154158@kernel.org>
User-Agent: Mutt/1.10.1 (2018-07-13)
X-BeenThere: linuxppc-dev@lists.ozlabs.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Linux on PowerPC Developers Mail List <linuxppc-dev.lists.ozlabs.org>
List-Unsubscribe: <https://lists.ozlabs.org/options/linuxppc-dev>,
 <mailto:linuxppc-dev-request@lists.ozlabs.org?subject=unsubscribe>
List-Archive: <http://lists.ozlabs.org/pipermail/linuxppc-dev/>
List-Post: <mailto:linuxppc-dev@lists.ozlabs.org>
List-Help: <mailto:linuxppc-dev-request@lists.ozlabs.org?subject=help>
List-Subscribe: <https://lists.ozlabs.org/listinfo/linuxppc-dev>,
 <mailto:linuxppc-dev-request@lists.ozlabs.org?subject=subscribe>
Cc: "david@redhat.com" <david@redhat.com>,
 "peterz@infradead.org" <peterz@infradead.org>,
 "catalin.marinas@arm.com" <catalin.marinas@arm.com>,
 "dave.hansen@linux.intel.com" <dave.hansen@linux.intel.com>,
 "linux-mm@kvack.org" <linux-mm@kvack.org>,
 "paulus@samba.org" <paulus@samba.org>, "pavel@ucw.cz" <pavel@ucw.cz>,
 "hpa@zytor.com" <hpa@zytor.com>,
 "sparclinux@vger.kernel.org" <sparclinux@vger.kernel.org>,
 "cl@linux.com" <cl@linux.com>,
 "linux-riscv@lists.infradead.org" <linux-riscv@lists.infradead.org>,
 "linux-s390@vger.kernel.org" <linux-s390@vger.kernel.org>,
 "x86@kernel.org" <x86@kernel.org>, "rppt@linux.ibm.com" <rppt@linux.ibm.com>,
 "borntraeger@de.ibm.com" <borntraeger@de.ibm.com>,
 "mingo@redhat.com" <mingo@redhat.com>,
 "rientjes@google.com" <rientjes@google.com>, "Brown,
 Len" <len.brown@intel.com>, "aou@eecs.berkeley.edu" <aou@eecs.berkeley.edu>,
 "gor@linux.ibm.com" <gor@linux.ibm.com>,
 "linux-pm@vger.kernel.org" <linux-pm@vger.kernel.org>,
 "hca@linux.ibm.com" <hca@linux.ibm.com>, "bp@alien8.de" <bp@alien8.de>,
 "luto@kernel.org" <luto@kernel.org>,
 "paul.walmsley@sifive.com" <paul.walmsley@sifive.com>,
 "kirill@shutemov.name" <kirill@shutemov.name>,
 "tglx@linutronix.de" <tglx@linutronix.de>,
 "iamjoonsoo.kim@lge.com" <iamjoonsoo.kim@lge.com>,
 "linux-arm-kernel@lists.infradead.org" <linux-arm-kernel@lists.infradead.org>,
 "rjw@rjwysocki.net" <rjw@rjwysocki.net>,
 "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
 "penberg@kernel.org" <penberg@kernel.org>,
 "palmer@dabbelt.com" <palmer@dabbelt.com>,
 "akpm@linux-foundation.org" <akpm@linux-foundation.org>, "Edgecombe,
 Rick P" <rick.p.edgecombe@intel.com>,
 "linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>,
 "davem@davemloft.net" <davem@davemloft.net>
Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org
Sender: "Linuxppc-dev"
 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

On Tue, Oct 27, 2020 at 10:38:16AM +0200, Mike Rapoport wrote:
> On Mon, Oct 26, 2020 at 06:05:30PM +0000, Edgecombe, Rick P wrote:
> > On Mon, 2020-10-26 at 11:05 +0200, Mike Rapoport wrote:
> > > On Mon, Oct 26, 2020 at 01:13:52AM +0000, Edgecombe, Rick P wrote:
> > > > On Sun, 2020-10-25 at 12:15 +0200, Mike Rapoport wrote:
> > > > > Indeed, for architectures that define
> > > > > CONFIG_ARCH_HAS_SET_DIRECT_MAP
> > > > > it is
> > > > > possible that __kernel_map_pages() would fail, but since this
> > > > > function is
> > > > > void, the failure will go unnoticed.
> > > > 
> > > > Could you elaborate on how this could happen? Do you mean during
> > > > runtime today or if something new was introduced?
> > > 
> > > A failure in__kernel_map_pages() may happen today. For instance, on
> > > x86
> > > if the kernel is built with DEBUG_PAGEALLOC.
> > > 
> > >         __kernel_map_pages(page, 1, 0);
> > > 
> > > will need to split, say, 2M page and during the split an allocation
> > > of
> > > page table could fail.
> > 
> > On x86 at least, DEBUG_PAGEALLOC expects to never have to break a page
> > on the direct map and even disables locking in cpa because it assumes
> > this. If this is happening somehow anyway then we should probably fix
> > that. Even if it's a debug feature, it will not be as useful if it is
> > causing its own crashes.
> > 
> > I'm still wondering if there is something I'm missing here. It seems
> > like you are saying there is a bug in some arch's, so let's add a WARN
> > in cross-arch code to log it as it crashes. A warn and making things
> > clearer seem like good ideas, but if there is a bug we should fix it.
> > The code around the callers still functionally assume re-mapping can't
> > fail.
> 
> Oh, I've meant x86 kernel *without* DEBUG_PAGEALLOC, and indeed the call
> that unmaps pages back in safe_copy_page will just reset a 4K page to
> NP because whatever made it NP at the first place already did the split.
> 
> Still, on arm64 with DEBUG_PAGEALLOC=n there is a possibility of a race
> between map/unmap dance in __vunmap() and safe_copy_page() that may
> cause access to unmapped memory:
> 
> __vunmap()
>     vm_remove_mappings()
>         set_direct_map_invalid()
> 					safe_copy_page()	
> 					    __kernel_map_pages()
> 					    	return
> 					    do_copy_page() -> fault
> 					   	
> This is a theoretical bug, but it is still not nice :) 							

Just to clarify: this patch series fixes this problem, right?

Will