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=-4.1 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS 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 22C5CC388F7 for ; Thu, 29 Oct 2020 00:35:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CA78C20791 for ; Thu, 29 Oct 2020 00:35:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1603931714; bh=gjv0U3NBKOnTNW44zMRkwPjIj1Z7l37xu//HQaF8TcY=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=hZgceJTgEVqfox3PdnO0lDLyoOR3z2KFR6IIArUX1nl3vsQerIbvYt9uHgGp/aQsb d2gQpPjukvowaCS0as4+0Fla/KtHcM6E7pRH6UM2es1VykFsgJvnNBeO0hisedD5Io 5zoDGBmLnUaYU1ZSw10w2W5ocvErGb+phRBiEjhU= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731939AbgJ1WT2 (ORCPT ); Wed, 28 Oct 2020 18:19:28 -0400 Received: from mail.kernel.org ([198.145.29.99]:60522 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731776AbgJ1WRp (ORCPT ); Wed, 28 Oct 2020 18:17:45 -0400 Received: from kernel.org (unknown [87.70.96.83]) (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 E6741246C3; Wed, 28 Oct 2020 11:31:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1603884674; bh=gjv0U3NBKOnTNW44zMRkwPjIj1Z7l37xu//HQaF8TcY=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=cUnXipj6SCyD+eplDp9tJgGGwn2XxiBxnaCPOYRKRcF6AcrwUEHeSRRr9vZ0ie5Ek O9QqqyW5F35vu7he5aXY8rhMDZYP4dyGxJF4jP4TGOEc1CbbkAGYzd/w5djNh6qS70 Sw2BJFOdgVYfPE9KAiE15TI5bQsRgWsjOYlHfBMk= Date: Wed, 28 Oct 2020 13:30:59 +0200 From: Mike Rapoport To: Will Deacon Cc: "Edgecombe, Rick P" , "david@redhat.com" , "cl@linux.com" , "gor@linux.ibm.com" , "hpa@zytor.com" , "peterz@infradead.org" , "catalin.marinas@arm.com" , "dave.hansen@linux.intel.com" , "borntraeger@de.ibm.com" , "penberg@kernel.org" , "linux-mm@kvack.org" , "iamjoonsoo.kim@lge.com" , "aou@eecs.berkeley.edu" , "kirill@shutemov.name" , "rientjes@google.com" , "rppt@linux.ibm.com" , "paulus@samba.org" , "hca@linux.ibm.com" , "bp@alien8.de" , "pavel@ucw.cz" , "sparclinux@vger.kernel.org" , "akpm@linux-foundation.org" , "luto@kernel.org" , "davem@davemloft.net" , "mpe@ellerman.id.au" , "benh@kernel.crashing.org" , "linuxppc-dev@lists.ozlabs.org" , "rjw@rjwysocki.net" , "tglx@linutronix.de" , "linux-riscv@lists.infradead.org" , "x86@kernel.org" , "linux-pm@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" , "palmer@dabbelt.com" , "Brown, Len" , "mingo@redhat.com" , "linux-s390@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "paul.walmsley@sifive.com" Subject: Re: [PATCH 0/4] arch, mm: improve robustness of direct map manipulation Message-ID: <20201028113059.GG1428094@kernel.org> References: <20201025101555.3057-1-rppt@kernel.org> <20201026090526.GA1154158@kernel.org> <20201027083816.GG1154158@kernel.org> <20201028112011.GB27927@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201028112011.GB27927@willie-the-truck> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Oct 28, 2020 at 11:20:12AM +0000, Will Deacon wrote: > 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? Yes. > Will -- Sincerely yours, Mike.