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=-7.5 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,NICE_REPLY_A,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 05C3CC433DB for ; Fri, 19 Mar 2021 14:46:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B598364F4D for ; Fri, 19 Mar 2021 14:46:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230190AbhCSOqH (ORCPT ); Fri, 19 Mar 2021 10:46:07 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:51673 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230336AbhCSOps (ORCPT ); Fri, 19 Mar 2021 10:45:48 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1616165146; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=8KGbLZQ7O+JfyAjTpujJt4QAW5NJjPJMsJ4wbUnlwTM=; b=am0tla4yt6NyH3/n0ysHeAFAxbHuD6Nghr/nrYZmETnzcsu4hAgRAoeusLCX95pqMElS4q YB/ZO+TNuwUM09n06K/3ObNQ6qkR7B03kEIKbzsPN7cuoiWTKLgUmLQy3kaevY5QZ3RSWQ Aa9/5QMw/ocE70P4XyGYdagXUD65Ndw= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-489-XIN7XlhROw6nZkbmTGNd6A-1; Fri, 19 Mar 2021 10:45:45 -0400 X-MC-Unique: XIN7XlhROw6nZkbmTGNd6A-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 21154801597; Fri, 19 Mar 2021 14:45:41 +0000 (UTC) Received: from [10.36.112.11] (ovpn-112-11.ams2.redhat.com [10.36.112.11]) by smtp.corp.redhat.com (Postfix) with ESMTP id 9858D6090F; Fri, 19 Mar 2021 14:45:26 +0000 (UTC) Subject: Re: [PATCH RFC 0/3] drivers/char: remove /dev/kmem for good To: linux-kernel@vger.kernel.org Cc: linux-mm@kvack.org, "Alexander A. Klimov" , Alexander Viro , Alexandre Belloni , Andrew Lunn , Andrew Morton , Andrey Zhizhikin , Arnd Bergmann , Benjamin Herrenschmidt , Brian Cain , Christian Borntraeger , Christophe Leroy , Chris Zankel , Corentin Labbe , "David S. Miller" , "Eric W. Biederman" , Geert Uytterhoeven , Gerald Schaefer , Greentime Hu , Greg Kroah-Hartman , Gregory Clement , Heiko Carstens , Helge Deller , Hillf Danton , huang ying , Ingo Molnar , Ivan Kokshaysky , "James E.J. Bottomley" , Jiaxun Yang , Jonas Bonn , Jonathan Corbet , Kairui Song , Krzysztof Kozlowski , Kuninori Morimoto , Linus Torvalds , Linux API , Liviu Dudau , Lorenzo Pieralisi , Luc Van Oostenryck , Luis Chamberlain , Matthew Wilcox , Matt Turner , Max Filippov , Michael Ellerman , Michal Hocko , Mike Rapoport , Mikulas Patocka , Minchan Kim , Niklas Schnelle , Oleksiy Avramchenko , Palmer Dabbelt , Paul Mackerras , "Pavel Machek (CIP)" , Pavel Machek , "Peter Zijlstra (Intel)" , Pierre Morel , Randy Dunlap , Richard Henderson , Rich Felker , Robert Richter , Rob Herring , Russell King , Sam Ravnborg , Sebastian Andrzej Siewior , Sebastian Hesselbarth , Stafford Horne , Stefan Kristiansson , Steven Rostedt , Sudeep Holla , Theodore Dubois , Thomas Bogendoerfer , Thomas Gleixner , Vasily Gorbik , Viresh Kumar , William Cohen , Xiaoming Ni , Yoshinori Sato References: <20210319143452.25948-1-david@redhat.com> From: David Hildenbrand Organization: Red Hat GmbH Message-ID: <39b945bc-003c-4fb3-17ad-3484b6196bd3@redhat.com> Date: Fri, 19 Mar 2021 15:45:25 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.0 MIME-Version: 1.0 In-Reply-To: <20210319143452.25948-1-david@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Precedence: bulk List-ID: X-Mailing-List: linux-api@vger.kernel.org On 19.03.21 15:34, David Hildenbrand wrote: > Let's start a discussion if /dev/kmem is worth keeping around and > fixing/maintaining or if we should just remove it now for good. > > More details / findings in patch #1. Patch #2 and #3 perform minor cleanups > based on removed /dev/kmem support. As some arch maintainers are only cced on patch #2 (grml), patch #1 is https://lkml.kernel.org/r/20210319143452.25948-2-david@redhat.com and the description is: " Exploring /dev/kmem and /dev/mem in the context of memory hot(un)plug and memory ballooning, I started questioning the existance of /dev/kmem. Comparing it with the /proc/kcore implementation, it does not seem to be able to deal with things like a) Pages unmapped from the direct mapping (e.g., to be used by secretmem) -> kern_addr_valid(). virt_addr_valid() is not sufficient. b) Special cases like gart aperture memory that is not to be touched -> mem_pfn_is_ram() Unless I am missing something, it's at least broken in some cases and might fault/crash the machine. Looks like its existance has been questioned before in 2005 and 2010 [1], after ~11 additional years, it might make sense to revive the discussion. CONFIG_DEVKMEM is only enabled in a single defconfig (on purpose or by mistake?). All distributions I looked at disable it. 1) /dev/kmem was popular for rootkits [2] before it got disabled basically everywhere. Ubuntu documents [3] "There is no modern user of /dev/kmem any more beyond attackers using it to load kernel rootkits.". RHEL documents in a BZ [5] "it served no practical purpose other than to serve as a potential security problem or to enable binary module drivers to access structures/functions they shouldn't be touching" 2) /proc/kcore is a decent interface to have a controlled way to read kernel memory for debugging puposes. (will need some extensions to deal with memory offlining/unplug, memory ballooning, and poisoned pages, though) 3) It might be useful for corner case debugging [1]. KDB/KGDB might be a better fit, especially, to write random memory; harder to shoot yourself into the foot. 4) "Kernel Memory Editor" hasn't seen any updates since 2000 and seems to be incompatible with 64bit [1]. For educational purposes, /proc/kcore might be used to monitor value updates -- or older kernels can be used. 5) It's broken on arm64, and therefore, completely disabled there. Looks like it's essentially unused and has been replaced by better suited interfaces for individual tasks (/proc/kcore, KDB/KGDB). Let's just remove it. [1] https://lwn.net/Articles/147901/ [2] https://www.linuxjournal.com/article/10505 [3] https://wiki.ubuntu.com/Security/Features#A.2Fdev.2Fkmem_disabled [4] https://sourceforge.net/projects/kme/ [5] https://bugzilla.redhat.com/show_bug.cgi?id=154796 " -- Thanks, David / dhildenb