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Message-ID: <YFidLVQs+/zw4aIF@dhcp22.suse.cz>
Date: Mon, 22 Mar 2021 14:35:41 +0100
From: Michal Hocko <mhocko@...e.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
Andrew Morton <akpm@...ux-foundation.org>,
Hillf Danton <hdanton@...a.com>,
Matthew Wilcox <willy@...radead.org>,
Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>,
Steven Rostedt <rostedt@...dmis.org>,
Minchan Kim <minchan@...nel.org>,
huang ying <huang.ying.caritas@...il.com>,
Jonathan Corbet <corbet@....net>,
Russell King <linux@...linux.org.uk>,
Liviu Dudau <liviu.dudau@....com>,
Sudeep Holla <sudeep.holla@....com>,
Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
Andrew Lunn <andrew@...n.ch>,
Gregory Clement <gregory.clement@...tlin.com>,
Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>,
Yoshinori Sato <ysato@...rs.sourceforge.jp>,
Brian Cain <bcain@...eaurora.org>,
Geert Uytterhoeven <geert@...ux-m68k.org>,
Jonas Bonn <jonas@...thpole.se>,
Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>,
Stafford Horne <shorne@...il.com>,
Rich Felker <dalias@...c.org>,
"David S. Miller" <davem@...emloft.net>,
Chris Zankel <chris@...kel.net>,
Max Filippov <jcmvbkbc@...il.com>,
Arnd Bergmann <arnd@...db.de>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Alexander Viro <viro@...iv.linux.org.uk>,
Rob Herring <robh@...nel.org>,
"Pavel Machek (CIP)" <pavel@...x.de>,
Theodore Dubois <tblodt@...oud.com>,
"Alexander A. Klimov" <grandmaster@...klimov.de>,
Pavel Machek <pavel@....cz>, Sam Ravnborg <sam@...nborg.org>,
Alexandre Belloni <alexandre.belloni@...tlin.com>,
Andrey Zhizhikin <andrey.zhizhikin@...ca-geosystems.com>,
Randy Dunlap <rdunlap@...radead.org>,
Krzysztof Kozlowski <krzk@...nel.org>,
Viresh Kumar <viresh.kumar@...aro.org>,
"Eric W. Biederman" <ebiederm@...ssion.com>,
Thomas Gleixner <tglx@...utronix.de>,
Xiaoming Ni <nixiaoming@...wei.com>,
Robert Richter <rric@...nel.org>,
William Cohen <wcohen@...hat.com>,
Corentin Labbe <clabbe@...libre.com>,
Kairui Song <kasong@...hat.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
linux-doc@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
uclinux-h8-devel@...ts.sourceforge.jp,
linux-hexagon@...r.kernel.org, linux-m68k@...ts.linux-m68k.org,
openrisc@...ts.librecores.org, linux-sh@...r.kernel.org,
sparclinux@...r.kernel.org, linux-xtensa@...ux-xtensa.org,
linux-fsdevel@...r.kernel.org,
Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH RFC 1/3] drivers/char: remove /dev/kmem for good
On Fri 19-03-21 15:34:50, David Hildenbrand wrote:
> 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
>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Hillf Danton <hdanton@...a.com>
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: Matthew Wilcox <willy@...radead.org>
> Cc: Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Minchan Kim <minchan@...nel.org>
> Cc: huang ying <huang.ying.caritas@...il.com>
> Cc: Jonathan Corbet <corbet@....net>
> Cc: Russell King <linux@...linux.org.uk>
> Cc: Liviu Dudau <liviu.dudau@....com>
> Cc: Sudeep Holla <sudeep.holla@....com>
> Cc: Lorenzo Pieralisi <lorenzo.pieralisi@....com>
> Cc: Andrew Lunn <andrew@...n.ch>
> Cc: Gregory Clement <gregory.clement@...tlin.com>
> Cc: Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>
> Cc: Yoshinori Sato <ysato@...rs.sourceforge.jp>
> Cc: Brian Cain <bcain@...eaurora.org>
> Cc: Geert Uytterhoeven <geert@...ux-m68k.org>
> Cc: Jonas Bonn <jonas@...thpole.se>
> Cc: Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>
> Cc: Stafford Horne <shorne@...il.com>
> Cc: Rich Felker <dalias@...c.org>
> Cc: "David S. Miller" <davem@...emloft.net>
> Cc: Chris Zankel <chris@...kel.net>
> Cc: Max Filippov <jcmvbkbc@...il.com>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Alexander Viro <viro@...iv.linux.org.uk>
> Cc: Rob Herring <robh@...nel.org>
> Cc: "Pavel Machek (CIP)" <pavel@...x.de>
> Cc: Theodore Dubois <tblodt@...oud.com>
> Cc: "Alexander A. Klimov" <grandmaster@...klimov.de>
> Cc: Pavel Machek <pavel@....cz>
> Cc: Sam Ravnborg <sam@...nborg.org>
> Cc: Alexandre Belloni <alexandre.belloni@...tlin.com>
> Cc: Andrey Zhizhikin <andrey.zhizhikin@...ca-geosystems.com>
> Cc: Randy Dunlap <rdunlap@...radead.org>
> Cc: Krzysztof Kozlowski <krzk@...nel.org>
> Cc: Viresh Kumar <viresh.kumar@...aro.org>
> Cc: "Eric W. Biederman" <ebiederm@...ssion.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Xiaoming Ni <nixiaoming@...wei.com>
> Cc: Robert Richter <rric@...nel.org>
> Cc: William Cohen <wcohen@...hat.com>
> Cc: Corentin Labbe <clabbe@...libre.com>
> Cc: Kairui Song <kasong@...hat.com>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: linux-doc@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> Cc: linux-arm-kernel@...ts.infradead.org
> Cc: uclinux-h8-devel@...ts.sourceforge.jp
> Cc: linux-hexagon@...r.kernel.org
> Cc: linux-m68k@...ts.linux-m68k.org
> Cc: openrisc@...ts.librecores.org
> Cc: linux-sh@...r.kernel.org
> Cc: sparclinux@...r.kernel.org
> Cc: linux-xtensa@...ux-xtensa.org
> Cc: linux-fsdevel@...r.kernel.org
> Cc: Linux API <linux-api@...r.kernel.org>
> Signed-off-by: David Hildenbrand <david@...hat.com>
Acked-by: Michal Hocko <mhocko@...e.com>
--
Michal Hocko
SUSE Labs
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