Messages in this thread | | | Date | Mon, 22 Mar 2021 14:35:41 +0100 | From | Michal Hocko <> | 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 Looks like it's essentially unused and has been replaced by better 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@linux-foundation.org> > Cc: Hillf Danton <hdanton@sina.com> > Cc: Michal Hocko <mhocko@suse.com> > Cc: Matthew Wilcox <willy@infradead.org> > Cc: Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com> > Cc: Steven Rostedt <rostedt@goodmis.org> > Cc: Minchan Kim <minchan@kernel.org> > Cc: huang ying <huang.ying.caritas@gmail.com> > Cc: Jonathan Corbet <corbet@lwn.net> > Cc: Russell King <linux@armlinux.org.uk> > Cc: Liviu Dudau <liviu.dudau@arm.com> > Cc: Sudeep Holla <sudeep.holla@arm.com> > Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com> > Cc: Andrew Lunn <andrew@lunn.ch> > Cc: Gregory Clement <gregory.clement@bootlin.com> > Cc: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com> > Cc: Yoshinori Sato <ysato@users.sourceforge.jp> > Cc: Brian Cain <bcain@codeaurora.org> > Cc: Geert Uytterhoeven <geert@linux-m68k.org> > Cc: Jonas Bonn <jonas@southpole.se> > Cc: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi> > Cc: Stafford Horne <shorne@gmail.com> > Cc: Rich Felker <dalias@libc.org> > Cc: "David S. Miller" <davem@davemloft.net> > Cc: Chris Zankel <chris@zankel.net> > Cc: Max Filippov <jcmvbkbc@gmail.com> > Cc: Arnd Bergmann <arnd@arndb.de> > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> > Cc: Alexander Viro <viro@zeniv.linux.org.uk> > Cc: Rob Herring <robh@kernel.org> > Cc: "Pavel Machek (CIP)" <pavel@denx.de> > Cc: Theodore Dubois <tblodt@icloud.com> > Cc: "Alexander A. Klimov" <grandmaster@al2klimov.de> > Cc: Pavel Machek <pavel@ucw.cz> > Cc: Sam Ravnborg <sam@ravnborg.org> > Cc: Alexandre Belloni <alexandre.belloni@bootlin.com> > Cc: Andrey Zhizhikin <andrey.zhizhikin@leica-geosystems.com> > Cc: Randy Dunlap <rdunlap@infradead.org> > Cc: Krzysztof Kozlowski <krzk@kernel.org> > Cc: Viresh Kumar <viresh.kumar@linaro.org> > Cc: "Eric W. Biederman" <ebiederm@xmission.com> > Cc: Thomas Gleixner <tglx@linutronix.de> > Cc: Xiaoming Ni <nixiaoming@huawei.com> > Cc: Robert Richter <rric@kernel.org> > Cc: William Cohen <wcohen@redhat.com> > Cc: Corentin Labbe <clabbe@baylibre.com> > Cc: Kairui Song <kasong@redhat.com> > Cc: Linus Torvalds <torvalds@linux-foundation.org> > Cc: linux-doc@vger.kernel.org > Cc: linux-kernel@vger.kernel.org > Cc: linux-arm-kernel@lists.infradead.org > Cc: uclinux-h8-devel@lists.sourceforge.jp > Cc: linux-hexagon@vger.kernel.org > Cc: linux-m68k@lists.linux-m68k.org > Cc: openrisc@lists.librecores.org > Cc: linux-sh@vger.kernel.org > Cc: sparclinux@vger.kernel.org > Cc: linux-xtensa@linux-xtensa.org > Cc: linux-fsdevel@vger.kernel.org > Cc: Linux API <linux-api@vger.kernel.org> > Signed-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Michal Hocko <mhocko@suse.com> -- Michal Hocko SUSE Labs
| |