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 Looks like it's essentially unused and has been replaced by better 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 <[email protected]> > Cc: Hillf Danton <[email protected]> > Cc: Michal Hocko <[email protected]> > Cc: Matthew Wilcox <[email protected]> > Cc: Oleksiy Avramchenko <[email protected]> > Cc: Steven Rostedt <[email protected]> > Cc: Minchan Kim <[email protected]> > Cc: huang ying <[email protected]> > Cc: Jonathan Corbet <[email protected]> > Cc: Russell King <[email protected]> > Cc: Liviu Dudau <[email protected]> > Cc: Sudeep Holla <[email protected]> > Cc: Lorenzo Pieralisi <[email protected]> > Cc: Andrew Lunn <[email protected]> > Cc: Gregory Clement <[email protected]> > Cc: Sebastian Hesselbarth <[email protected]> > Cc: Yoshinori Sato <[email protected]> > Cc: Brian Cain <[email protected]> > Cc: Geert Uytterhoeven <[email protected]> > Cc: Jonas Bonn <[email protected]> > Cc: Stefan Kristiansson <[email protected]> > Cc: Stafford Horne <[email protected]> > Cc: Rich Felker <[email protected]> > Cc: "David S. Miller" <[email protected]> > Cc: Chris Zankel <[email protected]> > Cc: Max Filippov <[email protected]> > Cc: Arnd Bergmann <[email protected]> > Cc: Greg Kroah-Hartman <[email protected]> > Cc: Alexander Viro <[email protected]> > Cc: Rob Herring <[email protected]> > Cc: "Pavel Machek (CIP)" <[email protected]> > Cc: Theodore Dubois <[email protected]> > Cc: "Alexander A. Klimov" <[email protected]> > Cc: Pavel Machek <[email protected]> > Cc: Sam Ravnborg <[email protected]> > Cc: Alexandre Belloni <[email protected]> > Cc: Andrey Zhizhikin <[email protected]> > Cc: Randy Dunlap <[email protected]> > Cc: Krzysztof Kozlowski <[email protected]> > Cc: Viresh Kumar <[email protected]> > Cc: "Eric W. Biederman" <[email protected]> > Cc: Thomas Gleixner <[email protected]> > Cc: Xiaoming Ni <[email protected]> > Cc: Robert Richter <[email protected]> > Cc: William Cohen <[email protected]> > Cc: Corentin Labbe <[email protected]> > Cc: Kairui Song <[email protected]> > Cc: Linus Torvalds <[email protected]> > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Cc: Linux API <[email protected]> > Signed-off-by: David Hildenbrand <[email protected]>
Acked-by: Michal Hocko <[email protected]> -- Michal Hocko SUSE Labs
