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Commit 7660a6fd authored by Kees Cook's avatar Kees Cook Committed by Linus Torvalds
Browse files

mm: allow slab_nomerge to be set at build time 

Some hardened environments want to build kernels with slab_nomerge
already set (so that they do not depend on remembering to set the kernel
command line option).  This is desired to reduce the risk of kernel heap
overflows being able to overwrite objects from merged caches and changes
the requirements for cache layout control, increasing the difficulty of
these attacks.  By keeping caches unmerged, these kinds of exploits can
usually only damage objects in the same cache (though the risk to
metadata exploitation is unchanged).

Link: http://lkml.kernel.org/r/20170620230911.GA25238@beast


Signed-off-by: default avatarKees Cook <keescook@chromium.org>
Cc: Daniel Micay <danielmicay@gmail.com>
Cc: David Windsor <dave@nullcore.net>
Cc: Eric Biggers <ebiggers3@gmail.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Daniel Micay <danielmicay@gmail.com>
Cc: David Windsor <dave@nullcore.net>
Cc: Eric Biggers <ebiggers3@gmail.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Nicolas Pitre <nicolas.pitre@linaro.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Daniel Mack <daniel@zonque.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Helge Deller <deller@gmx.de>
Cc: Rik van Riel <riel@redhat.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent e0771950

Documentation/admin-guide/kernel-parameters.txt

+8 −2
Original line number Diff line number Diff line
@@ -3760,8 +3760,14 @@ 
	slab_nomerge	[MM]

			Disable merging of slabs with similar size. May be

			necessary if there is some reason to distinguish

			allocs to different slabs. Debug options disable

			merging on their own.

			allocs to different slabs, especially in hardened

			environments where the risk of heap overflows and

			layout control by attackers can usually be

			frustrated by disabling merging. This will reduce

			most of the exposure of a heap attack to a single

			cache (risks via metadata attacks are mostly

			unchanged). Debug options disable merging on their

			own.

			For more information see Documentation/vm/slub.txt.



	slab_max_order=	[MM, SLAB]

init/Kconfig

+14 −0
Original line number Diff line number Diff line
@@ -1548,6 +1548,20 @@ config SLOB 


endchoice



config SLAB_MERGE_DEFAULT

	bool "Allow slab caches to be merged"

	default y

	help

	  For reduced kernel memory fragmentation, slab caches can be

	  merged when they share the same size and other characteristics.

	  This carries a risk of kernel heap overflows being able to

	  overwrite objects from merged caches (and more easily control

	  cache layout), which makes such heap attacks easier to exploit

	  by attackers. By keeping caches unmerged, these kinds of exploits

	  can usually only damage objects in the same cache. To disable

	  merging at runtime, "slab_nomerge" can be passed on the kernel

	  command line.



config SLAB_FREELIST_RANDOM

	default n

	depends on SLAB || SLUB

mm/slab_common.c

+2 −3
Original line number Diff line number Diff line
@@ -47,13 +47,12 @@ static DECLARE_WORK(slab_caches_to_rcu_destroy_work, 


/*

 * Merge control. If this is set then no merging of slab caches will occur.

 * (Could be removed. This was introduced to pacify the merge skeptics.)

 */

static int slab_nomerge;

static bool slab_nomerge = !IS_ENABLED(CONFIG_SLAB_MERGE_DEFAULT);



static int __init setup_slab_nomerge(char *str)

{

	slab_nomerge = 1;

	slab_nomerge = true;

	return 1;

}