mm: allow slab_nomerge to be set at build time (7660a6fd) · Commits · 戴 / test

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:

Kees 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:

Andrew Morton <akpm@linux-foundation.org> Signed-off-by:

Linus Torvalds <torvalds@linux-foundation.org>

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;
		}

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