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: Kees Cook <[email protected]>
Cc: Daniel Micay <[email protected]>
Cc: David Windsor <[email protected]>
Cc: Eric Biggers <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Daniel Micay <[email protected]>
Cc: David Windsor <[email protected]>
Cc: Eric Biggers <[email protected]>
Cc: Pekka Enberg <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: "Rafael J. Wysocki" <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Mauro Carvalho Chehab <[email protected]>
Cc: "Paul E. McKenney" <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Nicolas Pitre <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Daniel Mack <[email protected]>
Cc: Sebastian Andrzej Siewior <[email protected]>
Cc: Sergey Senozhatsky <[email protected]>
Cc: Helge Deller <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Randy Dunlap <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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