mm: prevent mapping slab pages to userspace

It's never appropriate to map a page allocated by SLAB into userspace.
A buggy device driver might try this, or an attacker might be able to
find a way to make it happen.

Christoph said:

: Let's just fail the code.  Currently this may work with SLUB.  But SLAB
: and SLOB overlay fields with mapcount.  So you would have a corrupted page
: struct if you mapped a slab page to user space.

Link: http://lkml.kernel.org/r/20190125173827.2658-1-willy@infradead.org


Signed-off-by: Matthew Wilcox <willy@infradead.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Pekka Enberg <penberg@kernel.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>