include/linux/poison.h: fix LIST_POISON{1,2} offset

Poison pointer values should be small enough to find a room in
non-mmap'able/hardly-mmap'able space.  E.g.  on x86 "poison pointer space"
is located starting from 0x0.  Given unprivileged users cannot mmap
anything below mmap_min_addr, it should be safe to use poison pointers
lower than mmap_min_addr.

The current poison pointer values of LIST_POISON{1,2} might be too big for
mmap_min_addr values equal or less than 1 MB (common case, e.g.  Ubuntu
uses only 0x10000).  There is little point to use such a big value given
the "poison pointer space" below 1 MB is not yet exhausted.  Changing it
to a smaller value solves the problem for small mmap_min_addr setups.

The values are suggested by Solar Designer:
http://www.openwall.com/lists/oss-security/2015/05/02/6

Signed-off-by: Vasily Kulikov <segoon@openwall.com>
Cc: Solar Designer <solar@openwall.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit 8a5e5e0)
Signed-off-by: David Riley <davidriley@chromium.org>
Change-Id: I4a93b354bf497a34b9d4d1874808e6759576ba8c
Reviewed-on: https://chrome-internal-review.googlesource.com/245578
Reviewed-by: Andrew Bresticker <abrestic@google.com>
Commit-Queue: David Riley <davidriley@google.com>
Tested-by: David Riley <davidriley@google.com>

include/linux/poison.h

@@ -19,8 +19,8 @@
  * under normal circumstances, used to verify that nobody uses
  * non-initialized list entries.
  */
-#define LIST_POISON1  ((void *) 0x00100100 + POISON_POINTER_DELTA)
-#define LIST_POISON2  ((void *) 0x00200200 + POISON_POINTER_DELTA)
+#define LIST_POISON1  ((void *) 0x100 + POISON_POINTER_DELTA)
+#define LIST_POISON2  ((void *) 0x200 + POISON_POINTER_DELTA)
 
 /********** include/linux/timer.h **********/
 /*