[PATCH 3.2 61/62] include/linux/poison.h: fix LIST_POISON{1,2} offset

[Ben Hutchings]

3.2.79-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Vasily Kulikov <segoon@xxxxxxxxxxxx>

commit 8a5e5e02fc83aaf67053ab53b359af08c6c49aaf upstream.

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@xxxxxxxxxxxx>
Cc: Solar Designer <solar@xxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 include/linux/poison.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/include/linux/poison.h
+++ b/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 **********/
 /*