security: use mmap_min_addr indepedently of security models

This patch removes the dependency of mmap_min_addr on CONFIG_SECURITY.
It also sets a default mmap_min_addr of 4096.

mmapping of addresses below 4096 will only be possible for processes
with CAP_SYS_RAWIO.

Signed-off-by: Christoph Lameter <cl@linux-foundation.org>
Acked-by: Eric Paris <eparis@redhat.com>
Looks-ok-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: James Morris <jmorris@namei.org>

1 files changed, 19 insertions, 0 deletions

diff --git a/mm/Kconfig b/mm/Kconfig
index c2b57d81e153..71830ba7b986 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -226,6 +226,25 @@ config HAVE_MLOCKED_PAGE_BIT
 config MMU_NOTIFIER
         bool
 
+config DEFAULT_MMAP_MIN_ADDR
+        int "Low address space to protect from user allocation"
+        default 4096
+        help
+          This is the portion of low virtual memory which should be protected
+          from userspace allocation.  Keeping a user from writing to low pages
+          can help reduce the impact of kernel NULL pointer bugs.
+
+          For most ia64, ppc64 and x86 users with lots of address space
+          a value of 65536 is reasonable and should cause no problems.
+          On arm and other archs it should not be higher than 32768.
+          Programs which use vm86 functionality would either need additional
+          permissions from either the LSM or the capabilities module or have
+          this protection disabled.
+
+          This value can be changed after boot using the
+          /proc/sys/vm/mmap_min_addr tunable.
+
+
 config NOMMU_INITIAL_TRIM_EXCESS
         int "Turn on mmap() excess space trimming before booting"
         depends on !MMU