security: allow Kconfig to set default mmap_min_addr protection

Since it was decided that low memory protection from userspace couldn't
be turned on by default add a Kconfig option to allow users/distros to
set a default at compile time.  This value is still tunable after boot
in /proc/sys/vm/mmap_min_addr

Discussion:
http://www.mail-archive.com/linux-security-module@vger.kernel.org/msg02543.html

Signed-off-by: Eric Paris <eparis@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>

2 files changed, 21 insertions, 1 deletions

diff --git a/security/Kconfig b/security/Kconfig
index 25ffe1b9dc98..5dfc206748cf 100644
--- a/security/Kconfig
+++ b/security/Kconfig
@@ -104,6 +104,24 @@ config SECURITY_ROOTPLUG
           
           If you are unsure how to answer this question, answer N.
 
+config SECURITY_DEFAULT_MMAP_MIN_ADDR
+        int "Low address space to protect from user allocation"
+        depends on SECURITY
+        default 0
+        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 users with lots of address space a value of 65536 is
+          reasonable and should cause no problems.  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.
+
+
 source security/selinux/Kconfig
 source security/smack/Kconfig
 
diff --git a/security/security.c b/security/security.c
index b6c57a6b2ff5..d15e56cbaade 100644
--- a/security/security.c
+++ b/security/security.c
@@ -23,7 +23,9 @@ extern struct security_operations dummy_security_ops;
 extern void security_fixup_ops(struct security_operations *ops);
 
 struct security_operations *security_ops;       /* Initialized to NULL */
-unsigned long mmap_min_addr;            /* 0 means no protection */
+
+/* amount of vm to protect from userspace access */
+unsigned long mmap_min_addr = CONFIG_SECURITY_DEFAULT_MMAP_MIN_ADDR;
 
 static inline int verify(struct security_operations *ops)
 {