security: Protection for exploiting null dereference using mmap

Add a new security check on mmap operations to see if the user is attempting
to mmap to low area of the address space.  The amount of space protected is
indicated by the new proc tunable /proc/sys/vm/mmap_min_addr and defaults to
0, preserving existing behavior.

This patch uses a new SELinux security class "memprotect."  Policy already
contains a number of allow rules like a_t self:process * (unconfined_t being
one of them) which mean that putting this check in the process class (its
best current fit) would make it useless as all user processes, which we also
want to protect against, would be allowed. By taking the memprotect name of
the new class it will also make it possible for us to move some of the other
memory protect permissions out of 'process' and into the new class next time
we bump the policy version number (which I also think is a good future idea)

Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Acked-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Eric Paris <eparis@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>

1 files changed, 8 insertions, 4 deletions

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index b29059ecc04..78c3f98fcdc 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2569,12 +2569,16 @@ static int file_map_prot_check(struct file *file, unsigned long prot, int shared
 }
 
 static int selinux_file_mmap(struct file *file, unsigned long reqprot,
-                             unsigned long prot, unsigned long flags)
+                             unsigned long prot, unsigned long flags,
+                             unsigned long addr, unsigned long addr_only)
 {
-        int rc;
+        int rc = 0;
+        u32 sid = ((struct task_security_struct*)(current->security))->sid;
 
-        rc = secondary_ops->file_mmap(file, reqprot, prot, flags);
-        if (rc)
+        if (addr < mmap_min_addr)
+                rc = avc_has_perm(sid, sid, SECCLASS_MEMPROTECT,
+                                  MEMPROTECT__MMAP_ZERO, NULL);
+        if (rc || addr_only)
                 return rc;
 
         if (selinux_checkreqprot)