CRED: Add a kernel_service object class to SELinux

Add a 'kernel_service' object class to SELinux and give this object class two
access vectors: 'use_as_override' and 'create_files_as'.

The first vector is used to grant a process the right to nominate an alternate
process security ID for the kernel to use as an override for the SELinux
subjective security when accessing stuff on behalf of another process.

For example, CacheFiles when accessing the cache on behalf on a process
accessing an NFS file needs to use a subjective security ID appropriate to the
cache rather then the one the calling process is using.  The cachefilesd
daemon will nominate the security ID to be used.

The second vector is used to grant a process the right to nominate a file
creation label for a kernel service to use.
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Signed-off-by: NJames Morris <jmorris@namei.org>

security/selinux/include/av_perm_to_string.h

@@ -176,3 +176,5 @@
    S_(SECCLASS_DCCP_SOCKET, DCCP_SOCKET__NAME_CONNECT, "name_connect")
    S_(SECCLASS_MEMPROTECT, MEMPROTECT__MMAP_ZERO, "mmap_zero")
    S_(SECCLASS_PEER, PEER__RECV, "recv")
+   S_(SECCLASS_KERNEL_SERVICE, KERNEL_SERVICE__USE_AS_OVERRIDE, "use_as_override")
+   S_(SECCLASS_KERNEL_SERVICE, KERNEL_SERVICE__CREATE_FILES_AS, "create_files_as")

security/selinux/include/av_permissions.h

@@ -841,3 +841,5 @@
 #define DCCP_SOCKET__NAME_CONNECT                 0x00800000UL
 #define MEMPROTECT__MMAP_ZERO                     0x00000001UL
 #define PEER__RECV                                0x00000001UL
+#define KERNEL_SERVICE__USE_AS_OVERRIDE           0x00000001UL
+#define KERNEL_SERVICE__CREATE_FILES_AS           0x00000002UL

security/selinux/include/class_to_string.h

@@ -72,3 +72,8 @@
     S_(NULL)
     S_("peer")
     S_("capability2")
+    S_(NULL)
+    S_(NULL)
+    S_(NULL)
+    S_(NULL)
+    S_("kernel_service")

security/selinux/include/flask.h

@@ -52,6 +52,7 @@
 #define SECCLASS_MEMPROTECT                              61
 #define SECCLASS_PEER                                    68
 #define SECCLASS_CAPABILITY2                             69
+#define SECCLASS_KERNEL_SERVICE                          74
 
 /*
  * Security identifier indices for initial entities