SELinux: add boundary support and thread context assignment

The purpose of this patch is to assign per-thread security context
under a constraint. It enables multi-threaded server application
to kick a request handler with its fair security context, and
helps some of userspace object managers to handle user's request.

When we assign a per-thread security context, it must not have wider
permissions than the original one. Because a multi-threaded process
shares a single local memory, an arbitary per-thread security context
also means another thread can easily refer violated information.

The constraint on a per-thread security context requires a new domain
has to be equal or weaker than its original one, when it tries to assign
a per-thread security context.

Bounds relationship between two types is a way to ensure a domain can
never have wider permission than its bounds. We can define it in two
explicit or implicit ways.

The first way is using new TYPEBOUNDS statement. It enables to define
a boundary of types explicitly. The other one expand the concept of
existing named based hierarchy. If we defines a type with "." separated
name like "httpd_t.php", toolchain implicitly set its bounds on "httpd_t".

This feature requires a new policy version.
The 24th version (POLICYDB_VERSION_BOUNDARY) enables to ship them into
kernel space, and the following patch enables to handle it.

Signed-off-by: KaiGai Kohei <kaigai@ak.jp.nec.com>
Acked-by:  Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: James Morris <jmorris@namei.org>

security/selinux/avc.c

@@ -136,7 +136,7 @@ static inline int avc_hash(u32 ssid, u32 tsid, u16 tclass)
  * @tclass: target security class
  * @av: access vector
  */
-static void avc_dump_av(struct audit_buffer *ab, u16 tclass, u32 av)
+void avc_dump_av(struct audit_buffer *ab, u16 tclass, u32 av)
 {
 	const char **common_pts = NULL;
 	u32 common_base = 0;

security/selinux/hooks.c

@@ -5226,20 +5226,29 @@ static int selinux_setprocattr(struct task_struct *p,
 
 		if (sid == 0)
 			return -EINVAL;
-
-		/* Only allow single threaded processes to change context */
+		/*
+		 * SELinux allows to change context in the following case only.
+		 *  - Single threaded processes.
+		 *  - Multi threaded processes intend to change its context into
+		 *    more restricted domain (defined by TYPEBOUNDS statement).
+		 */
 		if (atomic_read(&p->mm->mm_users) != 1) {
 			struct task_struct *g, *t;
 			struct mm_struct *mm = p->mm;
 			read_lock(&tasklist_lock);
 			do_each_thread(g, t) {
 				if (t->mm == mm && t != p) {
 					read_unlock(&tasklist_lock);
-					return -EPERM;
+					error = security_bounded_transition(tsec->sid, sid);
+					if (!error)
+						goto boundary_ok;
+
+					return error;
 				}
 			} while_each_thread(g, t);
 			read_unlock(&tasklist_lock);
 		}
+boundary_ok:
 
 		/* Check permissions for the transition. */
 		error = avc_has_perm(tsec->sid, sid, SECCLASS_PROCESS,

security/selinux/include/avc.h

@@ -12,6 +12,7 @@
 #include <linux/kdev_t.h>
 #include <linux/spinlock.h>
 #include <linux/init.h>
+#include <linux/audit.h>
 #include <linux/in6.h>
 #include <linux/path.h>
 #include <asm/system.h>
@@ -126,6 +127,9 @@ int avc_add_callback(int (*callback)(u32 event, u32 ssid, u32 tsid,
 		     u32 events, u32 ssid, u32 tsid,
 		     u16 tclass, u32 perms);
 
+/* Shows permission in human readable form */
+void avc_dump_av(struct audit_buffer *ab, u16 tclass, u32 av);
+
 /* Exported to selinuxfs */
 int avc_get_hash_stats(char *page);
 extern unsigned int avc_cache_threshold;

security/selinux/include/security.h

@@ -27,13 +27,14 @@
 #define POLICYDB_VERSION_RANGETRANS	21
 #define POLICYDB_VERSION_POLCAP		22
 #define POLICYDB_VERSION_PERMISSIVE	23
+#define POLICYDB_VERSION_BOUNDARY	24
 
 /* Range of policy versions we understand*/
 #define POLICYDB_VERSION_MIN   POLICYDB_VERSION_BASE
 #ifdef CONFIG_SECURITY_SELINUX_POLICYDB_VERSION_MAX
 #define POLICYDB_VERSION_MAX	CONFIG_SECURITY_SELINUX_POLICYDB_VERSION_MAX_VALUE
 #else
-#define POLICYDB_VERSION_MAX	POLICYDB_VERSION_PERMISSIVE
+#define POLICYDB_VERSION_MAX	POLICYDB_VERSION_BOUNDARY
 #endif
 
 #define CONTEXT_MNT	0x01
@@ -62,6 +63,16 @@ enum {
 extern int selinux_policycap_netpeer;
 extern int selinux_policycap_openperm;
 
+/*
+ * type_datum properties
+ * available at the kernel policy version >= POLICYDB_VERSION_BOUNDARY
+ */
+#define TYPEDATUM_PROPERTY_PRIMARY	0x0001
+#define TYPEDATUM_PROPERTY_ATTRIBUTE	0x0002
+
+/* limitation of boundary depth  */
+#define POLICYDB_BOUNDS_MAXDEPTH	4
+
 int security_load_policy(void *data, size_t len);
 
 int security_policycap_supported(unsigned int req_cap);
@@ -117,6 +128,8 @@ int security_node_sid(u16 domain, void *addr, u32 addrlen,
 int security_validate_transition(u32 oldsid, u32 newsid, u32 tasksid,
 				 u16 tclass);
 
+int security_bounded_transition(u32 oldsid, u32 newsid);
+
 int security_sid_mls_copy(u32 sid, u32 mls_sid, u32 *new_sid);
 
 int security_net_peersid_resolve(u32 nlbl_sid, u32 nlbl_type,