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>

1 files changed, 12 insertions, 3 deletions

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 6b5790bba8f9..89f446d86054 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -5226,8 +5226,12 @@ 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;
@@ -5235,11 +5239,16 @@ static int selinux_setprocattr(struct task_struct *p,
 			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,