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, 171 insertions, 1 deletions

diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
index 5a0536bddc63..4f233d9960e7 100644
--- a/security/selinux/ss/services.c
+++ b/security/selinux/ss/services.c
@@ -88,6 +88,11 @@ static u32 latest_granting;
 static int context_struct_to_string(struct context *context, char **scontext,
                                     u32 *scontext_len);
 
+static int context_struct_compute_av(struct context *scontext,
+                                     struct context *tcontext,
+                                     u16 tclass,
+                                     u32 requested,
+                                     struct av_decision *avd);
 /*
  * Return the boolean value of a constraint expression
  * when it is applied to the specified source and target
@@ -274,6 +279,100 @@ mls_ops:
 }
 
 /*
+ * security_boundary_permission - drops violated permissions
+ * on boundary constraint.
+ */
+static void type_attribute_bounds_av(struct context *scontext,
+                                     struct context *tcontext,
+                                     u16 tclass,
+                                     u32 requested,
+                                     struct av_decision *avd)
+{
+        struct context lo_scontext;
+        struct context lo_tcontext;
+        struct av_decision lo_avd;
+        struct type_datum *source
+                = policydb.type_val_to_struct[scontext->type - 1];
+        struct type_datum *target
+                = policydb.type_val_to_struct[tcontext->type - 1];
+        u32 masked = 0;
+
+        if (source->bounds) {
+                memset(&lo_avd, 0, sizeof(lo_avd));
+
+                memcpy(&lo_scontext, scontext, sizeof(lo_scontext));
+                lo_scontext.type = source->bounds;
+
+                context_struct_compute_av(&lo_scontext,
+                                          tcontext,
+                                          tclass,
+                                          requested,
+                                          &lo_avd);
+                if ((lo_avd.allowed & avd->allowed) == avd->allowed)
+                        return;         /* no masked permission */
+                masked = ~lo_avd.allowed & avd->allowed;
+        }
+
+        if (target->bounds) {
+                memset(&lo_avd, 0, sizeof(lo_avd));
+
+                memcpy(&lo_tcontext, tcontext, sizeof(lo_tcontext));
+                lo_tcontext.type = target->bounds;
+
+                context_struct_compute_av(scontext,
+                                          &lo_tcontext,
+                                          tclass,
+                                          requested,
+                                          &lo_avd);
+                if ((lo_avd.allowed & avd->allowed) == avd->allowed)
+                        return;         /* no masked permission */
+                masked = ~lo_avd.allowed & avd->allowed;
+        }
+
+        if (source->bounds && target->bounds) {
+                memset(&lo_avd, 0, sizeof(lo_avd));
+                /*
+                 * lo_scontext and lo_tcontext are already
+                 * set up.
+                 */
+
+                context_struct_compute_av(&lo_scontext,
+                                          &lo_tcontext,
+                                          tclass,
+                                          requested,
+                                          &lo_avd);
+                if ((lo_avd.allowed & avd->allowed) == avd->allowed)
+                        return;         /* no masked permission */
+                masked = ~lo_avd.allowed & avd->allowed;
+        }
+
+        if (masked) {
+                struct audit_buffer *ab;
+                char *stype_name
+                        = policydb.p_type_val_to_name[source->value - 1];
+                char *ttype_name
+                        = policydb.p_type_val_to_name[target->value - 1];
+                char *tclass_name
+                        = policydb.p_class_val_to_name[tclass - 1];
+
+                /* mask violated permissions */
+                avd->allowed &= ~masked;
+
+                /* notice to userspace via audit message */
+                ab = audit_log_start(current->audit_context,
+                                     GFP_ATOMIC, AUDIT_SELINUX_ERR);
+                if (!ab)
+                        return;
+
+                audit_log_format(ab, "av boundary violation: "
+                                 "source=%s target=%s tclass=%s",
+                                 stype_name, ttype_name, tclass_name);
+                avc_dump_av(ab, tclass, masked);
+                audit_log_end(ab);
+        }
+}
+
+/*
  * Compute access vectors based on a context structure pair for
  * the permissions in a particular class.
  */
@@ -404,6 +503,14 @@ static int context_struct_compute_av(struct context *scontext,
                                                         PROCESS__DYNTRANSITION);
         }
 
+        /*
+         * If the given source and target types have boundary
+         * constraint, lazy checks have to mask any violated
+         * permission and notice it to userspace via audit.
+         */
+        type_attribute_bounds_av(scontext, tcontext,
+                                 tclass, requested, avd);
+
         return 0;
 
 inval_class:
@@ -549,6 +656,69 @@ out:
         return rc;
 }
 
+/*
+ * security_bounded_transition - check whether the given
+ * transition is directed to bounded, or not.
+ * It returns 0, if @newsid is bounded by @oldsid.
+ * Otherwise, it returns error code.
+ *
+ * @oldsid : current security identifier
+ * @newsid : destinated security identifier
+ */
+int security_bounded_transition(u32 old_sid, u32 new_sid)
+{
+        struct context *old_context, *new_context;
+        struct type_datum *type;
+        int index;
+        int rc = -EINVAL;
+
+        read_lock(&policy_rwlock);
+
+        old_context = sidtab_search(&sidtab, old_sid);
+        if (!old_context) {
+                printk(KERN_ERR "SELinux: %s: unrecognized SID %u\n",
+                       __func__, old_sid);
+                goto out;
+        }
+
+        new_context = sidtab_search(&sidtab, new_sid);
+        if (!new_context) {
+                printk(KERN_ERR "SELinux: %s: unrecognized SID %u\n",
+                       __func__, new_sid);
+                goto out;
+        }
+
+        /* type/domain unchaned */
+        if (old_context->type == new_context->type) {
+                rc = 0;
+                goto out;
+        }
+
+        index = new_context->type;
+        while (true) {
+                type = policydb.type_val_to_struct[index - 1];
+                BUG_ON(!type);
+
+                /* not bounded anymore */
+                if (!type->bounds) {
+                        rc = -EPERM;
+                        break;
+                }
+
+                /* @newsid is bounded by @oldsid */
+                if (type->bounds == old_context->type) {
+                        rc = 0;
+                        break;
+                }
+                index = type->bounds;
+        }
+out:
+        read_unlock(&policy_rwlock);
+
+        return rc;
+}
+
+
 /**
  * security_compute_av - Compute access vector decisions.
  * @ssid: source security identifier
@@ -794,7 +964,7 @@ static int string_to_context_struct(struct policydb *pol,
         *p++ = 0;
 
         typdatum = hashtab_search(pol->p_types.table, scontextp);
-        if (!typdatum)
+        if (!typdatum || typdatum->attribute)
                 goto out;
 
         ctx->type = typdatum->value;