Store AppArmor label of bus during initialization

During dbus-daemon initialization, the AppArmor confinement context
should be stored for later use when checks are to be done on messages
to/from the bus itself.

AppArmor confinement contexts are documented in aa_getcon(2). They
contain a confinement string and a mode string. The confinement string
is typically the name of the AppArmor profile confining a given process.
The mode string gives the current enforcement mode of the process
confinement. For example, it may indicate that the confinement should be
enforced or it may indicate that the confinement should allow all
actions with the caveat that actions which would be denied should be
audited.

It is important to note that libapparmor mallocs a single buffer to
store the con and mode strings and separates them with a NUL terminator.
Because of this, only con should be freed.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=75113
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
[smcv: use BUS_SET_OOM]
[smcv: dbus_set_error doesn't need extra newlines]
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Tyler Hicks <tyhicks@canonical.com>

1 files changed, 86 insertions, 1 deletions

diff --git a/bus/apparmor.c b/bus/apparmor.c
index 645bb846..3b2be352 100644
--- a/bus/apparmor.c
+++ b/bus/apparmor.c
@@ -64,6 +64,67 @@ static AppArmorConfigMode apparmor_config_mode = APPARMOR_ENABLED;
 static int audit_fd = -1;
 #endif
 
+struct BusAppArmorConfinement
+{
+  int refcount; /* Reference count */
+
+  char *context; /* AppArmor confinement context (label) */
+  const char *mode; /* AppArmor confinement mode (freed by freeing *context) */
+};
+
+typedef struct BusAppArmorConfinement BusAppArmorConfinement;
+
+static BusAppArmorConfinement *bus_con = NULL;
+
+/**
+ * Callers of this function give up ownership of the *context and *mode
+ * pointers.
+ *
+ * Additionally, the responsibility of freeing *context and *mode becomes the
+ * responsibility of the bus_apparmor_confinement_unref() function. However, it
+ * does not free *mode because libapparmor's aa_getcon(), and libapparmor's
+ * other related functions, allocate a single buffer for *context and *mode and
+ * then separate the two char arrays with a NUL char. See the aa_getcon(2) man
+ * page for more details.
+ */
+static BusAppArmorConfinement*
+bus_apparmor_confinement_new (char *context, const char *mode)
+{
+  BusAppArmorConfinement *confinement;
+
+  confinement = dbus_new0 (BusAppArmorConfinement, 1);
+  if (confinement != NULL)
+    {
+      confinement->refcount = 1;
+      confinement->context = context;
+      confinement->mode = mode;
+    }
+
+  return confinement;
+}
+
+static void
+bus_apparmor_confinement_unref (BusAppArmorConfinement *confinement)
+{
+  if (!apparmor_enabled)
+    return;
+
+  _dbus_assert (confinement != NULL);
+  _dbus_assert (confinement->refcount > 0);
+
+  confinement->refcount -= 1;
+
+  if (confinement->refcount == 0)
+    {
+      /**
+       * Do not free confinement->mode, as libapparmor does a single malloc for
+       * both confinement->context and confinement->mode.
+       */
+      free (confinement->context);
+      dbus_free (confinement);
+    }
+}
+
 void
 bus_apparmor_audit_init (void)
 {
@@ -203,6 +264,8 @@ dbus_bool_t
 bus_apparmor_full_init (DBusError *error)
 {
 #ifdef HAVE_APPARMOR
+  char *context, *mode;
+
   if (apparmor_enabled)
     {
       if (apparmor_config_mode == APPARMOR_DISABLED)
@@ -210,13 +273,32 @@ bus_apparmor_full_init (DBusError *error)
           apparmor_enabled = FALSE;
           return TRUE;
         }
+
+      if (bus_con == NULL)
+        {
+          if (aa_getcon (&context, &mode) == -1)
+            {
+              dbus_set_error (error, DBUS_ERROR_FAILED,
+                              "Error getting AppArmor context of bus: %s",
+                              _dbus_strerror (errno));
+              return FALSE;
+            }
+
+          bus_con = bus_apparmor_confinement_new (context, mode);
+          if (bus_con == NULL)
+            {
+              BUS_SET_OOM (error);
+              free (context);
+              return FALSE;
+            }
+        }
     }
   else
     {
       if (apparmor_config_mode == APPARMOR_REQUIRED)
         {
           dbus_set_error (error, DBUS_ERROR_FAILED,
-                          "AppArmor mediation required but not present\n");
+                          "AppArmor mediation required but not present");
           return FALSE;
         }
       else if (apparmor_config_mode == APPARMOR_ENABLED)
@@ -238,6 +320,9 @@ bus_apparmor_shutdown (void)
 
   _dbus_verbose ("AppArmor shutdown\n");
 
+  bus_apparmor_confinement_unref (bus_con);
+  bus_con = NULL;
+
 #ifdef HAVE_LIBAUDIT
   audit_close (audit_fd);
 #endif /* HAVE_LIBAUDIT */