audit: add tty field to LOGIN event

The tty field was missing from AUDIT_LOGIN events.

Refactor code to create a new function audit_get_tty(), using it to
replace the call in audit_log_task_info() and to add it to
audit_log_set_loginuid().  Lock and bump the kref to protect it, adding
audit_put_tty() alias to decrement it.

Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>
diff --git a/kernel/audit.c b/kernel/audit.c
index f52fbef..384374a 100644
--- a/kernel/audit.c
+++ b/kernel/audit.c
@@ -64,7 +64,6 @@
 #include <linux/security.h>
 #endif
 #include <linux/freezer.h>
-#include <linux/tty.h>
 #include <linux/pid_namespace.h>
 #include <net/netns/generic.h>
 
@@ -1871,21 +1870,14 @@
 {
 	const struct cred *cred;
 	char comm[sizeof(tsk->comm)];
-	char *tty;
+	struct tty_struct *tty;
 
 	if (!ab)
 		return;
 
 	/* tsk == current */
 	cred = current_cred();
-
-	spin_lock_irq(&tsk->sighand->siglock);
-	if (tsk->signal && tsk->signal->tty && tsk->signal->tty->name)
-		tty = tsk->signal->tty->name;
-	else
-		tty = "(none)";
-	spin_unlock_irq(&tsk->sighand->siglock);
-
+	tty = audit_get_tty(tsk);
 	audit_log_format(ab,
 			 " ppid=%d pid=%d auid=%u uid=%u gid=%u"
 			 " euid=%u suid=%u fsuid=%u"
@@ -1901,11 +1893,11 @@
 			 from_kgid(&init_user_ns, cred->egid),
 			 from_kgid(&init_user_ns, cred->sgid),
 			 from_kgid(&init_user_ns, cred->fsgid),
-			 tty, audit_get_sessionid(tsk));
-
+			 tty ? tty_name(tty) : "(none)",
+			 audit_get_sessionid(tsk));
+	audit_put_tty(tty);
 	audit_log_format(ab, " comm=");
 	audit_log_untrustedstring(ab, get_task_comm(comm, tsk));
-
 	audit_log_d_path_exe(ab, tsk->mm);
 	audit_log_task_context(ab);
 }