audit: move calcs after alloc and check when logging set loginuid

BugLink: https://bugs.launchpad.net/bugs/1775477

commit 76a658c2 upstream.

Move the calculations of values after the allocation in case the
allocation fails.  This avoids wasting effort in the rare case that it
fails, but more importantly saves us extra logic to release the tty
ref.
Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>
Cc: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

kernel/auditsc.c

@@ -1981,14 +1981,15 @@ static void audit_log_set_loginuid(kuid_t koldloginuid, kuid_t kloginuid,
 	if (!audit_enabled)
 		return;
 
+	ab = audit_log_start(NULL, GFP_KERNEL, AUDIT_LOGIN);
+	if (!ab)
+		return;
+
 	uid = from_kuid(&init_user_ns, task_uid(current));
 	oldloginuid = from_kuid(&init_user_ns, koldloginuid);
 	loginuid = from_kuid(&init_user_ns, kloginuid),
 	tty = audit_get_tty(current);
 
-	ab = audit_log_start(NULL, GFP_KERNEL, AUDIT_LOGIN);
-	if (!ab)
-		return;
 	audit_log_format(ab, "pid=%d uid=%u", task_pid_nr(current), uid);
 	audit_log_task_context(ab);
 	audit_log_format(ab, " old-auid=%u auid=%u tty=%s old-ses=%u ses=%u res=%d",