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From d9c09138d91566af49ef8179d9995c84294e2486 Mon Sep 17 00:00:00 2001
From: Kaarle Ritvanen <kaarle.ritvanen@datakunkku.fi>
Date: Thu, 5 Nov 2015 16:27:36 +0200
Subject: [PATCH 10/12] su: FEATURE_SU_NULLOK_SECURE

When this feature is enabled, blank passwords are not accepted by su
unless the user is on a secure TTY defined in /etc/securetty. This
resembles the default PAM configuration of some Linux distros which
specify the nullok_secure option for pam_unix.so.
---
 loginutils/su.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/loginutils/su.c b/loginutils/su.c
index 24ffbde86..086445243 100644
--- a/loginutils/su.c
+++ b/loginutils/su.c
@@ -24,6 +24,11 @@
 //config:	bool "Enable su to check user's shell to be listed in /etc/shells"
 //config:	depends on SU
 //config:	default y
+//config:config FEATURE_SU_NULLOK_SECURE
+//config:       bool "Disallow blank passwords from TTYs other than specified in /etc/securetty"
+//config:       depends on SU
+//config:       default n
+
 
 //applet:/* Needs to be run by root or be suid root - needs to change uid and gid: */
 //applet:IF_SU(APPLET(su, BB_DIR_BIN, BB_SUID_REQUIRE))
@@ -76,6 +81,7 @@ int su_main(int argc UNUSED_PARAM, char **argv)
 	struct passwd *pw;
 	uid_t cur_uid = getuid();
 	const char *tty;
+	int allow_blank = 1;
 #if ENABLE_FEATURE_UTMP
 	char user_buf[64];
 #endif
@@ -100,6 +106,12 @@ int su_main(int argc UNUSED_PARAM, char **argv)
 		argv++;
 	}
 
+	tty = xmalloc_ttyname(STDIN_FILENO);
+	if (!tty) tty = "none";
+	tty = skip_dev_pfx(tty);
+
+	if (ENABLE_FEATURE_SU_NULLOK_SECURE) allow_blank = check_securetty(tty);
+
 	if (ENABLE_FEATURE_SU_SYSLOG) {
 		/* The utmp entry (via getlogin) is probably the best way to
 		 * identify the user, especially if someone su's from a su-shell.
@@ -113,16 +125,12 @@ int su_main(int argc UNUSED_PARAM, char **argv)
 			pw = getpwuid(cur_uid);
 			old_user = pw ? xstrdup(pw->pw_name) : "";
 		}
-		tty = xmalloc_ttyname(2);
-		if (!tty) {
-			tty = "none";
-		}
 		openlog(applet_name, 0, LOG_AUTH);
 	}
 
 	pw = xgetpwnam(opt_username);
 
-	if (cur_uid == 0 || ask_and_check_password(pw) > 0) {
+	if (cur_uid == 0 || ask_and_check_password_extended(pw, 0, allow_blank, "Password: ") > 0) {
 		if (ENABLE_FEATURE_SU_SYSLOG)
 			syslog(LOG_NOTICE, "%c %s %s:%s",
 				'+', tty, old_user, opt_username);
-- 
2.11.0