sulogin: improve support for locked root account

Some installations and distributions don't use a root account password
for security reasons and use sudo instead. In that case, asking for the
password makes no sense, and it is not even considered as valid as it's just
"*" or "!".

In these cases --force is required to just start a root shell and no
ask for password.

I don't think it's a good idea to automatically start root shell when
locked account is detected. It's possible that the machine is on
public place and for example Ubuntu uses root account disabled by
default (and also Fedora when installed by yum/dnf without anaconda).

The --force option forces admins to think about it...

The distro maintainers can also use --force in their initscripts or
systemd emergency.service if they believe that promiscuous setting is
the right thing for the distro.

Addresses: https://bugs.debian.org/326678
Signed-off-by: Karel Zak <kzak@redhat.com>

login-utils/sulogin.8

@@ -33,6 +33,8 @@ Give root password for system maintenance
 .br
 (or type Control\-D for normal startup):
 .PP
+If the root account is locked and --force is specified, no password is required.
+.PP
 .B sulogin
 will be connected to the current terminal, or to the optional \fItty\fR device that
 can be specified on the command line (typically
@@ -48,9 +50,11 @@ fails, then examine
 .I /etc/passwd
 and
 .I /etc/shadow
-to get the password.  If these files are damaged or nonexistent,
+to get the password.  If these files are damaged or nonexistent, or when
+root account is locked by '!' or '*' at the begin of the password then
 .B sulogin
-will start a root shell without asking for a password.
+will \fBstart a root shell without asking for a password\fP.
+.PP
 .IP
 Only use the
 .B \-e

login-utils/sulogin.c

@@ -81,6 +81,13 @@ static volatile sig_atomic_t sigchild;
 # define IUCLC		0
 #endif
 
+static int locked_account_password(const char *passwd)
+{
+	if (passwd && (*passwd == '*' || *passwd == '!'))
+		return 1;
+	return 0;
+}
+
 #ifdef TIOCGLCKTRMIOS
 /*
  * For the case plymouth is found on this system
@@ -485,7 +492,6 @@ static struct passwd *getrootpwent(int try_manually)
 		p = line;
 		break;
 	}
-
 	fclose(fp);
 
 	/*
@@ -522,7 +528,8 @@ static struct passwd *getrootpwent(int try_manually)
 		warnx(_("%s: no entry for root"), _PATH_SHADOW_PASSWD);
 		*pwd.pw_passwd = '\0';
 	}
-	if (!valid(pwd.pw_passwd)) {
+	/* locked accont passwords are valid too */
+	if (!locked_account_password(pwd.pw_passwd) && !valid(pwd.pw_passwd)) {
 		warnx(_("%s: root password garbled"), _PATH_SHADOW_PASSWD);
 		*pwd.pw_passwd = '\0';
 	}
@@ -532,7 +539,7 @@ static struct passwd *getrootpwent(int try_manually)
 /*
  * Ask by prompt for the password.
  */
-static void doprompt(const char *crypted, struct console *con)
+static void doprompt(const char *crypted, struct console *con, int deny)
 {
 	struct termios tty;
 
@@ -549,18 +556,25 @@ static void doprompt(const char *crypted, struct console *con)
 		if  ((con->file = fdopen(con->fd, "r+")) == (FILE*)0)
 			goto err;
 	}
+
+	if (deny)
+		fprintf(con->file, _("\nCannot open access to console, the root account is locked.\n"
+				     "See sulogin(8) man page for more details.\n\n"
+				     "Press Enter to continue.\n"));
+	else {
 #if defined(USE_ONELINE)
-	if (crypted[0])
-		fprintf(con->file, _("Give root password for login: "));
-	else
-		fprintf(con->file, _("Press Enter for login: "));
+		if (crypted[0] && !locked_account_password(crypted))
+			fprintf(con->file, _("Give root password for login: "));
+		else
+			fprintf(con->file, _("Press Enter for login: "));
 #else
-	if (crypted[0])
-		fprintf(con->file, _("Give root password for maintenance\n"));
-	else
-		fprintf(con->file, _("Press Enter for maintenance"));
-	fprintf(con->file, _("(or press Control-D to continue): "));
+		if (crypted[0] && !locked_account_password(crypted))
+			fprintf(con->file, _("Give root password for maintenance\n"));
+		else
+			fprintf(con->file, _("Press Enter for maintenance\n"));
+		fprintf(con->file, _("(or press Control-D to continue): "));
 #endif
+	}
 	fflush(con->file);
 err:
 	if (con->flags & CON_SERIAL)
@@ -980,6 +994,7 @@ int main(int argc, char **argv)
 		goto nofork;
 	}
 
+
 	mask_signal(SIGCHLD, chld_handler, &saved_sigchld);
 	do {
 		con = list_entry(ptr, struct console, entry);
@@ -996,12 +1011,17 @@ int main(int argc, char **argv)
 				const char *passwd = pwd->pw_passwd;
 				const char *answer;
 				int failed = 0, doshell = 0;
+				int deny = !opt_e && locked_account_password(pwd->pw_passwd);
+
+				doprompt(passwd, con, deny);
 
-				doprompt(passwd, con);
 				if ((answer = getpasswd(con)) == NULL)
 					break;
+				if (deny)
+					exit(EXIT_FAILURE);
 
-				if (passwd[0] == '\0')
+				/* no password or locked account */
+				if (!passwd[0] || locked_account_password(passwd))
 					doshell++;
 				else {
 					const char *cryptbuf;