agetty: fix chown(3), chmod(3) and open(3) race

Signed-off-by: Sami Kerola <kerolasa@iki.fi>

diff --git a/term-utils/agetty.c b/term-utils/agetty.c
index 0237b79d9eb4de04549b6600ed36a42636390907..a9c87854711bcfad07aaa3961c287f626c7d87f6 100644 (file)
--- a/term-utils/agetty.c
+++ b/term-utils/agetty.c
@@ -1004,23 +1004,23 @@ static void open_tty(char *tty, struct termios *tp, struct options *op)
                     (int)sizeof(buf)) || (len < 0))
                        log_err(_("/dev/%s: cannot open as standard input: %m"), tty);
 
+               /* Open the tty as standard input. */
+               if ((fd = open(buf, O_RDWR|O_NOCTTY|O_NONBLOCK, 0)) < 0)
+                       log_err(_("/dev/%s: cannot open as standard input: %m"), tty);
+
                /*
                 * There is always a race between this reset and the call to
                 * vhangup() that s.o. can use to get access to your tty.
                 * Linux login(1) will change tty permissions. Use root owner and group
                 * with permission -rw------- for the period between getty and login.
                 */
-               if (chown(buf, 0, gid) || chmod(buf, (gid ? 0620 : 0600))) {
+               if (fchown(fd, 0, gid) || fchmod(fd, (gid ? 0620 : 0600))) {
                        if (errno == EROFS)
                                log_warn("%s: %m", buf);
                        else
                                log_err("%s: %m", buf);
                }
 
-               /* Open the tty as standard input. */
-               if ((fd = open(buf, O_RDWR|O_NOCTTY|O_NONBLOCK, 0)) < 0)
-                       log_err(_("/dev/%s: cannot open as standard input: %m"), tty);
-
                /* Sanity checks... */
                if (fstat(fd, &st) < 0)
                        log_err("%s: %m", buf);