tty: fix data race between tty_init_dev and flush of buf

There can be a race, if receive_buf call comes before
tty initialization completes in n_tty_open and tty->disc_data
may be NULL.

CPU0					CPU1
----					----
 000|n_tty_receive_buf_common()   	n_tty_open()
-001|n_tty_receive_buf2()		tty_ldisc_open.isra.3()
-002|tty_ldisc_receive_buf(inline)	tty_ldisc_setup()

Using ldisc semaphore lock in tty_init_dev till disc_data
initializes completely.
Signed-off-by: NGaurav Kohli <gkohli@codeaurora.org>
Reviewed-by: NAlan Cox <alan@linux.intel.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty_io.c

@@ -1323,6 +1323,9 @@ struct tty_struct *tty_init_dev(struct tty_driver *driver, int idx)
 			"%s: %s driver does not set tty->port. This will crash the kernel later. Fix the driver!\n",
 			__func__, tty->driver->name);
 
+	retval = tty_ldisc_lock(tty, 5 * HZ);
+	if (retval)
+		goto err_release_lock;
 	tty->port->itty = tty;
 
 	/*
@@ -1333,6 +1336,7 @@ struct tty_struct *tty_init_dev(struct tty_driver *driver, int idx)
 	retval = tty_ldisc_setup(tty, tty->link);
 	if (retval)
 		goto err_release_tty;
+	tty_ldisc_unlock(tty);
 	/* Return the tty locked so that it cannot vanish under the caller */
 	return tty;
 
@@ -1345,9 +1349,11 @@ struct tty_struct *tty_init_dev(struct tty_driver *driver, int idx)
 
 	/* call the tty release_tty routine to clean out this slot */
 err_release_tty:
-	tty_unlock(tty);
+	tty_ldisc_unlock(tty);
 	tty_info_ratelimited(tty, "ldisc open failed (%d), clearing slot %d\n",
 			     retval, idx);
+err_release_lock:
+	tty_unlock(tty);
 	release_tty(tty, idx);
 	return ERR_PTR(retval);
 }

drivers/tty/tty_ldisc.c

@@ -337,7 +337,7 @@ static inline void __tty_ldisc_unlock(struct tty_struct *tty)
 	ldsem_up_write(&tty->ldisc_sem);
 }
 
-static int tty_ldisc_lock(struct tty_struct *tty, unsigned long timeout)
+int tty_ldisc_lock(struct tty_struct *tty, unsigned long timeout)
 {
 	int ret;
 
@@ -348,7 +348,7 @@ static int tty_ldisc_lock(struct tty_struct *tty, unsigned long timeout)
 	return 0;
 }
 
-static void tty_ldisc_unlock(struct tty_struct *tty)
+void tty_ldisc_unlock(struct tty_struct *tty)
 {
 	clear_bit(TTY_LDISC_HALTED, &tty->flags);
 	__tty_ldisc_unlock(tty);

include/linux/tty.h

@@ -405,6 +405,8 @@ extern const char *tty_name(const struct tty_struct *tty);
 extern struct tty_struct *tty_kopen(dev_t device);
 extern void tty_kclose(struct tty_struct *tty);
 extern int tty_dev_name_to_number(const char *name, dev_t *number);
+extern int tty_ldisc_lock(struct tty_struct *tty, unsigned long timeout);
+extern void tty_ldisc_unlock(struct tty_struct *tty);
 #else
 static inline void tty_kref_put(struct tty_struct *tty)
 { }