4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Gaurav Kohli <gko...@codeaurora.org>

commit b027e2298bd588d6fa36ed2eda97447fb3eac078 upstream.

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: Gaurav Kohli <gko...@codeaurora.org>
Reviewed-by: Alan Cox <a...@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>

---
 drivers/tty/tty_io.c    |    8 +++++++-
 drivers/tty/tty_ldisc.c |    4 ++--
 include/linux/tty.h     |    2 ++
 3 files changed, 11 insertions(+), 3 deletions(-)

--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -1543,6 +1543,9 @@ struct tty_struct *tty_init_dev(struct t
                        "%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;
 
        /*
@@ -1553,6 +1556,7 @@ struct tty_struct *tty_init_dev(struct t
        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;
 
@@ -1565,9 +1569,11 @@ err_module_put:
 
        /* 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);
 }
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -336,7 +336,7 @@ static inline void __tty_ldisc_unlock(st
        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;
 
@@ -347,7 +347,7 @@ static int tty_ldisc_lock(struct tty_str
        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);
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -394,6 +394,8 @@ extern struct tty_struct *get_current_tt
 /* tty_io.c */
 extern int __init tty_init(void);
 extern const char *tty_name(const struct tty_struct *tty);
+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 console_init(void)
 { }


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