The patch below does not apply to the 4.4-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From b027e2298bd588d6fa36ed2eda97447fb3eac078 Mon Sep 17 00:00:00 2001

From: Gaurav Kohli gkohli@codeaurora.org Date: Tue, 23 Jan 2018 13:16:34 +0530 Subject: [PATCH] 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: Gaurav Kohli gkohli@codeaurora.org Reviewed-by: Alan Cox alan@linux.intel.com Cc: stable stable@vger.kernel.org

Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c index a6ca634411e1..688bd25aa6b0 100644 --- a/drivers/tty/tty_io.c +++ b/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); } diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c index 24ec5c7e6b20..4e7946c0484b 100644 --- a/drivers/tty/tty_ldisc.c +++ b/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); diff --git a/include/linux/tty.h b/include/linux/tty.h index 7ac8ba208b1f..0a6c71e0ad01 100644 --- a/include/linux/tty.h +++ b/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) { }