tty: fix data race between tty_init_dev and flush of buf

Gaurav Kohli · gregkh · commit b027e2298bd5 · 2018-01-23T08:57:37.000+01:00
There can be a race, if receive_buf call comes before
tty initialization completes in n_tty_open and tty-&gt;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 &lt;gkohli@codeaurora.org&gt;
Reviewed-by: Alan Cox &lt;alan@linux.intel.com&gt;
Cc: stable &lt;stable@vger.kernel.org&gt;

Signed-off-by: Greg Kroah-Hartman &lt;gregkh@linuxfoundation.org&gt;
diff --git 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
@@ -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
@@ -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)
 { }

Original file line number	Diff line number	Diff line change
`@@ -337,7 +337,7 @@ static inline void __tty_ldisc_unlock(struct tty_struct *tty)`
`337`	`337`	`ldsem_up_write(&tty->ldisc_sem);`
`338`	`338`	`}`
`339`	`339`
`340`		`-static int tty_ldisc_lock(struct tty_struct *tty, unsigned long timeout)`
	`340`	`+int tty_ldisc_lock(struct tty_struct *tty, unsigned long timeout)`
`341`	`341`	`{`
`342`	`342`	`int ret;`
`343`	`343`
`@@ -348,7 +348,7 @@ static int tty_ldisc_lock(struct tty_struct *tty, unsigned long timeout)`
`348`	`348`	`return 0;`
`349`	`349`	`}`
`350`	`350`
`351`		`-static void tty_ldisc_unlock(struct tty_struct *tty)`
	`351`	`+void tty_ldisc_unlock(struct tty_struct *tty)`
`352`	`352`	`{`
`353`	`353`	`clear_bit(TTY_LDISC_HALTED, &tty->flags);`
`354`	`354`	`__tty_ldisc_unlock(tty);`