n_gsm: race between ld close and gsmtty open

ttyA has ld associated to n_gsm, when ttyA is closing, it triggers
to release gsmttyB's ld data dlci[B], then race would happen if gsmttyB
is opening in parallel.

(Note: This patch set differs from previous set in that it uses mutex
instead of spin lock to avoid race, so that it avoids sleeping in automic
context)

Here are race cases we found recently in test:

CASE #1
====================================================================
releasing dlci[B] race with gsmtty_install(gsmttyB), then panic
in gsmtty_open(gsmttyB), as below:

 tty_release(ttyA)                  tty_open(gsmttyB)
     |                                   |
   -----                           gsmtty_install(gsmttyB)
     |                                   |
   -----                    gsm_dlci_alloc(gsmttyB) => alloc dlci[B]
 tty_ldisc_release(ttyA)               -----
     |                                   |
 gsm_dlci_release(dlci[B])             -----
     |                                   |
 gsm_dlci_free(dlci[B])                -----
     |                                   |
   -----                           gsmtty_open(gsmttyB)

 gsmtty_open()
 {
     struct gsm_dlci *dlci = tty->driver_data; => here it uses dlci[B]
     ...
 }

 In gsmtty_open(gsmttyA), it uses dlci[B] which was release, so hit a panic.
=====================================================================

CASE #2
=====================================================================
releasing dlci[0] race with gsmtty_install(gsmttyB), then panic
in gsmtty_open(), as below:

 tty_release(ttyA)                  tty_open(gsmttyB)
     |                                   |
   -----                           gsmtty_install(gsmttyB)
     |                                   |
   -----                    gsm_dlci_alloc(gsmttyB) => alloc dlci[B]
     |                                   |
   -----                         gsmtty_open(gsmttyB) fail
     |                                   |
   -----                           tty_release(gsmttyB)
     |                                   |
   -----                           gsmtty_close(gsmttyB)
     |                                   |
   -----                        gsmtty_detach_dlci(dlci[B])
     |                                   |
   -----                             dlci_put(dlci[B])
     |                                   |
 tty_ldisc_release(ttyA)               -----
     |                                   |
 gsm_dlci_release(dlci[0])             -----
     |                                   |
 gsm_dlci_free(dlci[0])                -----
     |                                   |
   -----                             dlci_put(dlci[0])

 In gsmtty_detach_dlci(dlci[B]), it tries to use dlci[0] which was released,
 then hit panic.
=====================================================================

IMHO, n_gsm tty operations would refer released ldisc,  as long as
gsm_dlci_release() has chance to release ldisc data when some gsmtty operations
are ongoing..

This patch is try to avoid it by:

1) in n_gsm driver, use a global gsm mutex lock to avoid gsm_dlci_release() run in
parallel with gsmtty_install();

2) Increase dlci's ref count in gsmtty_install() instead of in gsmtty_open(), the
purpose is to prevent gsm_dlci_release() releasing dlci after gsmtty_install()
allocats dlci but before gsmtty_open increases dlci's ref count;

3) Decrease dlci's ref count in gsmtty_remove(), a tty framework API, this is the
opposite process of step 2).

Signed-off-by: Chao Bi <chao.bi@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_gsm.c

@@ -194,6 +194,7 @@ struct gsm_control {
 struct gsm_mux {
 	struct tty_struct *tty;		/* The tty our ldisc is bound to */
 	spinlock_t lock;
+	struct mutex mutex;
 	unsigned int num;
 	struct kref ref;
 
@@ -2054,9 +2055,11 @@ void gsm_cleanup_mux(struct gsm_mux *gsm)
 					dlci->state == DLCI_CLOSED);
 	}
 	/* Free up any link layer users */
+	mutex_lock(&gsm->mutex);
 	for (i = 0; i < NUM_DLCI; i++)
 		if (gsm->dlci[i])
 			gsm_dlci_release(gsm->dlci[i]);
+	mutex_unlock(&gsm->mutex);
 	/* Now wipe the queues */
 	list_for_each_entry_safe(txq, ntxq, &gsm->tx_list, list)
 		kfree(txq);
@@ -2170,6 +2173,7 @@ struct gsm_mux *gsm_alloc_mux(void)
 		return NULL;
 	}
 	spin_lock_init(&gsm->lock);
+	mutex_init(&gsm->mutex);
 	kref_init(&gsm->ref);
 	INIT_LIST_HEAD(&gsm->tx_list);
 
@@ -2910,23 +2914,33 @@ static int gsmtty_install(struct tty_driver *driver, struct tty_struct *tty)
 	This is ok from a locking
 	perspective as we don't have to worry about this
 	if DLCI0 is lost */
-	if (gsm->dlci[0] && gsm->dlci[0]->state != DLCI_OPEN)
+	mutex_lock(&gsm->mutex);
+	if (gsm->dlci[0] && gsm->dlci[0]->state != DLCI_OPEN) {
+		mutex_unlock(&gsm->mutex);
 		return -EL2NSYNC;
+	}
 	dlci = gsm->dlci[line];
 	if (dlci == NULL) {
 		alloc = true;
 		dlci = gsm_dlci_alloc(gsm, line);
 	}
-	if (dlci == NULL)
+	if (dlci == NULL) {
+		mutex_unlock(&gsm->mutex);
 		return -ENOMEM;
+	}
 	ret = tty_port_install(&dlci->port, driver, tty);
 	if (ret) {
 		if (alloc)
 			dlci_put(dlci);
+		mutex_unlock(&gsm->mutex);
 		return ret;
 	}
 
+	dlci_get(dlci);
+	dlci_get(gsm->dlci[0]);
+	mux_get(gsm);
 	tty->driver_data = dlci;
+	mutex_unlock(&gsm->mutex);
 
 	return 0;
 }
@@ -2937,9 +2951,6 @@ static int gsmtty_open(struct tty_struct *tty, struct file *filp)
 	struct tty_port *port = &dlci->port;
 
 	port->count++;
-	dlci_get(dlci);
-	dlci_get(dlci->gsm->dlci[0]);
-	mux_get(dlci->gsm);
 	tty_port_tty_set(port, tty);
 
 	dlci->modem_rx = 0;
@@ -2966,7 +2977,7 @@ static void gsmtty_close(struct tty_struct *tty, struct file *filp)
 	mutex_unlock(&dlci->mutex);
 	gsm = dlci->gsm;
 	if (tty_port_close_start(&dlci->port, tty, filp) == 0)
-		goto out;
+		return;
 	gsm_dlci_begin_close(dlci);
 	if (test_bit(ASYNCB_INITIALIZED, &dlci->port.flags)) {
 		if (C_HUPCL(tty))
@@ -2974,10 +2985,7 @@ static void gsmtty_close(struct tty_struct *tty, struct file *filp)
 	}
 	tty_port_close_end(&dlci->port, tty);
 	tty_port_tty_set(&dlci->port, NULL);
-out:
-	dlci_put(dlci);
-	dlci_put(gsm->dlci[0]);
-	mux_put(gsm);
+	return;
 }
 
 static void gsmtty_hangup(struct tty_struct *tty)
@@ -3154,6 +3162,16 @@ static int gsmtty_break_ctl(struct tty_struct *tty, int state)
 	return gsmtty_modem_update(dlci, encode);
 }
 
+static void gsmtty_remove(struct tty_driver *driver, struct tty_struct *tty)
+{
+	struct gsm_dlci *dlci = tty->driver_data;
+	struct gsm_mux *gsm = dlci->gsm;
+
+	dlci_put(dlci);
+	dlci_put(gsm->dlci[0]);
+	mux_put(gsm);
+	driver->ttys[tty->index] = NULL;
+}
 
 /* Virtual ttys for the demux */
 static const struct tty_operations gsmtty_ops = {
@@ -3173,6 +3191,7 @@ static const struct tty_operations gsmtty_ops = {
 	.tiocmget		= gsmtty_tiocmget,
 	.tiocmset		= gsmtty_tiocmset,
 	.break_ctl		= gsmtty_break_ctl,
+	.remove			= gsmtty_remove,
 };