tty: n_gsm: Modify gsmtty driver register method when config requester

jira VULN-676
cve-pre CVE-2023-6546
commit-author Zhenguo Zhao <Zhenguo.Zhao1@unisoc.com>
commit 5b87686

As requester,because n_gsm has no uevent report for application,the
application can't know dlci connect or disconnect.

The application will control every dlcl dev by uevent,when application
receive gsmtty0 dev remove uevent,it will close mux function,and change to
normal mode.

Example:

Before modify:

gsmld receive DLC0 DISC,no event report to application
gsmld receive DLC1 SABM,no event report to application
gsmld receive DLC1 DISC,no event report to application

After modify:

Receive DLC0 DISC,report "/devices/virtual/tty/gsmtty0" remove uevent

Receive DLC1 SABM,report "/devices/virtual/tty/gsmtty1" add uevent
Receive DLC1 DISC,report "/devices/virtual/tty/gsmtty1" remove uevent

	Signed-off-by: Zhenguo Zhao <Zhenguo.Zhao1@unisoc.com>
Link: https://lore.kernel.org/r/1629461872-26965-7-git-send-email-zhenguo6858@gmail.com
	Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 5b87686)
	Signed-off-by: David Gomez <dgomez@ciq.com>

Author: dvdgomez

drivers/tty/n_gsm.c

@@ -1429,6 +1429,8 @@ static void gsm_dlci_close(struct gsm_dlci *dlci)
 		kfifo_reset(&dlci->fifo);
 	} else
 		dlci->gsm->dead = true;
+	/* Unregister gsmtty driver,report gsmtty dev remove uevent for user */
+	tty_unregister_device(gsm_tty_driver, dlci->addr);
 	wake_up(&dlci->gsm->event);
 	/* A DLCI 0 close is a MUX termination so we need to kick that
 	   back to userspace somehow */
@@ -1450,6 +1452,8 @@ static void gsm_dlci_open(struct gsm_dlci *dlci)
 	dlci->state = DLCI_OPEN;
 	if (debug & 8)
 		pr_debug("DLCI %d goes open.\n", dlci->addr);
+	/* Register gsmtty driver,report gsmtty dev add uevent for user */
+	tty_register_device(gsm_tty_driver, dlci->addr, NULL);
 	wake_up(&dlci->gsm->event);
 }
 
@@ -2384,17 +2388,19 @@ static int gsmld_attach_gsm(struct tty_struct *tty, struct gsm_mux *gsm)
 	else {
 		/* Don't register device 0 - this is the control channel and not
 		   a usable tty interface */
-		base = mux_num_to_base(gsm); /* Base for this MUX */
-		for (i = 1; i < NUM_DLCI; i++) {
-			struct device *dev;
+		if (gsm->initiator) {
+			base = mux_num_to_base(gsm); /* Base for this MUX */
+			for (i = 1; i < NUM_DLCI; i++) {
+				struct device *dev;
 
-			dev = tty_register_device(gsm_tty_driver,
+				dev = tty_register_device(gsm_tty_driver,
 							base + i, NULL);
-			if (IS_ERR(dev)) {
-				for (i--; i >= 1; i--)
-					tty_unregister_device(gsm_tty_driver,
-								base + i);
-				return PTR_ERR(dev);
+				if (IS_ERR(dev)) {
+					for (i--; i >= 1; i--)
+						tty_unregister_device(gsm_tty_driver,
+									base + i);
+					return PTR_ERR(dev);
+				}
 			}
 		}
 	}
@@ -2416,8 +2422,10 @@ static void gsmld_detach_gsm(struct tty_struct *tty, struct gsm_mux *gsm)
 	int i;
 
 	WARN_ON(tty != gsm->tty);
-	for (i = 1; i < NUM_DLCI; i++)
-		tty_unregister_device(gsm_tty_driver, base + i);
+	if (gsm->initiator) {
+		for (i = 1; i < NUM_DLCI; i++)
+			tty_unregister_device(gsm_tty_driver, base + i);
+	}
 	gsm_cleanup_mux(gsm);
 	tty_kref_put(gsm->tty);
 	gsm->tty = NULL;