[PATCH] tty: n_gsm: fix memory leak in gsm_activate_mux

[Minu Jin <s9430939@naver.com>]

I found a memory leak in gsm_activate_mux() from the syzbot dashboard.
I have analyzed the root cause and created a fix.
Please test the following patch.

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git tty-next

---
diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index c13e050de83b..de3d30eac86e 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -2645,7 +2645,12 @@ static int gsm_dlci_config(struct gsm_dlci *dlci, struct gsm_dlci_config *dc, in

 static struct gsm_dlci *gsm_dlci_alloc(struct gsm_mux *gsm, int addr)
 {
-       struct gsm_dlci *dlci = kzalloc_obj(struct gsm_dlci, GFP_ATOMIC);
+       struct gsm_dlci *dlci;
+
+       if (gsm->dlci[addr])
+               return gsm->dlci[addr];
+
+       dlci = kzalloc_obj(struct gsm_dlci, GFP_ATOMIC);
        if (dlci == NULL)
                return NULL;
        spin_lock_init(&dlci->lock);
@@ -3196,8 +3201,10 @@ static int gsm_activate_mux(struct gsm_mux *gsm)
                gsm->receive = gsm1_receive;

        ret = gsm_register_devices(gsm_tty_driver, gsm->num);
-       if (ret)
+       if (ret) {
+               gsm_dlci_free(&dlci->port);
                return ret;
+       }

        gsm->has_devices = true;
        gsm->dead = false;              /* Tty opens are now permissible */