[PATCH 1/4] tty: n_gsm: add some locking around gsm_mux[]

[Dan Carpenter <dan.carpenter@oracle.com>]

We should take "gsm_mux_lock" when we access gsm_mux[].

Reported-by: Sun Peng <sun_peng@topsec.com.cn>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index 3b3e1f6632d7..cc7f68814200 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -2898,18 +2898,22 @@ static int gsmtty_install(struct tty_driver *driver, struct tty_struct *tty)
 	bool alloc = false;
 	int ret;
 
-	line = line & 0x3F;
 
 	if (mux >= MAX_MUX)
 		return -ENXIO;
-	/* FIXME: we need to lock gsm_mux for lifetimes of ttys eventually */
-	if (gsm_mux[mux] == NULL)
-		return -EUNATCH;
+
+	line = line & 0x3F;
 	if (line == 0 || line > 61)	/* 62/63 reserved */
 		return -ECHRNG;
+
+	spin_lock(&gsm_mux_lock);
 	gsm = gsm_mux[mux];
+	spin_unlock(&gsm_mux_lock);
+	if (!gsm)
+		return -EUNATCH;
 	if (gsm->dead)
 		return -EL2HLT;
+
 	/* If DLCI 0 is not yet fully open return an error.
 	This is ok from a locking
 	perspective as we don't have to worry about this