[PATCH 2/4] tty: n_gsm: Prevent a potential use after free

From: Dan Carpenter
Date: Thu Apr 26 2018 - 01:53:56 EST

We're freeing the gsm->dlci[] array elements but leaving the freed
pointers hanging around.

My concern here is if we use the ioctl to change the config, it triggers
a restart in gsmld_config(). In that case, we would only reset the
first ->dlci[0] element and not the others so it does look to me like a
possible use after free.

Reported-by: Sun Peng <sun_peng@xxxxxxxxxxxxx>
Signed-off-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index cc7f68814200..1f2fd9e76fe0 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -2075,9 +2075,11 @@ static void gsm_cleanup_mux(struct gsm_mux *gsm)

/* Free up any link layer users */
- for (i = 0; i < NUM_DLCI; i++)
+ for (i = 0; i < NUM_DLCI; i++) {
if (gsm->dlci[i])
+ gsm->dlci[i] = NULL;
+ }
/* Now wipe the queues */
list_for_each_entry_safe(txq, ntxq, &gsm->tx_list, list)