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Date:   Thu, 26 Apr 2018 08:53:21 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Sun Peng <sun_peng@...sec.com.cn>
Cc:     Jiri Slaby <jslaby@...e.com>, linux-kernel@...r.kernel.org,
        security@...nel.org, Tony Lindgren <tony@...mide.com>,
        Lars Poeschel <poeschel@...onage.de>,
        Sascha Hauer <s.hauer@...gutronix.de>
Subject: [PATCH 2/4] tty: n_gsm: Prevent a potential use after free

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@...sec.com.cn>
Signed-off-by: Dan Carpenter <dan.carpenter@...cle.com>

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 */
 	mutex_lock(&gsm->mutex);
-	for (i = 0; i < NUM_DLCI; i++)
+	for (i = 0; i < NUM_DLCI; i++) {
 		if (gsm->dlci[i])
 			gsm_dlci_release(gsm->dlci[i]);
+		gsm->dlci[i] = NULL;
+	}
 	mutex_unlock(&gsm->mutex);
 	/* Now wipe the queues */
 	list_for_each_entry_safe(txq, ntxq, &gsm->tx_list, list)

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