[PATCH 3.4 136/214] n_gsm: avoid accessing freed memory during CMD_FCOFF condition

[Greg Kroah-Hartman]

3.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Russ Gorby <russ.gorby@xxxxxxxxx>

commit b4338e1efc339986cf6c0a3652906e914a86e2d3 upstream.

gsm_data_kick was recently modified to allow messages on the
tx queue bound for DLCI0 to flow even during FCOFF conditions.
Unfortunately we introduced a bug discovered by code inspection
where subsequent list traversers can access freed memory if
the DLCI0 messages were not all at the head of the list.

Replaced singly linked tx list w/ a list_head and used
provided interfaces for traversing and deleting members.

Signed-off-by: Russ Gorby <russ.gorby@xxxxxxxxx>
Tested-by: Yin, Fengwei <fengwei.yin@xxxxxxxxx>
Signed-off-by: Alan Cox <alan@xxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
Cc: Rui Xiang <rui.xiang@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 drivers/tty/n_gsm.c |   40 +++++++++++++---------------------------
 1 file changed, 13 insertions(+), 27 deletions(-)

--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -108,7 +108,7 @@ struct gsm_mux_net {
  */
 
 struct gsm_msg {
-	struct gsm_msg *next;
+	struct list_head list;
 	u8 addr;		/* DLCI address + flags */
 	u8 ctrl;		/* Control byte + flags */
 	unsigned int len;	/* Length of data block (can be zero) */
@@ -245,8 +245,7 @@ struct gsm_mux {
 	unsigned int tx_bytes;		/* TX data outstanding */
 #define TX_THRESH_HI		8192
 #define TX_THRESH_LO		2048
-	struct gsm_msg *tx_head;	/* Pending data packets */
-	struct gsm_msg *tx_tail;
+	struct list_head tx_list;	/* Pending data packets */
 
 	/* Control messages */
 	struct timer_list t2_timer;	/* Retransmit timer for commands */
@@ -663,7 +662,7 @@ static struct gsm_msg *gsm_data_alloc(st
 	m->len = len;
 	m->addr = addr;
 	m->ctrl = ctrl;
-	m->next = NULL;
+	INIT_LIST_HEAD(&m->list);
 	return m;
 }
 
@@ -681,16 +680,13 @@ static struct gsm_msg *gsm_data_alloc(st
 
 static void gsm_data_kick(struct gsm_mux *gsm)
 {
-	struct gsm_msg *msg = gsm->tx_head;
-	struct gsm_msg *free_msg;
+	struct gsm_msg *msg, *nmsg;
 	int len;
 	int skip_sof = 0;
 
-	while (msg) {
-		if (gsm->constipated && msg->addr) {
-			msg = msg->next;
+	list_for_each_entry_safe(msg, nmsg, &gsm->tx_list, list) {
+		if (gsm->constipated && msg->addr)
 			continue;
-		}
 		if (gsm->encoding != 0) {
 			gsm->txframe[0] = GSM1_SOF;
 			len = gsm_stuff_frame(msg->data,
@@ -718,14 +714,9 @@ static void gsm_data_kick(struct gsm_mux
 		   burst */
 		skip_sof = 1;
 
-		if (gsm->tx_head == msg)
-			gsm->tx_head = msg->next;
-		free_msg = msg;
-		msg = msg->next;
-		kfree(free_msg);
+		list_del(&msg->list);
+		kfree(msg);
 	}
-	if (!gsm->tx_head)
-		gsm->tx_tail = NULL;
 }
 
 /**
@@ -774,11 +765,7 @@ static void __gsm_data_queue(struct gsm_
 	msg->data = dp;
 
 	/* Add to the actual output queue */
-	if (gsm->tx_tail)
-		gsm->tx_tail->next = msg;
-	else
-		gsm->tx_head = msg;
-	gsm->tx_tail = msg;
+	list_add_tail(&msg->list, &gsm->tx_list);
 	gsm->tx_bytes += msg->len;
 	gsm_data_kick(gsm);
 }
@@ -2052,7 +2039,7 @@ void gsm_cleanup_mux(struct gsm_mux *gsm
 {
 	int i;
 	struct gsm_dlci *dlci = gsm->dlci[0];
-	struct gsm_msg *txq;
+	struct gsm_msg *txq, *utxq;
 	struct gsm_control *gc;
 
 	gsm->dead = 1;
@@ -2087,11 +2074,9 @@ void gsm_cleanup_mux(struct gsm_mux *gsm
 		if (gsm->dlci[i])
 			gsm_dlci_release(gsm->dlci[i]);
 	/* Now wipe the queues */
-	for (txq = gsm->tx_head; txq != NULL; txq = gsm->tx_head) {
-		gsm->tx_head = txq->next;
+	list_for_each_entry_safe(txq, ntxq, &gsm->tx_list, list)
 		kfree(txq);
-	}
-	gsm->tx_tail = NULL;
+	INIT_LIST_HEAD(&gsm->tx_list);
 }
 EXPORT_SYMBOL_GPL(gsm_cleanup_mux);
 
@@ -2202,6 +2187,7 @@ struct gsm_mux *gsm_alloc_mux(void)
 	}
 	spin_lock_init(&gsm->lock);
 	kref_init(&gsm->ref);
+	INIT_LIST_HEAD(&gsm->tx_list);
 
 	gsm->t1 = T1;
 	gsm->t2 = T2;


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