Bluetooth: L2CAP: Fix use-after-free caused by l2cap_chan_put

This fixes the following trace which is caused by hci_rx_work starting up
*after* the final channel reference has been put() during sock_close() but
*before* the references to the channel have been destroyed, so instead
the code now rely on kref_get_unless_zero/l2cap_chan_hold_unless_zero to
prevent referencing a channel that is about to be destroyed.

  refcount_t: increment on 0; use-after-free.
  BUG: KASAN: use-after-free in refcount_dec_and_test+0x20/0xd0
  Read of size 4 at addr ffffffc114f5bf18 by task kworker/u17:14/705

  CPU: 4 PID: 705 Comm: kworker/u17:14 Tainted: G S      W
  4.14.234-00003-g1fb6d0bd49a4-dirty #28
  Hardware name: Qualcomm Technologies, Inc. SM8150 V2 PM8150
  Google Inc. MSM sm8150 Flame DVT (DT)
  Workqueue: hci0 hci_rx_work
  Call trace:
   dump_backtrace+0x0/0x378
   show_stack+0x20/0x2c
   dump_stack+0x124/0x148
   print_address_description+0x80/0x2e8
   __kasan_report+0x168/0x188
   kasan_report+0x10/0x18
   __asan_load4+0x84/0x8c
   refcount_dec_and_test+0x20/0xd0
   l2cap_chan_put+0x48/0x12c
   l2cap_recv_frame+0x4770/0x6550
   l2cap_recv_acldata+0x44c/0x7a4
   hci_acldata_packet+0x100/0x188
   hci_rx_work+0x178/0x23c
   process_one_work+0x35c/0x95c
   worker_thread+0x4cc/0x960
   kthread+0x1a8/0x1c4
   ret_from_fork+0x10/0x18

Cc: stable@kernel.org
Reported-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Tested-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>

include/net/bluetooth/l2cap.h

@@ -847,6 +847,7 @@ enum {
 };
 
 void l2cap_chan_hold(struct l2cap_chan *c);
+struct l2cap_chan *l2cap_chan_hold_unless_zero(struct l2cap_chan *c);
 void l2cap_chan_put(struct l2cap_chan *c);
 
 static inline void l2cap_chan_lock(struct l2cap_chan *chan)

net/bluetooth/l2cap_core.c

@@ -111,7 +111,8 @@ static struct l2cap_chan *__l2cap_get_chan_by_scid(struct l2cap_conn *conn,
 }
 
 /* Find channel with given SCID.
- * Returns locked channel. */
+ * Returns a reference locked channel.
+ */
 static struct l2cap_chan *l2cap_get_chan_by_scid(struct l2cap_conn *conn,
 						 u16 cid)
 {
@@ -119,15 +120,19 @@ static struct l2cap_chan *l2cap_get_chan_by_scid(struct l2cap_conn *conn,
 
 	mutex_lock(&conn->chan_lock);
 	c = __l2cap_get_chan_by_scid(conn, cid);
+	if (c) {
+		/* Only lock if chan reference is not 0 */
+		c = l2cap_chan_hold_unless_zero(c);
 		if (c)
 			l2cap_chan_lock(c);
+	}
 	mutex_unlock(&conn->chan_lock);
 
 	return c;
 }
 
 /* Find channel with given DCID.
- * Returns locked channel.
+ * Returns a reference locked channel.
  */
 static struct l2cap_chan *l2cap_get_chan_by_dcid(struct l2cap_conn *conn,
 						 u16 cid)
@@ -136,8 +141,12 @@ static struct l2cap_chan *l2cap_get_chan_by_dcid(struct l2cap_conn *conn,
 
 	mutex_lock(&conn->chan_lock);
 	c = __l2cap_get_chan_by_dcid(conn, cid);
+	if (c) {
+		/* Only lock if chan reference is not 0 */
+		c = l2cap_chan_hold_unless_zero(c);
 		if (c)
 			l2cap_chan_lock(c);
+	}
 	mutex_unlock(&conn->chan_lock);
 
 	return c;
@@ -162,8 +171,12 @@ static struct l2cap_chan *l2cap_get_chan_by_ident(struct l2cap_conn *conn,
 
 	mutex_lock(&conn->chan_lock);
 	c = __l2cap_get_chan_by_ident(conn, ident);
+	if (c) {
+		/* Only lock if chan reference is not 0 */
+		c = l2cap_chan_hold_unless_zero(c);
 		if (c)
 			l2cap_chan_lock(c);
+	}
 	mutex_unlock(&conn->chan_lock);
 
 	return c;
@@ -497,6 +510,16 @@ void l2cap_chan_hold(struct l2cap_chan *c)
 	kref_get(&c->kref);
 }
 
+struct l2cap_chan *l2cap_chan_hold_unless_zero(struct l2cap_chan *c)
+{
+	BT_DBG("chan %p orig refcnt %u", c, kref_read(&c->kref));
+
+	if (!kref_get_unless_zero(&c->kref))
+		return NULL;
+
+	return c;
+}
+
 void l2cap_chan_put(struct l2cap_chan *c)
 {
 	BT_DBG("chan %p orig refcnt %u", c, kref_read(&c->kref));
@@ -1968,7 +1991,10 @@ static struct l2cap_chan *l2cap_global_chan_by_psm(int state, __le16 psm,
 			src_match = !bacmp(&c->src, src);
 			dst_match = !bacmp(&c->dst, dst);
 			if (src_match && dst_match) {
-				l2cap_chan_hold(c);
+				c = l2cap_chan_hold_unless_zero(c);
+				if (!c)
+					continue;
+
 				read_unlock(&chan_list_lock);
 				return c;
 			}
@@ -1983,7 +2009,7 @@ static struct l2cap_chan *l2cap_global_chan_by_psm(int state, __le16 psm,
 	}
 
 	if (c1)
-		l2cap_chan_hold(c1);
+		c1 = l2cap_chan_hold_unless_zero(c1);
 
 	read_unlock(&chan_list_lock);
 
@@ -4463,6 +4489,7 @@ static inline int l2cap_config_req(struct l2cap_conn *conn,
 
 unlock:
 	l2cap_chan_unlock(chan);
+	l2cap_chan_put(chan);
 	return err;
 }
 
@@ -4577,6 +4604,7 @@ static inline int l2cap_config_rsp(struct l2cap_conn *conn,
 
 done:
 	l2cap_chan_unlock(chan);
+	l2cap_chan_put(chan);
 	return err;
 }
 
@@ -5304,6 +5332,7 @@ static inline int l2cap_move_channel_req(struct l2cap_conn *conn,
 	l2cap_send_move_chan_rsp(chan, result);
 
 	l2cap_chan_unlock(chan);
+	l2cap_chan_put(chan);
 
 	return 0;
 }
@@ -5396,6 +5425,7 @@ static void l2cap_move_continue(struct l2cap_conn *conn, u16 icid, u16 result)
 	}
 
 	l2cap_chan_unlock(chan);
+	l2cap_chan_put(chan);
 }
 
 static void l2cap_move_fail(struct l2cap_conn *conn, u8 ident, u16 icid,
@@ -5425,6 +5455,7 @@ static void l2cap_move_fail(struct l2cap_conn *conn, u8 ident, u16 icid,
 	l2cap_send_move_chan_cfm(chan, L2CAP_MC_UNCONFIRMED);
 
 	l2cap_chan_unlock(chan);
+	l2cap_chan_put(chan);
 }
 
 static int l2cap_move_channel_rsp(struct l2cap_conn *conn,
@@ -5488,6 +5519,7 @@ static int l2cap_move_channel_confirm(struct l2cap_conn *conn,
 	l2cap_send_move_chan_cfm_rsp(conn, cmd->ident, icid);
 
 	l2cap_chan_unlock(chan);
+	l2cap_chan_put(chan);
 
 	return 0;
 }
@@ -5523,6 +5555,7 @@ static inline int l2cap_move_channel_confirm_rsp(struct l2cap_conn *conn,
 	}
 
 	l2cap_chan_unlock(chan);
+	l2cap_chan_put(chan);
 
 	return 0;
 }
@@ -5895,12 +5928,11 @@ static inline int l2cap_le_credits(struct l2cap_conn *conn,
 	if (credits > max_credits) {
 		BT_ERR("LE credits overflow");
 		l2cap_send_disconn_req(chan, ECONNRESET);
-		l2cap_chan_unlock(chan);
 
 		/* Return 0 so that we don't trigger an unnecessary
 		 * command reject packet.
 		 */
-		return 0;
+		goto unlock;
 	}
 
 	chan->tx_credits += credits;
@@ -5911,7 +5943,9 @@ static inline int l2cap_le_credits(struct l2cap_conn *conn,
 	if (chan->tx_credits)
 		chan->ops->resume(chan);
 
+unlock:
 	l2cap_chan_unlock(chan);
+	l2cap_chan_put(chan);
 
 	return 0;
 }
@@ -7597,6 +7631,7 @@ static void l2cap_data_channel(struct l2cap_conn *conn, u16 cid,
 
 done:
 	l2cap_chan_unlock(chan);
+	l2cap_chan_put(chan);
 }
 
 static void l2cap_conless_channel(struct l2cap_conn *conn, __le16 psm,
@@ -8085,7 +8120,7 @@ static struct l2cap_chan *l2cap_global_fixed_chan(struct l2cap_chan *c,
 		if (src_type != c->src_type)
 			continue;
 
-		l2cap_chan_hold(c);
+		c = l2cap_chan_hold_unless_zero(c);
 		read_unlock(&chan_list_lock);
 		return c;
 	}