[RFC] Bluetooth: L2CAP: Fix to handling fragmented header

[Luiz Augusto von Dentz <luiz.dentz@gmail.com>]

From: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>

Bluetooth Core Specification v5.2, Vol. 3, Part A, section 1.4, table
1.1:

 'Start Fragments always either begin with the first octet of the Basic
  L2CAP header of a PDU or they have a length of zero (see [Vol 2] Part
  B, Section 6.6.2).'

This text has been changed recently as it previously stated:

 'Start Fragments always begin with the Basic L2CAP header of a PDU.'

Apparently this was changed by the following errata:

https://www.bluetooth.org/tse/errata_view.cfm?errata_id=10216

In past this has not been a problem but it seems new controllers are
apparently doing it as it has been reported in Zephyr:

https://github.com/zephyrproject-rtos/zephyr/issues/26900

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
---
 net/bluetooth/l2cap_core.c | 104 +++++++++++++++++++++++++++++--------
 1 file changed, 83 insertions(+), 21 deletions(-)

diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
index ade83e224567..193bea314222 100644
--- a/net/bluetooth/l2cap_core.c
+++ b/net/bluetooth/l2cap_core.c
@@ -8269,6 +8269,63 @@ static void l2cap_security_cfm(struct hci_conn *hcon, u8 status, u8 encrypt)
 	mutex_unlock(&conn->chan_lock);
 }
 
+/* Append fragment into frame respecting the maximum len of rx_skb */
+static int l2cap_recv_frag(struct l2cap_conn *conn, struct sk_buff *skb,
+			   u16 len)
+{
+	if (!conn->rx_skb) {
+		/* Allocate skb for the complete frame (with header) */
+		conn->rx_skb = bt_skb_alloc(len, GFP_KERNEL);
+		if (!conn->rx_skb)
+			return -ENOMEM;
+		/* Init rx_len */
+		conn->rx_len = len;
+	}
+
+	/* Copy as much as the rx_skb can hold */
+	len = min_t(u16, len, skb->len);
+	skb_copy_from_linear_data(skb, skb_put(conn->rx_skb, len), len);
+	skb_pull(skb, len);
+	conn->rx_len -= len;
+
+	return len;
+}
+
+static int l2cap_recv_header(struct l2cap_conn *conn, struct sk_buff *skb)
+{
+	struct l2cap_hdr *hdr;
+	struct sk_buff *rx_skb;
+	int len;
+
+	/* Append just enough to complete the header */
+	len = l2cap_recv_frag(conn, skb, L2CAP_HDR_SIZE - conn->rx_skb->len);
+
+	/* If header could not be read just continue */
+	if (len < 0 || conn->rx_skb->len < L2CAP_HDR_SIZE)
+		return len;
+
+	rx_skb = conn->rx_skb;
+	conn->rx_skb = NULL;
+
+	hdr = (struct l2cap_hdr *) rx_skb->data;
+
+	/* Append existing rx_skb as that allocates only enough for the
+	 * headers.
+	 */
+	len = l2cap_recv_frag(conn, rx_skb,
+			      __le16_to_cpu(hdr->len) + L2CAP_HDR_SIZE);
+	kfree_skb(rx_skb);
+
+	return len;
+}
+
+static void l2cap_recv_reset(struct l2cap_conn *conn)
+{
+	kfree_skb(conn->rx_skb);
+	conn->rx_skb = NULL;
+	conn->rx_len = 0;
+}
+
 void l2cap_recv_acldata(struct hci_conn *hcon, struct sk_buff *skb, u16 flags)
 {
 	struct l2cap_conn *conn = hcon->l2cap_data;
@@ -8291,19 +8348,19 @@ void l2cap_recv_acldata(struct hci_conn *hcon, struct sk_buff *skb, u16 flags)
 	case ACL_START:
 	case ACL_START_NO_FLUSH:
 	case ACL_COMPLETE:
-		if (conn->rx_len) {
+		if (conn->rx_skb) {
 			BT_ERR("Unexpected start frame (len %d)", skb->len);
-			kfree_skb(conn->rx_skb);
-			conn->rx_skb = NULL;
-			conn->rx_len = 0;
+			l2cap_recv_reset(conn);
 			l2cap_conn_unreliable(conn, ECOMM);
 		}
 
-		/* Start fragment always begin with Basic L2CAP header */
+		/* Start fragment may not contain the L2CAP header so just
+		 * copy the initial byte when that happens.
+		 */
 		if (skb->len < L2CAP_HDR_SIZE) {
-			BT_ERR("Frame is too short (len %d)", skb->len);
-			l2cap_conn_unreliable(conn, ECOMM);
-			goto drop;
+			if (l2cap_recv_frag(conn, skb, L2CAP_HDR_SIZE) < 0)
+				goto drop;
+			return;
 		}
 
 		hdr = (struct l2cap_hdr *) skb->data;
@@ -8324,38 +8381,43 @@ void l2cap_recv_acldata(struct hci_conn *hcon, struct sk_buff *skb, u16 flags)
 			goto drop;
 		}
 
-		/* Allocate skb for the complete frame (with header) */
-		conn->rx_skb = bt_skb_alloc(len, GFP_KERNEL);
-		if (!conn->rx_skb)
+		/* Append fragment into frame (with header) */
+		if (l2cap_recv_frag(conn, skb, len) < 0)
 			goto drop;
 
-		skb_copy_from_linear_data(skb, skb_put(conn->rx_skb, skb->len),
-					  skb->len);
-		conn->rx_len = len - skb->len;
 		break;
 
 	case ACL_CONT:
 		BT_DBG("Cont: frag len %d (expecting %d)", skb->len, conn->rx_len);
 
-		if (!conn->rx_len) {
+		if (!conn->rx_skb) {
 			BT_ERR("Unexpected continuation frame (len %d)", skb->len);
 			l2cap_conn_unreliable(conn, ECOMM);
 			goto drop;
 		}
 
+		/* Complete the L2CAP header if it has not been read */
+		if (conn->rx_skb->len < L2CAP_HDR_SIZE) {
+			if (l2cap_recv_header(conn, skb) < 0) {
+				l2cap_conn_unreliable(conn, ECOMM);
+				goto drop;
+			}
+
+			/* Header still could not be read just continue */
+			if (conn->rx_skb->len < L2CAP_HDR_SIZE)
+				return;
+		}
+
 		if (skb->len > conn->rx_len) {
 			BT_ERR("Fragment is too long (len %d, expected %d)",
 			       skb->len, conn->rx_len);
-			kfree_skb(conn->rx_skb);
-			conn->rx_skb = NULL;
-			conn->rx_len = 0;
+			l2cap_recv_reset(conn);
 			l2cap_conn_unreliable(conn, ECOMM);
 			goto drop;
 		}
 
-		skb_copy_from_linear_data(skb, skb_put(conn->rx_skb, skb->len),
-					  skb->len);
-		conn->rx_len -= skb->len;
+		/* Append fragment into frame (with header) */
+		l2cap_recv_frag(conn, skb, skb->len);
 
 		if (!conn->rx_len) {
 			/* Complete frame received. l2cap_recv_frame
-- 
2.26.2