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

[Marcel Holtmann <marcel@holtmann.org>]

Hi Luiz,

> 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;

so I think it is pointless to insist on getting the complete header. We really just need the first 2 octets.

struct l2cap_hdr {                                                               
        __le16     len;                                                          
        __le16     cid;                                                          
} __packed;

Once we have received at least 2 octets, we can get_unaligned_le16(rx_skb->data) and then just continue.

Regards

Marcel