[V3] Bluetooth: rfcomm: Fix null-ptr-deref in rfcomm_check_security

During our fuzz testing of the connection and disconnection process at the
RFCOMM layer, we discovered this bug. By comparing the packets from a normal
connection and disconnection process with the testcase that triggered a
KASAN report. We analyzed the cause of this bug as follows:

1. In the packets captured during a normal connection, the host sends a
`Read Encryption Key Size` type of `HCI_CMD` packet (Command Opcode: 0x1408)
to the controller to inquire the length of encryption key.After receiving
this packet, the controller immediately replies with a Command Complete
packet (Event Code: 0x0e) to return the Encryption Key Size.

2. In our fuzz test case, the timing of the controller's response to this
packet was delayed to an unexpected point: after the RFCOMM and L2CAP
layers had disconnected but before the HCI layer had disconnected.

3. After receiving the Encryption Key Size Response at the time described
in point 2, the host still called the rfcomm_check_security function.
However, by this time `struct l2cap_conn *conn = l2cap_pi(sk)->chan->conn;`
had already been released, and when the function executed
`return hci_conn_security(conn->hcon, d->sec_level, auth_type, d->out);`,
specifically when accessing `conn->hcon`, a null-ptr-deref error occurred.

To fix this bug, check if `sk->sk_state` is BT_CLOSED before calling
rfcomm_recv_frame in rfcomm_process_rx.

Signed-off-by: Yuxuan Hu <20373622@buaa.edu.cn>
---
V1 -> V2: Check earlier on rfcomm_process_rx
V2 -> V3: Fixed formatting errors in the commit

 net/bluetooth/rfcomm/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Dear All:

I hope this email finds you well. I hope you haven't missed my previous 
email, as I understand that everyone has a busy schedule. I just wanted 
to follow up on my previous message sent.

I understand that you may be occupied with other tasks or priorities. 
However, I would greatly appreciate it if you could spare a few moments 
to check the patch in my previous email. Your prompt response would be 
highly valuable to me.

Thank you for your attention to this matter, and I look forward to 
hearing from you soon.


On 2024/1/3 17:02, Yuxuan Hu wrote:
> During our fuzz testing of the connection and disconnection process at the
> RFCOMM layer, we discovered this bug. By comparing the packets from a normal
> connection and disconnection process with the testcase that triggered a
> KASAN report. We analyzed the cause of this bug as follows:
>
> 1. In the packets captured during a normal connection, the host sends a
> `Read Encryption Key Size` type of `HCI_CMD` packet (Command Opcode: 0x1408)
> to the controller to inquire the length of encryption key.After receiving
> this packet, the controller immediately replies with a Command Complete
> packet (Event Code: 0x0e) to return the Encryption Key Size.
>
> 2. In our fuzz test case, the timing of the controller's response to this
> packet was delayed to an unexpected point: after the RFCOMM and L2CAP
> layers had disconnected but before the HCI layer had disconnected.
>
> 3. After receiving the Encryption Key Size Response at the time described
> in point 2, the host still called the rfcomm_check_security function.
> However, by this time `struct l2cap_conn *conn = l2cap_pi(sk)->chan->conn;`
> had already been released, and when the function executed
> `return hci_conn_security(conn->hcon, d->sec_level, auth_type, d->out);`,
> specifically when accessing `conn->hcon`, a null-ptr-deref error occurred.
>
> To fix this bug, check if `sk->sk_state` is BT_CLOSED before calling
> rfcomm_recv_frame in rfcomm_process_rx.
>
> Signed-off-by: Yuxuan Hu <20373622@buaa.edu.cn>
> ---
> V1 -> V2: Check earlier on rfcomm_process_rx
> V2 -> V3: Fixed formatting errors in the commit
>
>   net/bluetooth/rfcomm/core.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/bluetooth/rfcomm/core.c b/net/bluetooth/rfcomm/core.c
> index 053ef8f25fae..1d34d8497033 100644
> --- a/net/bluetooth/rfcomm/core.c
> +++ b/net/bluetooth/rfcomm/core.c
> @@ -1941,7 +1941,7 @@ static struct rfcomm_session *rfcomm_process_rx(struct rfcomm_session *s)
>   	/* Get data directly from socket receive queue without copying it. */
>   	while ((skb = skb_dequeue(&sk->sk_receive_queue))) {
>   		skb_orphan(skb);
> -		if (!skb_linearize(skb)) {
> +		if (!skb_linearize(skb) && sk->sk_state != BT_CLOSED) {
>   			s = rfcomm_recv_frame(s, skb);
>   			if (!s)
>   				break;