概要

In the Linux kernel, the following vulnerability has been resolved:Bluetooth: rfcomm: Fix null-ptr-deref in rfcomm_check_securityDuring our fuzz testing of the connection and disconnection process at theRFCOMM layer, we discovered this bug. By comparing the packets from anormal connection and disconnection process with the testcase thattriggered 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 ofencryption key.After receiving this packet, the controller immediatelyreplies with a Command Completepacket (Event Code: 0x0e) to return theEncryption Key Size.2. In our fuzz test case, the timing of the controller s response to thispacket was delayed to an unexpected point: after the RFCOMM and L2CAPlayers had disconnected but before the HCI layer had disconnected.3. After receiving the Encryption Key Size Response at the time describedin 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 callingrfcomm_recv_frame in rfcomm_process_rx.

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