scsi: target: iscsi: Fix a race condition between login_work and the login thread

In case a malicious initiator sends some random data immediately after a
login PDU; the iscsi_target_sk_data_ready() callback will schedule the
login_work and, at the same time, the negotiation may end without clearing
the LOGIN_FLAGS_INITIAL_PDU flag (because no additional PDU exchanges are
required to complete the login).

The login has been completed but the login_work function will find the
LOGIN_FLAGS_INITIAL_PDU flag set and will never stop from rescheduling
itself; at this point, if the initiator drops the connection, the
iscsit_conn structure will be freed, login_work will dereference a released
socket structure and the kernel crashes.

BUG: kernel NULL pointer dereference, address: 0000000000000230
PF: supervisor write access in kernel mode
PF: error_code(0x0002) - not-present page
Workqueue: events iscsi_target_do_login_rx [iscsi_target_mod]
RIP: 0010:_raw_read_lock_bh+0x15/0x30
Call trace:
 iscsi_target_do_login_rx+0x75/0x3f0 [iscsi_target_mod]
 process_one_work+0x1e8/0x3c0

Fix this bug by forcing login_work to stop after the login has been
completed and the socket callbacks have been restored.

Add a comment to clearify the return values of iscsi_target_do_login()
Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
Link: https://lore.kernel.org/r/20221115125638.102517-1-mlombard@redhat.comReviewed-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/target/iscsi/iscsi_target_nego.c

@@ -1018,6 +1018,13 @@ static int iscsi_target_handle_csg_one(struct iscsit_conn *conn, struct iscsi_lo
 	return 0;
 }
 
+/*
+ * RETURN VALUE:
+ *
+ *  1 = Login successful
+ * -1 = Login failed
+ *  0 = More PDU exchanges required
+ */
 static int iscsi_target_do_login(struct iscsit_conn *conn, struct iscsi_login *login)
 {
 	int pdu_count = 0;
@@ -1363,12 +1370,13 @@ int iscsi_target_start_negotiation(
 		ret = -1;
 
 	if (ret < 0) {
-		cancel_delayed_work_sync(&conn->login_work);
 		iscsi_target_restore_sock_callbacks(conn);
 		iscsi_remove_failed_auth_entry(conn);
 	}
-	if (ret != 0)
+	if (ret != 0) {
+		cancel_delayed_work_sync(&conn->login_work);
 		iscsi_target_nego_release(conn);
+	}
 
 	return ret;
 }