ksmbd: throttle session setup failures to avoid dictionary attacks

To avoid dictionary attacks (repeated session setups rapidly sent) to
connect to server, ksmbd make a delay of a 5 seconds on session setup
failure to make it harder to send enough random connection requests
to break into a server if a user insert the wrong password 10 times
in a row.

Signed-off-by: Namjae Jeon <[e-mail]>
Signed-off-by: Steve French <[e-mail]>

fs/ksmbd/ksmbd_netlink.h

···
  */
 struct ksmbd_logout_request {
 	__s8	account[KSMBD_REQ_MAX_ACCOUNT_NAME_SZ]; /* user account name */
+	__u32	account_flags;
 };
 
 /*
···
 #define KSMBD_USER_FLAG_BAD_UID		BIT(2)
 #define KSMBD_USER_FLAG_BAD_USER	BIT(3)
 #define KSMBD_USER_FLAG_GUEST_ACCOUNT	BIT(4)
+#define KSMBD_USER_FLAG_DELAY_SESSION	BIT(5)
 
 /*
  * Share config flags.

fs/ksmbd/mgmt/user_config.c

···
 
 void ksmbd_free_user(struct ksmbd_user *user)
 {
-	ksmbd_ipc_logout_request(user->name);
+	ksmbd_ipc_logout_request(user->name, user->flags);
 	kfree(user->name);
 	kfree(user->passkey);
 	kfree(user);

fs/ksmbd/mgmt/user_config.h

···
 
 	size_t			passkey_sz;
 	char			*passkey;
+	unsigned int		failed_login_count;
 };
 
 static inline bool user_guest(struct ksmbd_user *user)

fs/ksmbd/smb2pdu.c

···
 		conn->mechToken = NULL;
 	}
 
-	if (rc < 0 && sess) {
-		ksmbd_session_destroy(sess);
-		work->sess = NULL;
+	if (rc < 0) {
+		/*
+		 * SecurityBufferOffset should be set to zero
+		 * in session setup error response.
+		 */
+		rsp->SecurityBufferOffset = 0;
+
+		if (sess) {
+			bool try_delay = false;
+
+			/*
+			 * To avoid dictionary attacks (repeated session setups rapidly sent) to
+			 * connect to server, ksmbd make a delay of a 5 seconds on session setup
+			 * failure to make it harder to send enough random connection requests
+			 * to break into a server.
+			 */
+			if (sess->user && sess->user->flags & KSMBD_USER_FLAG_DELAY_SESSION)
+				try_delay = true;
+
+			ksmbd_session_destroy(sess);
+			work->sess = NULL;
+			if (try_delay)
+				ssleep(5);
+		}
 	}
 
 	return rc;

fs/ksmbd/transport_ipc.c

···
 	return ret;
 }
 
-int ksmbd_ipc_logout_request(const char *account)
+int ksmbd_ipc_logout_request(const char *account, int flags)
 {
 	struct ksmbd_ipc_msg *msg;
 	struct ksmbd_logout_request *req;
···
 
 	msg->type = KSMBD_EVENT_LOGOUT_REQUEST;
 	req = (struct ksmbd_logout_request *)msg->payload;
+	req->account_flags = flags;
 	strscpy(req->account, account, KSMBD_REQ_MAX_ACCOUNT_NAME_SZ);
 
 	ret = ipc_msg_send(msg);

fs/ksmbd/transport_ipc.h

···
 			       struct sockaddr *peer_addr);
 int ksmbd_ipc_tree_disconnect_request(unsigned long long session_id,
 				      unsigned long long connect_id);
-int ksmbd_ipc_logout_request(const char *account);
+int ksmbd_ipc_logout_request(const char *account, int flags);
 struct ksmbd_share_config_response *
 ksmbd_ipc_share_config_request(const char *name);
 struct ksmbd_spnego_authen_response *