Xous API: names

xous-names resolves plaintext server names to 128-bit randomly assigned server IDs. It is also the front-line gatekeeper for restricting access to services by preventing connections or discoverability (or more accurately, discoverability is inherently hard because it requires brute-forcing the server ID in a 128-bit space of randomly assigned numbers).

See the Xous Book for more details. The Xous Book should be considered normative; the specification below is historical.

Specification

Server IDs (SIDs) are used by processes to send messages to servers. It is thus an attack surface. Furthermore, if a process can forge an SID before a server can claim it, it can “become” the server. Thus, it’s helpful to keep the SID a secret.

In Xous, an SID is a 128-bit number that, with two exceptions, is only known to the server itself, and an oracle known as xous-name-server.

SIDs are never revealed to a process. Processes establish connections to servers using a descriptive, human-readable string name. Since the SIDs are random numbers, there is no way to turn the descriptive string into the SID except by resolving it with xous-name-server.

On boot, a trusted set of processes are started which form the operating system. These must all claim names in the name space before any further processes are started, to prevent later processes from claiming their names. Servers can also optionally limit the total number of connections allowed, which effectively makes them unreachable by less trusted code that is run after the core trusted set of processes have started up. The trusted_init_done() libary call on the name server will return true if all the servers that have a connection limit have fully populated all of their connections.

The exception to random SIDs are the xous-name-server, xous-log-server (note the trailing space), and ticktimer-server. These are three well known names that have a defined, fixed SID so that all processes can talk to them; xous-name-server is necessarily well-known as it is the mechanism to resolve further names, xous-log-server is necessary for debugging, so that bugs upstream of name resolution can be logged. xous-name-server was picked because name-server already has a meaning in the context of DNS. ticktimer-server is necessary for implementing deterministic timing delays in xous-name-server upon failure, and also for other processes to wait a fixed period of time on initialization while the initial set of name registrations occur.

A new process that intends to receive messages uses the register_name convenience function in the xous-names/src/lib.rs file, with the following procedure.

1. It calls register_name with a preferred ASCII name string, limited to 64 characters. It also specifies how many connections the server will allow. None on the specifier means no limit.

2. xous-name-server returns the borrowed memory to the server, where the buffer has been replaced with a response field. In the case that the registration is affirmed, the SID field in Registration contains the assigned SID of calling process. In the case that the name is determined to be invalid (perhaps because it is already reserved or registered), it will return an error code.

3. If the registration is denied, the server can attempt to re-register its SID with a different ASCII name string by repeating steps 2-3.

A process that would like to send a server a message must first request the name server to broker a connection to the target process. It does this by typically calling the request_conneciton_blocking convenience function with the registered name of the server.

1. request_connection_blocking is called with the maximum 64-character ASCII name string.

2. The convencience function creates a Lookup message which is lent to the name server.

3. xous-name-server can respond with one of three results: A. affirm the connection by returning a connection ID B. a flat denial of the connection; or C. a slot containing a request to authenticate.

Here are the cases worked out:

A. affirming the connection: xous-name-server would use MessageSender.pid() to extract the sender’s PID, and call ConnectForProcess(PID,SID) on behalf of the sender. The sender can then use the CID as the first argument to send_message(). This is the common case, and many servers follow this path, such as those asking for access to the ticktimer or other public services.

B. flat denial: xous-name-server simply returns a message saying the request was denied. This is also the case when the request is malformed or incorrect. No information shall be leaked about the nature of the denial. Denials are also delayed to the nearest 0.1 second interval since boot to eliminate side channels and to rate limit fuzzing requests. Some services (such as the key server) are restricted to only a set of trusted process loaded at boot, and therefore it should not be discoverable.

C. request to authenticate: xous-name-server responds with success set to false, but authenticate_request to true. The pubkey_id field is populated with the ID of an acceptable Ed25519 public key for authentication, and a 256-bit challenge nonce is provided in the challenge field. Authentication consists of the requesting server proving that it has knowledge of a shared secret, namely, an Ed25519 private key.

Upon generating the request to authenticate, xous-name-server computes the correct response to the challenge and stores it in a table with a timestamp.

The sending process must then sign the challenge and return an Authenticate message, constructed similarly to the Lookup message but with the response_to_challenge field filled out. It must do this before AUTHENTICATE_TIMEOUT milliseconds have passed. The server, upon receipt of an Authenticate message, merely checks if the response_to_challenge matches any response stored in its table, and if it does, it accepts the process as authenticated. It is cryptographically unlikely for there to be a collision in the table; however, this implementation is weak to an attacker potentially stealing the response and using it. That being said, if an attacker already has that level of control in the calling process, there are bigger problems.

The AUTHENTICATE_TIMEOUT field is used to give xous-name-server a chance to depopulate the response table over time, so that it does not "leak" memory.

Current Implementation

The current implementation is a hash map that matches randomly generated names with a list of names each server selects for itself. Currently, any request to lookup and connect to a server will succeed up to the limit of connections (if any) specified by a server, but the hooks are there to enforce permissions and deny connections, and/or request authentication for connection.

Server names are crate-local, and are bound through library functions called during the creation of server access objects. In other words, there is no global name space for servers.