fix(auth): prevent token collisions and optimise lookups

Ensures access tokens are unique across both user and appservice tables to
prevent authentication ambiguity and potential security issues.

Changes:
- On startup, automatically logout any user devices using tokens that
  conflict with appservice tokens (resolves in favour of appservices)
  and log a warning with affected user/device details
- When creating new user tokens, check for conflicts with appservice tokens
  and generate a new token if a collision would occur
- When registering new appservices, reject registration if the token is
  already in use by a user device
- Use futures::select_ok to race token lookups concurrently for better
  performance (adapted from tuwunel commit 066097a8)

This fix-forward approach resolves existing token collisions on startup
whilst preventing new ones from being created, without breaking existing
valid authentications.

The find_token optimisation is adapted from tuwunel (matrix-construct/tuwunel)
commit 066097a8: "Optimize user and appservice token queries" by Jason Volk.

src/api/router/auth.rs

@@ -5,6 +5,14 @@ use axum_extra::{
 	typed_header::TypedHeaderRejectionReason,
 };
 use conduwuit::{Err, Error, Result, debug_error, err, warn};
+use futures::{
+	TryFutureExt,
+	future::{
+		Either::{Left, Right},
+		select_ok,
+	},
+	pin_mut,
+};
 use ruma::{
 	CanonicalJsonObject, CanonicalJsonValue, OwnedDeviceId, OwnedServerName, OwnedUserId, UserId,
 	api::{
@@ -54,17 +62,7 @@ pub(super) async fn auth(
 		| None => request.query.access_token.as_deref(),
 	};
 
-	let token = if let Some(token) = token {
-		match services.appservice.find_from_token(token).await {
-			| Some(reg_info) => Token::Appservice(Box::new(reg_info)),
-			| _ => match services.users.find_from_token(token).await {
-				| Ok((user_id, device_id)) => Token::User((user_id, device_id)),
-				| _ => Token::Invalid,
-			},
-		}
-	} else {
-		Token::None
-	};
+	let token = find_token(services, token).await?;
 
 	if metadata.authentication == AuthScheme::None {
 		match metadata {
@@ -342,3 +340,24 @@ async fn parse_x_matrix(request: &mut Request) -> Result<XMatrix> {
 
 	Ok(x_matrix)
 }
+
+async fn find_token(services: &Services, token: Option<&str>) -> Result<Token> {
+	let Some(token) = token else {
+		return Ok(Token::None);
+	};
+
+	let user_token = services.users.find_from_token(token).map_ok(Token::User);
+
+	let appservice_token = services
+		.appservice
+		.find_from_token(token)
+		.map_ok(Box::new)
+		.map_ok(Token::Appservice);
+
+	pin_mut!(user_token, appservice_token);
+	match select_ok([Left(user_token), Right(appservice_token)]).await {
+		| Err(e) if !e.is_not_found() => Err(e),
+		| Ok((token, _)) => Ok(token),
+		| _ => Ok(Token::Invalid),
+	}
+}

src/service/appservice/mod.rs

@@ -133,10 +133,10 @@ impl Service {
 			.await
 			.is_ok()
 		{
-			return err!(Request(InvalidParam(
+			return Err(err!(Request(InvalidParam(
 				"Cannot register appservice: The provided token is already in use by a user \
 				 device. Please generate a different token for the appservice."
-			)));
+			))));
 		}
 
 		self.db
@@ -182,12 +182,13 @@ impl Service {
 			.map(|info| info.registration)
 	}
 
-	pub async fn find_from_token(&self, token: &str) -> Option<RegistrationInfo> {
+	pub async fn find_from_token(&self, token: &str) -> Result<RegistrationInfo> {
 		self.read()
 			.await
 			.values()
 			.find(|info| info.registration.as_token == token)
 			.cloned()
+			.ok_or_else(|| err!(Request(NotFound("Appservice token not found"))))
 	}
 
 	/// Checks if a given user id matches any exclusive appservice regex

src/service/users/mod.rs

@@ -19,7 +19,7 @@ use ruma::{
 use serde::{Deserialize, Serialize};
 use serde_json::json;
 
-use crate::{Dep, account_data, admin, globals, rooms};
+use crate::{Dep, account_data, admin, appservice, globals, rooms};
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct UserSuspension {
@@ -40,6 +40,7 @@ struct Services {
 	server: Arc<Server>,
 	account_data: Dep<account_data::Service>,
 	admin: Dep<admin::Service>,
+	appservice: Dep<appservice::Service>,
 	globals: Dep<globals::Service>,
 	state_accessor: Dep<rooms::state_accessor::Service>,
 	state_cache: Dep<rooms::state_cache::Service>,
@@ -76,6 +77,7 @@ impl crate::Service for Service {
 				server: args.server.clone(),
 				account_data: args.depend::<account_data::Service>("account_data"),
 				admin: args.depend::<admin::Service>("admin"),
+				appservice: args.depend::<appservice::Service>("appservice"),
 				globals: args.depend::<globals::Service>("globals"),
 				state_accessor: args
 					.depend::<rooms::state_accessor::Service>("rooms::state_accessor"),
@@ -407,6 +409,26 @@ impl Service {
 			)));
 		}
 
+		// Prevent token collisions with appservice tokens
+		let final_token = if self
+			.services
+			.appservice
+			.find_from_token(token)
+			.await
+			.is_ok()
+		{
+			let new_token = utils::random_string(32);
+			conduwuit::debug_warn!(
+				"Token collision prevented: Generated new token for user '{}' device '{}' \
+				 (original token conflicted with an appservice)",
+				user_id.localpart(),
+				device_id
+			);
+			new_token
+		} else {
+			token.to_owned()
+		};
+
 		// Remove old token
 		if let Ok(old_token) = self.db.userdeviceid_token.qry(&key).await {
 			self.db.token_userdeviceid.remove(&old_token);
@@ -414,8 +436,8 @@ impl Service {
 		}
 
 		// Assign token to user device combination
-		self.db.userdeviceid_token.put_raw(key, token);
-		self.db.token_userdeviceid.raw_put(token, key);
+		self.db.userdeviceid_token.put_raw(key, &final_token);
+		self.db.token_userdeviceid.raw_put(&final_token, key);
 
 		Ok(())
 	}