Make maximum resident credential count configurable

Previously, we estimated that we can handle 100 resident keys when
returning the number of remaining resident keys in the credential
management command.

This patch introduces a config option to set a maximum count of resident
keys that is used to report the number of remaining resident keys and
that is enforced when trying to create a new resident key.

CHANGELOG.md

@@ -4,12 +4,15 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## Unreleased
+- Add config option for setting a maximum number of resident credentials.
+
 ## [0.1.1] - 2022-08-22
 - Fix bug that treated U2F payloads as APDU over APDU in NFC transport @conorpp
 - Add config option to skip UP when device was just booted,
   as insertion is a kind of UP check @robin-nitrokey
 
-## [Unreleased]
+## [0.1.0] - 2022-03-17
 
 - use 2021 edition
 - use @szszszsz's credential ID shortening

src/constants.rs

@@ -7,3 +7,5 @@ pub const U2F_UP_TIMEOUT: u32 = 250;
 
 pub const ATTESTATION_CERT_ID: CertId = CertId::from_special(0);
 pub const ATTESTATION_KEY_ID: KeyId = KeyId::from_special(0);
+
+pub const MAX_RESIDENT_CREDENTIALS_GUESSTIMATE: u32 = 100;

src/ctap2.rs

@@ -375,6 +375,16 @@ impl<UP: UserPresence, T: TrussedRequirements> Authenticator for crate::Authenti
             self.delete_resident_key_by_user_id(&rp_id_hash, &credential.user.id)
                 .ok();
 
+            // then check the maximum number of RK credentials
+            if let Some(max_count) = self.config.max_resident_credential_count {
+                let mut cm = credential_management::CredentialManagement::new(self);
+                let metadata = cm.get_creds_metadata()?;
+                let count = metadata.existing_resident_credentials_count.unwrap_or(max_count);
+                if count >= max_count {
+                    return Err(Error::KeyStoreFull);
+                }
+            }
+
             // then store key, making it resident
             let credential_id_hash = self.hash(credential_id.0.as_ref());
             try_syscall!(self.trussed.write_file(

src/ctap2/credential_management.rs

@@ -18,6 +18,7 @@ use ctap_types::{
 use littlefs2::path::{Path, PathBuf};
 
 use crate::{
+    constants::MAX_RESIDENT_CREDENTIALS_GUESSTIMATE,
     credential::Credential,
     state::{CredentialManagementEnumerateCredentials, CredentialManagementEnumerateRps},
     Authenticator, Result, TrussedRequirements, UserPresence,
@@ -67,9 +68,12 @@ where
         info!("get metadata");
         let mut response: Response = Default::default();
 
-        let guesstimate = self.state.persistent.max_resident_credentials_guesstimate();
+        let max_resident_credentials = self
+            .config
+            .max_resident_credential_count
+            .unwrap_or(MAX_RESIDENT_CREDENTIALS_GUESSTIMATE);
         response.existing_resident_credentials_count = Some(0);
-        response.max_possible_remaining_residential_credentials_count = Some(guesstimate);
+        response.max_possible_remaining_residential_credentials_count = Some(max_resident_credentials);
 
         let dir = PathBuf::from(b"rk");
         let maybe_first_rp =
@@ -98,11 +102,7 @@ where
                 None => {
                     response.existing_resident_credentials_count = Some(num_rks);
                     response.max_possible_remaining_residential_credentials_count =
-                        Some(if num_rks >= guesstimate {
-                            0
-                        } else {
-                            guesstimate - num_rks
-                        });
+                        Some(max_resident_credentials.saturating_sub(num_rks));
                     return Ok(response);
                 }
                 Some(rp) => {

src/lib.rs

@@ -76,6 +76,8 @@ pub struct Config {
     /// If set, the first Get Assertion or Authenticate request within the specified time after
     /// boot is accepted without additional user presence verification.
     pub skip_up_timeout: Option<Duration>,
+    /// The maximum number of resident credentials.
+    pub max_resident_credential_count: Option<u32>,
 }
 
 // impl Default for Config {

src/state.rs

@@ -273,11 +273,6 @@ pub struct PersistentState {
 impl PersistentState {
     const RESET_RETRIES: u8 = 8;
     const FILENAME: &'static [u8] = b"persistent-state.cbor";
-    const MAX_RESIDENT_CREDENTIALS_GUESSTIMATE: u32 = 100;
-
-    pub fn max_resident_credentials_guesstimate(&self) -> u32 {
-        Self::MAX_RESIDENT_CREDENTIALS_GUESSTIMATE
-    }
 
     pub fn load<T: client::Client + client::Chacha8Poly1305>(trussed: &mut T) -> Result<Self> {
         // TODO: add "exists_file" method instead?