ca/user: Add certificate extensions

According to the *sshd(8)* manual page:

> Certificates may encode access restrictions similar to these key
> options.  If both certificate restrictions and key options are
> present, the most restrictive union of the two is applied.

This would seem to apply that if a certificate has no restrictions, all
features are allowed unless restricted in the `authorized_keys` file.
Unfortunately, this is not actually the case.  A certificate with no
extensions apparently trumps all other configuration.  As such,
certificates need to explicitly list the features users will need.

The list of extensions to add to user certificates is configurable via
the `ca.user.extensions` array.  The default set should provide a good
user experience without being overly permissive.

src/ca.rs

@@ -144,7 +144,20 @@ pub fn sign_host_cert(
     privkey: &PrivateKey,
     alias: &[&str],
 ) -> Result<Certificate, CertError> {
-    sign_cert(hostname, pubkey, duration, privkey, alias, CertType::Host)
+    let now = SystemTime::now();
+    let not_before = now.duration_since(UNIX_EPOCH)?.as_secs();
+    let not_after = not_before + duration.as_secs();
+
+    let mut builder = Builder::new_with_random_nonce(
+        &mut OsRng, pubkey, not_before, not_after,
+    )?;
+    builder.cert_type(CertType::Host)?;
+    builder.valid_principal(hostname)?;
+    for a in alias {
+        builder.valid_principal(*a)?;
+    }
+
+    Ok(builder.sign(privkey)?)
 }
 
 /// Create a signed SSH certificate for a user public key
@@ -158,17 +171,7 @@ pub fn sign_user_cert(
     duration: Duration,
     privkey: &PrivateKey,
     alias: &[&str],
-) -> Result<Certificate, CertError> {
-    sign_cert(username, pubkey, duration, privkey, alias, CertType::User)
-}
-
-fn sign_cert(
-    principal: &str,
-    pubkey: &PublicKey,
-    duration: Duration,
-    privkey: &PrivateKey,
-    alias: &[&str],
-    cert_type: CertType,
+    extensions: &[impl AsRef<str>],
 ) -> Result<Certificate, CertError> {
     let now = SystemTime::now();
     let not_before = now.duration_since(UNIX_EPOCH)?.as_secs();
@@ -177,11 +180,14 @@ fn sign_cert(
     let mut builder = Builder::new_with_random_nonce(
         &mut OsRng, pubkey, not_before, not_after,
     )?;
-    builder.cert_type(cert_type)?;
-    builder.valid_principal(principal)?;
+    builder.cert_type(CertType::User)?;
+    builder.valid_principal(username)?;
     for a in alias {
         builder.valid_principal(*a)?;
     }
+    for e in extensions {
+        builder.extension(e.as_ref(), "")?;
+    }
 
     Ok(builder.sign(privkey)?)
 }
@@ -201,9 +207,9 @@ mod test {
         let host_pub_key = host_key.public_key();
         let duration = Duration::from_secs(86400 * 30);
         let hostname = "cloud0.example.org";
-        let cert = sign_cert(
+        let cert = sign_host_cert(
             hostname,
-            &host_pub_key,
+            host_pub_key,
             duration,
             &ca_key,
             &["nextcloud.example.org"],

src/config.rs

@@ -75,6 +75,10 @@ pub struct UserCaConfig {
     /// Duration of issued user certificates
     #[serde(default = "default_user_cert_duration")]
     pub cert_duration: u64,
+
+    /// Certificate extensions
+    #[serde(default = "default_user_cert_extensions")]
+    pub extensions: Vec<String>,
 }
 
 impl Default for UserCaConfig {
@@ -83,6 +87,7 @@ impl Default for UserCaConfig {
             private_key_file: default_user_ca_key(),
             private_key_passphrase_file: None,
             cert_duration: default_user_cert_duration(),
+            extensions: default_user_cert_extensions(),
         }
     }
 }
@@ -171,6 +176,16 @@ fn default_user_cert_duration() -> u64 {
     3600
 }
 
+fn default_user_cert_extensions() -> Vec<String> {
+    vec![
+        "permit-X11-forwarding".into(),
+        "permit-agent-forwarding".into(),
+        "permit-port-forwarding".into(),
+        "permit-pty".into(),
+        "permit-user-rc".into(),
+    ]
+}
+
 /// Load configuration from a TOML file
 ///
 /// If `path` is provided, the configuration will be loaded from the

src/server/user.rs

@@ -168,6 +168,7 @@ pub(super) async fn sign_user_cert(
 
     let config = &ctx.config;
     let duration = Duration::from_secs(config.ca.user.cert_duration);
+    let extensions = &config.ca.user.extensions;
     let privkey = ca::load_private_key(
         &config.ca.user.private_key_file,
         config.ca.user.private_key_passphrase_file.as_ref(),
@@ -192,7 +193,14 @@ pub(super) async fn sign_user_cert(
         pubkey.algorithm().as_str(),
         username
     );
-    let cert = ca::sign_user_cert(username, &pubkey, duration, &privkey, &alias)?;
+    let cert = ca::sign_user_cert(
+        username,
+        &pubkey,
+        duration,
+        &privkey,
+        &alias,
+        &extensions[..],
+    )?;
     info!(
         "Signed {} key for {}",
         pubkey.algorithm().as_str(),