Redirect back to the originating page after sign-in, not always /

Sign in link now carries ?redirect=<question_id>; /auth/login stashes
it in the session (validated same-site-path-only to avoid an open
redirect via a crafted query param), /auth/callback reads it back for
the final redirect instead of a hardcoded "/".

src/server/oidc.rs

@@ -40,6 +40,7 @@ pub struct Oidc {
 const PKCE_KEY: &str = "oidc_pkce_verifier";
 const CSRF_KEY: &str = "oidc_csrf_state";
 const NONCE_KEY: &str = "oidc_nonce";
+const REDIRECT_KEY: &str = "oidc_post_login_redirect";
 
 impl Oidc {
     /// Discovers the provider and builds the client from environment:
@@ -92,8 +93,31 @@ fn internal(err: impl std::fmt::Display) -> HandlerError {
     )
 }
 
-/// GET /auth/login — stash PKCE/state/nonce in the session and bounce to Kanidm.
-pub async fn login(State(state): State<AppState>, session: Session) -> Result<Redirect, HandlerError> {
+#[derive(Deserialize)]
+pub struct LoginParams {
+    redirect: Option<String>,
+}
+
+/// A same-site, single-segment-rooted path only - never an absolute
+/// URL or scheme-relative one (`//evil.com/...` is a valid "path" to a
+/// browser but resolves to an external host), since this value comes
+/// straight from a query param an attacker could craft into a phishing
+/// link that still passes through this app's own real login flow.
+fn is_safe_redirect(path: &str) -> bool {
+    path.starts_with('/') && !path.starts_with("//") && !path.contains("://")
+}
+
+/// GET /auth/login — stash PKCE/state/nonce (and where to return to
+/// after) in the session and bounce to Kanidm.
+pub async fn login(
+    State(state): State<AppState>,
+    session: Session,
+    Query(params): Query<LoginParams>,
+) -> Result<Redirect, HandlerError> {
+    if let Some(redirect) = params.redirect.filter(|r| is_safe_redirect(r)) {
+        session.insert(REDIRECT_KEY, redirect).await.map_err(internal)?;
+    }
+
     let (pkce_challenge, pkce_verifier) = PkceCodeChallenge::new_random_sha256();
 
     let (auth_url, csrf_state, nonce) = state
@@ -209,8 +233,11 @@ pub async fn callback(
         .await
         .map_err(internal)?;
 
-    tracing::info!(user = %user.username, "signed in");
-    Ok(Redirect::to("/"))
+    let redirect: Option<String> = session.remove(REDIRECT_KEY).await.map_err(internal)?;
+    let target = redirect.filter(|r| is_safe_redirect(r)).unwrap_or_else(|| "/".to_string());
+
+    tracing::info!(user = %user.username, redirect = %target, "signed in");
+    Ok(Redirect::to(&target))
 }
 
 /// GET /auth/logout — drop the session.