pansy/internal/api/oidc.go

package api

import (
	"context"
	"crypto/rand"
	"crypto/subtle"
	"encoding/base64"
	"encoding/json"
	"log/slog"
	"net/http"
	"sync"
	"time"

	"github.com/coreos/go-oidc/v3/oidc"
	"github.com/gin-gonic/gin"
	"golang.org/x/oauth2"

	"gitea.stevedudenhoeffer.com/steve/pansy/internal/config"
	"gitea.stevedudenhoeffer.com/steve/pansy/internal/service"
)

const (
	// oidcCallbackPath is appended to PANSY_BASE_URL to form the redirect URI
	// registered with the IdP.
	oidcCallbackPath = "/api/v1/auth/oidc/callback"
	// oidcTxCookie holds the short-lived per-login transaction state (CSRF state,
	// PKCE verifier, nonce) between /oidc/login and /oidc/callback.
	oidcTxCookie = "pansy_oidc_tx"
	oidcTxMaxAge = 10 * time.Minute
	// oidcDiscoveryTimeout bounds a single lazy discovery attempt so a hung IdP
	// can't wedge a request goroutine.
	oidcDiscoveryTimeout = 10 * time.Second
)

// oidcClient lazily performs OIDC discovery and holds the derived verifier and
// oauth2 config. Discovery is deferred to the first login/callback (and retried
// on the next request if it fails) so an IdP that's momentarily unreachable at
// boot doesn't stop the server — or local auth — from starting.
type oidcClient struct {
	issuer       string
	clientID     string
	clientSecret string
	redirectURL  string

	mu       sync.Mutex
	provider *oidc.Provider
	verifier *oidc.IDTokenVerifier
	oauth    *oauth2.Config
}

func newOIDCClient(cfg *config.Config) *oidcClient {
	return &oidcClient{
		issuer:       cfg.OIDC.Issuer,
		clientID:     cfg.OIDC.ClientID,
		clientSecret: cfg.OIDC.ClientSecret,
		redirectURL:  cfg.BaseURL + oidcCallbackPath,
	}
}

// ensure performs discovery once (idempotent) and builds the verifier + oauth2
// config. Safe for concurrent callers; a failure leaves the client uninitialized
// so the next request retries.
func (o *oidcClient) ensure(ctx context.Context) error {
	o.mu.Lock()
	defer o.mu.Unlock()
	if o.provider != nil {
		return nil
	}

	dctx, cancel := context.WithTimeout(ctx, oidcDiscoveryTimeout)
	defer cancel()

	provider, err := oidc.NewProvider(dctx, o.issuer)
	if err != nil {
		return err
	}
	o.provider = provider
	o.verifier = provider.Verifier(&oidc.Config{ClientID: o.clientID})
	o.oauth = &oauth2.Config{
		ClientID:     o.clientID,
		ClientSecret: o.clientSecret,
		Endpoint:     provider.Endpoint(),
		RedirectURL:  o.redirectURL,
		Scopes:       []string{oidc.ScopeOpenID, "email", "profile"},
	}
	return nil
}

// oidcTx is the per-login transaction stashed in the tx cookie. It never leaves
// the browser as anything an attacker can forge (HttpOnly, our domain), and the
// callback checks the returned state against it (CSRF), uses the PKCE verifier
// for the code exchange, and checks the ID-token nonce.
type oidcTx struct {
	State    string `json:"s"`
	Verifier string `json:"v"`
	Nonce    string `json:"n"`
}

// oidcLogin starts the authorization-code + PKCE flow: it stashes fresh state,
// PKCE verifier, and nonce in a short-lived cookie, then redirects to the IdP.
func (h *handlers) oidcLogin(c *gin.Context) {
	if err := h.oidc.ensure(c.Request.Context()); err != nil {
		slog.Error("api: oidc discovery failed", "error", err)
		c.Redirect(http.StatusFound, "/login?error=oidc_unavailable")
		return
	}

	state, err1 := randToken()
	nonce, err2 := randToken()
	if err1 != nil || err2 != nil {
		slog.Error("api: oidc token generation failed", "state_err", err1, "nonce_err", err2)
		c.Redirect(http.StatusFound, "/login?error=oidc")
		return
	}
	verifier := oauth2.GenerateVerifier()

	h.setOIDCTxCookie(c, oidcTx{State: state, Verifier: verifier, Nonce: nonce})

	authURL := h.oidc.oauth.AuthCodeURL(state,
		oauth2.S256ChallengeOption(verifier),
		oidc.Nonce(nonce),
	)
	c.Redirect(http.StatusFound, authURL)
}

// oidcCallback completes the flow: verify state, exchange the code (with the PKCE
// verifier), verify the ID token and nonce, provision/link the user, and start a
// pansy session. Every failure clears the tx cookie and redirects to the login
// page with an error code rather than leaking details to the browser.
func (h *handlers) oidcCallback(c *gin.Context) {
	ctx := c.Request.Context()

	tx, haveTx := h.readOIDCTxCookie(c)
	h.clearOIDCTxCookie(c)

	// A provider-side error (e.g. user denied consent) comes back as ?error=.
	if e := c.Query("error"); e != "" {
		slog.Warn("api: oidc provider returned error", "error", e)
		c.Redirect(http.StatusFound, "/login?error=oidc")
		return
	}

	// State must be present and match the cookie (CSRF protection). Constant-time
	// compare avoids leaking via timing.
	state := c.Query("state")
	if !haveTx || state == "" || subtle.ConstantTimeCompare([]byte(state), []byte(tx.State)) != 1 {
		slog.Warn("api: oidc state mismatch or missing transaction")
		c.Redirect(http.StatusFound, "/login?error=state")
		return
	}

	if err := h.oidc.ensure(ctx); err != nil {
		slog.Error("api: oidc discovery failed on callback", "error", err)
		c.Redirect(http.StatusFound, "/login?error=oidc_unavailable")
		return
	}

	code := c.Query("code")
	if code == "" {
		c.Redirect(http.StatusFound, "/login?error=oidc")
		return
	}

	token, err := h.oidc.oauth.Exchange(ctx, code, oauth2.VerifierOption(tx.Verifier))
	if err != nil {
		slog.Error("api: oidc code exchange failed", "error", err)
		c.Redirect(http.StatusFound, "/login?error=oidc")
		return
	}

	rawIDToken, ok := token.Extra("id_token").(string)
	if !ok || rawIDToken == "" {
		slog.Error("api: oidc token response missing id_token")
		c.Redirect(http.StatusFound, "/login?error=oidc")
		return
	}

	idToken, err := h.oidc.verifier.Verify(ctx, rawIDToken)
	if err != nil {
		slog.Error("api: oidc id_token verification failed", "error", err)
		c.Redirect(http.StatusFound, "/login?error=oidc")
		return
	}
	if subtle.ConstantTimeCompare([]byte(idToken.Nonce), []byte(tx.Nonce)) != 1 {
		slog.Warn("api: oidc nonce mismatch")
		c.Redirect(http.StatusFound, "/login?error=oidc")
		return
	}

	var claims struct {
		Email             string `json:"email"`
		EmailVerified     bool   `json:"email_verified"`
		Name              string `json:"name"`
		PreferredUsername string `json:"preferred_username"`
	}
	if err := idToken.Claims(&claims); err != nil {
		slog.Error("api: oidc claims decode failed", "error", err)
		c.Redirect(http.StatusFound, "/login?error=oidc")
		return
	}
	name := claims.Name
	if name == "" {
		name = claims.PreferredUsername
	}

	// Issuer/Subject come from the *verified* token, not raw claims.
	user, err := h.svc.LoginOIDC(ctx, service.OIDCIdentity{
		Issuer:        idToken.Issuer,
		Subject:       idToken.Subject,
		Email:         claims.Email,
		EmailVerified: claims.EmailVerified,
		Name:          name,
	})
	if err != nil {
		slog.Warn("api: oidc provisioning failed", "error", err)
		c.Redirect(http.StatusFound, "/login?error=oidc")
		return
	}

	if err := h.startSession(c, user.ID); err != nil {
		slog.Error("api: oidc session start failed", "user_id", user.ID, "error", err)
		c.Redirect(http.StatusFound, "/login?error=oidc")
		return
	}
	c.Redirect(http.StatusFound, "/gardens")
}

// setOIDCTxCookie writes the login transaction as a compact base64 JSON cookie.
func (h *handlers) setOIDCTxCookie(c *gin.Context, tx oidcTx) {
	b, err := json.Marshal(tx)
	if err != nil {
		// tx holds only our own generated strings, so this cannot fail in practice.
		slog.Error("api: marshal oidc tx", "error", err)
		return
	}
	value := base64.RawURLEncoding.EncodeToString(b)
	// SameSite=Lax so the cookie survives the IdP's top-level redirect back to the
	// callback (Strict would drop it on that cross-site navigation).
	c.SetSameSite(http.SameSiteLaxMode)
	c.SetCookie(oidcTxCookie, value, int(oidcTxMaxAge.Seconds()), "/", "", h.cookieSecure(), true)
}

func (h *handlers) readOIDCTxCookie(c *gin.Context) (oidcTx, bool) {
	value, err := c.Cookie(oidcTxCookie)
	if err != nil || value == "" {
		return oidcTx{}, false
	}
	raw, err := base64.RawURLEncoding.DecodeString(value)
	if err != nil {
		return oidcTx{}, false
	}
	var tx oidcTx
	if err := json.Unmarshal(raw, &tx); err != nil || tx.State == "" || tx.Verifier == "" {
		return oidcTx{}, false
	}
	return tx, true
}

func (h *handlers) clearOIDCTxCookie(c *gin.Context) {
	c.SetSameSite(http.SameSiteLaxMode)
	c.SetCookie(oidcTxCookie, "", -1, "/", "", h.cookieSecure(), true)
}

// randToken returns 32 bytes of URL-safe randomness for state/nonce values.
func randToken() (string, error) {
	b := make([]byte, 32)
	if _, err := rand.Read(b); err != nil {
		return "", err
	}
	return base64.RawURLEncoding.EncodeToString(b), nil
}