feature: switch to simple user

internal/mailcloak/config.go

@@ -8,6 +8,10 @@ import (
 )
 
 type Config struct {
+	Daemon struct {
+		User string `yaml:"user"`
+	} `yaml:"daemon"`
+
 	Keycloak struct {
 		BaseURL      string `yaml:"base_url"`
 		Realm        string `yaml:"realm"`
@@ -58,5 +62,8 @@ func LoadConfig(path string) (*Config, error) {
 	if cfg.Policy.KeycloakFailureMode == "" {
 		cfg.Policy.KeycloakFailureMode = "tempfail"
 	}
+	if cfg.Daemon.User == "" {
+		cfg.Daemon.User = "mailcloak"
+	}
 	return &cfg, nil
 }

internal/mailcloak/policy.go

@@ -38,20 +38,26 @@ func (c *Cache) Put(key, val string, ok bool) {
 	c.m[key] = cacheItem{val: val, ok: ok, expires: time.Now().Add(c.ttl)}
 }
 
-func RunPolicy(ctx context.Context, cfg *Config, db *AliasDB, kc *Keycloak, cache *Cache) error {
+func OpenPolicyListener(cfg *Config) (net.Listener, error) {
 	sock := cfg.Sockets.PolicySocket
 	_ = os.Remove(sock)
 
 	l, err := net.Listen("unix", sock)
 	if err != nil {
-		return err
+		return nil, err
 	}
-	defer l.Close()
 
 	if err := ChownChmodSocket(sock, cfg); err != nil {
-		return err
+		_ = l.Close()
+		return nil, err
 	}
 
+	return l, nil
+}
+
+func ServePolicy(ctx context.Context, cfg *Config, db *AliasDB, kc *Keycloak, cache *Cache, l net.Listener) error {
+	defer l.Close()
+
 	for {
 		conn, err := l.Accept()
 		if err != nil {
@@ -66,6 +72,14 @@ func RunPolicy(ctx context.Context, cfg *Config, db *AliasDB, kc *Keycloak, cach
 	}
 }
 
+func RunPolicy(ctx context.Context, cfg *Config, db *AliasDB, kc *Keycloak, cache *Cache) error {
+	l, err := OpenPolicyListener(cfg)
+	if err != nil {
+		return err
+	}
+	return ServePolicy(ctx, cfg, db, kc, cache, l)
+}
+
 func handlePolicyConn(conn net.Conn, cfg *Config, db *AliasDB, kc *Keycloak, cache *Cache) {
 	defer conn.Close()
 	r := bufio.NewReader(conn)

internal/mailcloak/privileges.go

@@ -0,0 +1,60 @@
+package mailcloak
+
+import (
+	"fmt"
+	"os"
+	"os/user"
+	"strconv"
+	"syscall"
+)
+
+func DropPrivileges(cfg *Config) error {
+	userName := cfg.Daemon.User
+	if userName == "" {
+		userName = "mailcloak"
+	}
+
+	u, err := user.Lookup(userName)
+	if err != nil {
+		return fmt.Errorf("lookup user %s: %w", userName, err)
+	}
+	uid, err := strconv.Atoi(u.Uid)
+	if err != nil {
+		return fmt.Errorf("bad uid for %s: %w", userName, err)
+	}
+	gid, err := strconv.Atoi(u.Gid)
+	if err != nil {
+		return fmt.Errorf("bad gid for %s: %w", userName, err)
+	}
+
+	euid := os.Geteuid()
+	if euid == uid {
+		return nil
+	}
+	if euid != 0 {
+		return fmt.Errorf("must run as root to switch user to %s", userName)
+	}
+
+	if groups, err := u.GroupIds(); err == nil {
+		gids := make([]int, 0, len(groups))
+		for _, g := range groups {
+			if id, err := strconv.Atoi(g); err == nil {
+				gids = append(gids, id)
+			}
+		}
+		if len(gids) > 0 {
+			if err := syscall.Setgroups(gids); err != nil {
+				return fmt.Errorf("setgroups: %w", err)
+			}
+		}
+	}
+
+	if err := syscall.Setgid(gid); err != nil {
+		return fmt.Errorf("setgid: %w", err)
+	}
+	if err := syscall.Setuid(uid); err != nil {
+		return fmt.Errorf("setuid: %w", err)
+	}
+
+	return nil
+}

internal/mailcloak/socketmap.go

@@ -11,20 +11,26 @@ import (
 	"strings"
 )
 
-func RunSocketmap(ctx context.Context, cfg *Config, db *AliasDB) error {
+func OpenSocketmapListener(cfg *Config) (net.Listener, error) {
 	sock := cfg.Sockets.SocketmapSocket
 	_ = os.Remove(sock)
 
 	l, err := net.Listen("unix", sock)
 	if err != nil {
-		return err
+		return nil, err
 	}
-	defer l.Close()
 
 	if err := ChownChmodSocket(sock, cfg); err != nil {
-		return err
+		_ = l.Close()
+		return nil, err
 	}
 
+	return l, nil
+}
+
+func ServeSocketmap(ctx context.Context, cfg *Config, db *AliasDB, l net.Listener) error {
+	defer l.Close()
+
 	for {
 		conn, err := l.Accept()
 		if err != nil {
@@ -39,6 +45,14 @@ func RunSocketmap(ctx context.Context, cfg *Config, db *AliasDB) error {
 	}
 }
 
+func RunSocketmap(ctx context.Context, cfg *Config, db *AliasDB) error {
+	l, err := OpenSocketmapListener(cfg)
+	if err != nil {
+		return err
+	}
+	return ServeSocketmap(ctx, cfg, db, l)
+}
+
 // Postfix socketmap framing: "<len>:<payload>,"
 func handleSocketmapConn(conn net.Conn, cfg *Config, db *AliasDB) {
 	defer conn.Close()