app: support mapping target uid as privileged uid in sandbox

Chromium's D-Bus client implementation refuses to work when its getuid call returns a different value than what the D-Bus server is running as. The reason behind this is not fully understood, but this workaround is implemented to support chromium and electron apps. This is not used by default since it has many side effects that break many other programs, like SSH on NixOS.

Signed-off-by: Ophestra Umiker <cat@ophivana.moe>

internal/app/config.go

@@ -8,11 +8,6 @@ import (
 	"git.ophivana.moe/security/fortify/internal/system"
 )
 
-const (
-	mappedID       = 65534
-	mappedIDString = "65534"
-)
-
 // Config is used to seal an *App
 type Config struct {
 	// D-Bus application ID
@@ -54,6 +49,8 @@ type SandboxConfig struct {
 	Net bool `json:"net,omitempty"`
 	// do not run in new session
 	NoNewSession bool `json:"no_new_session,omitempty"`
+	// map target user uid to privileged user uid in the user namespace
+	UseRealUID bool `json:"use_real_uid"`
 	// mediated access to wayland socket
 	Wayland bool `json:"wayland,omitempty"`
 
@@ -82,11 +79,15 @@ type FilesystemConfig struct {
 
 // Bwrap returns the address of the corresponding bwrap.Config to s.
 // Note that remaining tmpfs entries must be queued by the caller prior to launch.
-func (s *SandboxConfig) Bwrap() *bwrap.Config {
+func (s *SandboxConfig) Bwrap(uid int) *bwrap.Config {
 	if s == nil {
 		return nil
 	}
 
+	if !s.UseRealUID {
+		uid = 65534
+	}
+
 	conf := (&bwrap.Config{
 		Net:           s.Net,
 		UserNS:        s.UserNS,
@@ -100,7 +101,7 @@ func (s *SandboxConfig) Bwrap() *bwrap.Config {
 		// initialise map
 		Chmod: make(map[string]os.FileMode),
 	}).
-		SetUID(mappedID).SetGID(mappedID).
+		SetUID(uid).SetGID(uid).
 		Procfs("/proc").DevTmpfs("/dev").Mqueue("/dev/mqueue").
 		Tmpfs("/dev/fortify", 4*1024)
 

internal/app/seal.go

@@ -129,6 +129,15 @@ func (a *app) Seal(config *Config) error {
 	// create seal system component
 	seal.sys = new(appSealSys)
 
+	// mapped uid
+	if config.Confinement.Sandbox != nil && config.Confinement.Sandbox.UseRealUID {
+		seal.sys.mappedID = a.os.Geteuid()
+	} else {
+		seal.sys.mappedID = 65534
+	}
+	seal.sys.mappedIDString = strconv.Itoa(seal.sys.mappedID)
+	seal.sys.runtime = path.Join("/run/user", seal.sys.mappedIDString)
+
 	// look up user from system
 	if u, err := a.os.Lookup(config.User); err != nil {
 		if errors.As(err, new(user.UnknownUserError)) {
@@ -139,7 +148,6 @@ func (a *app) Seal(config *Config) error {
 		}
 	} else {
 		seal.sys.user = u
-		seal.sys.runtime = path.Join("/run/user", mappedIDString)
 	}
 
 	// map sandbox config to bwrap
@@ -228,7 +236,7 @@ func (a *app) Seal(config *Config) error {
 
 		config.Confinement.Sandbox = conf
 	}
-	seal.sys.bwrap = config.Confinement.Sandbox.Bwrap()
+	seal.sys.bwrap = config.Confinement.Sandbox.Bwrap(a.os.Geteuid())
 	seal.sys.override = config.Confinement.Sandbox.Override
 	if seal.sys.bwrap.SetEnv == nil {
 		seal.sys.bwrap.SetEnv = make(map[string]string)

internal/app/share.system.go

@@ -58,12 +58,12 @@ func (seal *appSeal) sharePasswd(os linux.System) {
 		homeDir = seal.sys.user.HomeDir
 		seal.sys.bwrap.SetEnv["HOME"] = seal.sys.user.HomeDir
 	}
-	passwd := username + ":x:" + mappedIDString + ":" + mappedIDString + ":Fortify:" + homeDir + ":" + sh + "\n"
+	passwd := username + ":x:" + seal.sys.mappedIDString + ":" + seal.sys.mappedIDString + ":Fortify:" + homeDir + ":" + sh + "\n"
 	seal.sys.Write(passwdPath, passwd)
 
 	// write /etc/group
 	groupPath := path.Join(seal.share, "group")
-	seal.sys.Write(groupPath, "fortify:x:"+mappedIDString+":\n")
+	seal.sys.Write(groupPath, "fortify:x:"+seal.sys.mappedIDString+":\n")
 
 	// bind /etc/passwd and /etc/group
 	seal.sys.bwrap.Bind(passwdPath, "/etc/passwd")

internal/app/system.go

@@ -20,6 +20,11 @@ type appSealSys struct {
 	// target user sealed from config
 	user *user.User
 
+	// mapped uid and gid in user namespace
+	mappedID int
+	// string representation of mappedID
+	mappedIDString string
+
 	needRevert bool
 	saveState  bool
 	*system.I