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ipn/{ipnauth,ipnlocal,ipnserver,localapi}: start baby step toward moving access checks from the localapi.Handler to the LocalBackend

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Currently, we use PermitRead/PermitWrite/PermitCert permission flags to determine which operations are allowed for a LocalAPI client.
These checks are performed when localapi.Handler handles a request. Additionally, certain operations (e.g., changing the serve config)
requires the connected user to be a local admin. This approach is inherently racey and is subject to TOCTOU issues.
We consider it to be more critical on Windows environments, which are inherently multi-user, and therefore we prevent more than one
OS user from connecting and utilizing the LocalBackend at the same time. However, the same type of issues is also applicable to other
platforms when switching between profiles that have different OperatorUser values in ipn.Prefs.

We'd like to allow more than one Windows user to connect, but limit what they can see and do based on their access rights on the device
(e.g., an local admin or not) and to the currently active LoginProfile (e.g., owner/operator or not), while preventing TOCTOU issues on Windows
and other platforms. Therefore, we'd like to pass an actor from the LocalAPI to the LocalBackend to represent the user performing the operation.
The LocalBackend, or the profileManager down the line, will then check the actor's access rights to perform a given operation on the device
and against the current (and/or the target) profile.

This PR does not change the current permission model in any way, but it introduces the concept of an actor and includes some preparatory
work to pass it around. Temporarily, the ipnauth.Actor interface has methods like IsLocalSystem and IsLocalAdmin, which are only relevant
to the current permission model. It also lacks methods that will actually be used in the new model. We'll be adding these gradually in the next
PRs and removing the deprecated methods and the Permit* flags at the end of the transition.

Updates tailscale/corp#18342

Signed-off-by: Nick Khyl <nickk@tailscale.com>
This commit is contained in:
Nick Khyl
2024-08-27 15:22:56 -05:00
committed by Nick Khyl
parent 8b23ba7d05
commit 961ee321e8
8 changed files with 333 additions and 215 deletions
Download Patch File Download Diff File Expand all files Collapse all files

112
ipn/localapi/localapi.go
View File

@@ -23,7 +23,6 @@ import (
"net/netip"
"net/url"
"os"
"os/exec"
"path"
"runtime"
"slices"
@@ -60,7 +59,6 @@ import (
"tailscale.com/util/httpm"
"tailscale.com/util/mak"
"tailscale.com/util/osdiag"
"tailscale.com/util/osuser"
"tailscale.com/util/progresstracking"
"tailscale.com/util/rands"
"tailscale.com/util/testenv"
@@ -183,12 +181,8 @@ type Handler struct {
// cert fetching access.
PermitCert bool
// ConnIdentity is the identity of the client connected to the Handler.
ConnIdentity *ipnauth.ConnIdentity
// Test-only override for connIsLocalAdmin method. If non-nil,
// connIsLocalAdmin returns this value.
testConnIsLocalAdmin *bool
// Actor is the identity of the client connected to the Handler.
Actor ipnauth.Actor
b *ipnlocal.LocalBackend
logf logger.Logf
@@ -1065,7 +1059,7 @@ func authorizeServeConfigForGOOSAndUserContext(goos string, configIn *ipn.ServeC
if !configIn.HasPathHandler() {
return nil
}
if h.connIsLocalAdmin() {
if h.Actor.IsLocalAdmin(h.b.OperatorUserID()) {
return nil
}
switch goos {
@@ -1081,104 +1075,6 @@ func authorizeServeConfigForGOOSAndUserContext(goos string, configIn *ipn.ServeC
}
// connIsLocalAdmin reports whether the connected client has administrative
// access to the local machine, for whatever that means with respect to the
// current OS.
//
// This is useful because tailscaled itself always runs with elevated rights:
// we want to avoid privilege escalation for certain mutative operations.
func (h *Handler) connIsLocalAdmin() bool {
if h.testConnIsLocalAdmin != nil {
return *h.testConnIsLocalAdmin
}
if h.ConnIdentity == nil {
h.logf("[unexpected] missing ConnIdentity in LocalAPI Handler")
return false
}
switch runtime.GOOS {
case "windows":
tok, err := h.ConnIdentity.WindowsToken()
if err != nil {
if !errors.Is(err, ipnauth.ErrNotImplemented) {
h.logf("ipnauth.ConnIdentity.WindowsToken() error: %v", err)
}
return false
}
defer tok.Close()
return tok.IsElevated()
case "darwin":
// Unknown, or at least unchecked on sandboxed macOS variants. Err on
// the side of less permissions.
//
// authorizeServeConfigForGOOSAndUserContext should not call
// connIsLocalAdmin on sandboxed variants anyway.
if version.IsSandboxedMacOS() {
return false
}
// This is a standalone tailscaled setup, use the same logic as on
// Linux.
fallthrough
case "linux":
uid, ok := h.ConnIdentity.Creds().UserID()
if !ok {
return false
}
// root is always admin.
if uid == "0" {
return true
}
// if non-root, must be operator AND able to execute "sudo tailscale".
operatorUID := h.b.OperatorUserID()
if operatorUID != "" && uid != operatorUID {
return false
}
u, err := osuser.LookupByUID(uid)
if err != nil {
return false
}
// Short timeout just in case sudo hangs for some reason.
ctx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
defer cancel()
if err := exec.CommandContext(ctx, "sudo", "--other-user="+u.Name, "--list", "tailscale").Run(); err != nil {
return false
}
return true
default:
return false
}
}
func (h *Handler) getUsername() (string, error) {
if h.ConnIdentity == nil {
h.logf("[unexpected] missing ConnIdentity in LocalAPI Handler")
return "", errors.New("missing ConnIdentity")
}
switch runtime.GOOS {
case "windows":
tok, err := h.ConnIdentity.WindowsToken()
if err != nil {
return "", fmt.Errorf("get windows token: %w", err)
}
defer tok.Close()
return tok.Username()
case "darwin", "linux":
uid, ok := h.ConnIdentity.Creds().UserID()
if !ok {
return "", errors.New("missing user ID")
}
u, err := osuser.LookupByUID(uid)
if err != nil {
return "", fmt.Errorf("lookup user: %w", err)
}
return u.Username, nil
default:
return "", errors.New("unsupported OS")
}
}
func (h *Handler) serveCheckIPForwarding(w http.ResponseWriter, r *http.Request) {
if !h.PermitRead {
http.Error(w, "IP forwarding check access denied", http.StatusForbidden)
@@ -2859,7 +2755,7 @@ func (h *Handler) serveShares(w http.ResponseWriter, r *http.Request) {
}
if drive.AllowShareAs() {
// share as the connected user
username, err := h.getUsername()
username, err := h.Actor.Username()
if err != nil {
http.Error(w, err.Error(), http.StatusInternalServerError)
return

20
ipn/localapi/localapi_test.go
View File

@@ -26,6 +26,7 @@ import (
"tailscale.com/client/tailscale/apitype"
"tailscale.com/ipn"
"tailscale.com/ipn/ipnauth"
"tailscale.com/ipn/ipnlocal"
"tailscale.com/ipn/store/mem"
"tailscale.com/tailcfg"
@@ -38,6 +39,23 @@ import (
"tailscale.com/wgengine"
)
var _ ipnauth.Actor = (*testActor)(nil)
type testActor struct {
uid ipn.WindowsUserID
name string
isLocalSystem bool
isLocalAdmin bool
}
func (u *testActor) UserID() ipn.WindowsUserID { return u.uid }
func (u *testActor) Username() (string, error) { return u.name, nil }
func (u *testActor) IsLocalSystem() bool { return u.isLocalSystem }
func (u *testActor) IsLocalAdmin(operatorUID string) bool { return u.isLocalAdmin }
func TestValidHost(t *testing.T) {
tests := []struct {
host string
@@ -189,7 +207,7 @@ func TestWhoIsArgTypes(t *testing.T) {
func TestShouldDenyServeConfigForGOOSAndUserContext(t *testing.T) {
newHandler := func(connIsLocalAdmin bool) *Handler {
return &Handler{testConnIsLocalAdmin: &connIsLocalAdmin}
return &Handler{Actor: &testActor{isLocalAdmin: connIsLocalAdmin}, b: newTestLocalBackend(t)}
}
tests := []struct {
name string