Reject Tailscale SSH user switch without privilege

protocol/tailscale/tailssh/server_helper_unix.go

@@ -11,6 +11,7 @@ import (
 
 	gliderssh "github.com/sagernet/gliderssh"
 	"github.com/sagernet/sing-box/adapter"
+	E "github.com/sagernet/sing/common/exceptions"
 )
 
 func isPrivilegedUser() bool {
@@ -21,10 +22,21 @@ func requestedUserMatchesProcess(localUser *adapter.PlatformUser) (bool, error)
 	return localUser.Uid == os.Getuid() && localUser.Gid == os.Getgid(), nil
 }
 
-// verifyShellIdentity is a no-op on Unix: spawned shells and sftp-server drop to the
-// requested user via setCredential, so the child already runs as that user.
-func verifyShellIdentity(_ adapter.PlatformInterface, _ *adapter.PlatformUser) error {
-	return nil
+func verifyShellIdentity(platformInterface adapter.PlatformInterface, localUser *adapter.PlatformUser) error {
+	if platformInterface != nil && platformInterface.UsePlatformShell() {
+		return nil
+	}
+	match, err := requestedUserMatchesProcess(localUser)
+	if err != nil {
+		return err
+	}
+	if match {
+		return nil
+	}
+	if !isPrivilegedUser() {
+		return E.New("switching to local user ", localUser.Username, " requires sing-box to run as root")
+	}
+	return checkSwitchUserPrivilege(localUser)
 }
 
 func systemHostKeyPath() string {

protocol/tailscale/tailssh/switch_user_linux.go

@@ -0,0 +1,24 @@
+//go:build with_gvisor && linux
+
+package tailssh
+
+import (
+	"github.com/sagernet/sing-box/adapter"
+	E "github.com/sagernet/sing/common/exceptions"
+
+	"golang.org/x/sys/unix"
+)
+
+func checkSwitchUserPrivilege(localUser *adapter.PlatformUser) error {
+	header := unix.CapUserHeader{Version: unix.LINUX_CAPABILITY_VERSION_3}
+	var data [2]unix.CapUserData
+	err := unix.Capget(&header, &data[0])
+	if err != nil {
+		return E.Cause(err, "query process capabilities")
+	}
+	const required = 1<<unix.CAP_SETUID | 1<<unix.CAP_SETGID
+	if data[0].Effective&required != required {
+		return E.New("switching to local user ", localUser.Username, " requires CAP_SETUID and CAP_SETGID")
+	}
+	return nil
+}

protocol/tailscale/tailssh/switch_user_other.go

@@ -0,0 +1,9 @@
+//go:build with_gvisor && !windows && !linux
+
+package tailssh
+
+import "github.com/sagernet/sing-box/adapter"
+
+func checkSwitchUserPrivilege(_ *adapter.PlatformUser) error {
+	return nil
+}