[client] Use setsid to avoid the parent process from being killed via HUP by login (#4900)

client/ssh/server/command_execution_js.go

@@ -42,6 +42,11 @@ func (s *Server) detectSuPtySupport(context.Context) bool {
 	return false
 }
 
+// detectUtilLinuxLogin always returns false on JS/WASM
+func (s *Server) detectUtilLinuxLogin(context.Context) bool {
+	return false
+}
+
 // executeCommandWithPty is not supported on JS/WASM
 func (s *Server) executeCommandWithPty(logger *log.Entry, session ssh.Session, execCmd *exec.Cmd, privilegeResult PrivilegeCheckResult, ptyReq ssh.Pty, winCh <-chan ssh.Window) bool {
 	logger.Errorf("PTY command execution not supported on JS/WASM")

client/ssh/server/command_execution_unix.go

@@ -10,6 +10,7 @@ import (
 	"os"
 	"os/exec"
 	"os/user"
+	"runtime"
 	"strings"
 	"sync"
 	"syscall"
@@ -75,6 +76,29 @@ func (s *Server) detectSuPtySupport(ctx context.Context) bool {
 	return supported
 }
 
+// detectUtilLinuxLogin checks if login is from util-linux (vs shadow-utils).
+// util-linux login uses vhangup() which requires setsid wrapper to avoid killing parent.
+// See https://bugs.debian.org/1078023 for details.
+func (s *Server) detectUtilLinuxLogin(ctx context.Context) bool {
+	if runtime.GOOS != "linux" {
+		return false
+	}
+
+	ctx, cancel := context.WithTimeout(ctx, 500*time.Millisecond)
+	defer cancel()
+
+	cmd := exec.CommandContext(ctx, "login", "--version")
+	output, err := cmd.CombinedOutput()
+	if err != nil {
+		log.Debugf("login --version failed (likely shadow-utils): %v", err)
+		return false
+	}
+
+	isUtilLinux := strings.Contains(string(output), "util-linux")
+	log.Debugf("util-linux login detected: %v", isUtilLinux)
+	return isUtilLinux
+}
+
 // createSuCommand creates a command using su -l -c for privilege switching
 func (s *Server) createSuCommand(session ssh.Session, localUser *user.User, hasPty bool) (*exec.Cmd, error) {
 	suPath, err := exec.LookPath("su")
@@ -144,7 +168,7 @@ func (s *Server) handlePty(logger *log.Entry, session ssh.Session, privilegeResu
 		return false
 	}
 
-	logger.Infof("starting interactive shell: %s", execCmd.Path)
+	logger.Infof("starting interactive shell: %s", strings.Join(execCmd.Args, " "))
 	return s.runPtyCommand(logger, session, execCmd, ptyReq, winCh)
 }
 

client/ssh/server/command_execution_windows.go

@@ -383,6 +383,11 @@ func (s *Server) detectSuPtySupport(context.Context) bool {
 	return false
 }
 
+// detectUtilLinuxLogin always returns false on Windows
+func (s *Server) detectUtilLinuxLogin(context.Context) bool {
+	return false
+}
+
 // executeCommandWithPty executes a command with PTY allocation on Windows using ConPty
 func (s *Server) executeCommandWithPty(logger *log.Entry, session ssh.Session, execCmd *exec.Cmd, privilegeResult PrivilegeCheckResult, ptyReq ssh.Pty, winCh <-chan ssh.Window) bool {
 	command := session.RawCommand()

client/ssh/server/server.go

@@ -138,7 +138,8 @@ type Server struct {
 	jwtExtractor *jwt.ClaimsExtractor
 	jwtConfig    *JWTConfig
 
-	suSupportsPty bool
+	suSupportsPty    bool
+	loginIsUtilLinux bool
 }
 
 type JWTConfig struct {
@@ -193,6 +194,7 @@ func (s *Server) Start(ctx context.Context, addr netip.AddrPort) error {
 	}
 
 	s.suSupportsPty = s.detectSuPtySupport(ctx)
+	s.loginIsUtilLinux = s.detectUtilLinuxLogin(ctx)
 
 	ln, addrDesc, err := s.createListener(ctx, addr)
 	if err != nil {

client/ssh/server/userswitching_unix.go

@@ -87,11 +87,8 @@ func (s *Server) getLoginCmd(username string, remoteAddr net.Addr) (string, []st
 
 	switch runtime.GOOS {
 	case "linux":
-		// Special handling for Arch Linux without /etc/pam.d/remote
+		p, a := s.getLinuxLoginCmd(loginPath, username, addrPort.Addr().String())
-		if s.fileExists("/etc/arch-release") && !s.fileExists("/etc/pam.d/remote") {
+		return p, a, nil
-			return loginPath, []string{"-f", username, "-p"}, nil
-		}
-		return loginPath, []string{"-f", username, "-h", addrPort.Addr().String(), "-p"}, nil
 	case "darwin", "freebsd", "openbsd", "netbsd", "dragonfly":
 		return loginPath, []string{"-fp", "-h", addrPort.Addr().String(), username}, nil
 	default:
@@ -99,7 +96,37 @@ func (s *Server) getLoginCmd(username string, remoteAddr net.Addr) (string, []st
 	}
 }
 
-// fileExists checks if a file exists (helper for login command logic)
+// getLinuxLoginCmd returns the login command for Linux systems.
+// Handles differences between util-linux and shadow-utils login implementations.
+func (s *Server) getLinuxLoginCmd(loginPath, username, remoteIP string) (string, []string) {
+	// Special handling for Arch Linux without /etc/pam.d/remote
+	var loginArgs []string
+	if s.fileExists("/etc/arch-release") && !s.fileExists("/etc/pam.d/remote") {
+		loginArgs = []string{"-f", username, "-p"}
+	} else {
+		loginArgs = []string{"-f", username, "-h", remoteIP, "-p"}
+	}
+
+	// util-linux login requires setsid -c to create a new session and set the
+	// controlling terminal. Without this, vhangup() kills the parent process.
+	// See https://bugs.debian.org/1078023 for details.
+	// TODO: handle this via the executor using syscall.Setsid() + TIOCSCTTY + syscall.Exec()
+	// to avoid external setsid dependency.
+	if !s.loginIsUtilLinux {
+		return loginPath, loginArgs
+	}
+
+	setsidPath, err := exec.LookPath("setsid")
+	if err != nil {
+		log.Warnf("setsid not available but util-linux login detected, login may fail: %v", err)
+		return loginPath, loginArgs
+	}
+
+	args := append([]string{"-w", "-c", loginPath}, loginArgs...)
+	return setsidPath, args
+}
+
+// fileExists checks if a file exists
 func (s *Server) fileExists(path string) bool {
 	_, err := os.Stat(path)
 	return err == nil