[client,management] Rewrite the SSH feature (#4015)

client/ssh/server/sftp_unix.go

@@ -0,0 +1,71 @@
+//go:build !windows
+
+package server
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"os/exec"
+	"os/user"
+	"strconv"
+
+	"github.com/gliderlabs/ssh"
+	log "github.com/sirupsen/logrus"
+)
+
+// executeSftpWithPrivilegeDrop executes SFTP using Unix privilege dropping
+func (s *Server) executeSftpWithPrivilegeDrop(sess ssh.Session, targetUser *user.User) error {
+	uid, gid, groups, err := s.parseUserCredentials(targetUser)
+	if err != nil {
+		return fmt.Errorf("parse user credentials: %w", err)
+	}
+
+	sftpCmd, err := s.createSftpExecutorCommand(sess, uid, gid, groups, targetUser.HomeDir)
+	if err != nil {
+		return fmt.Errorf("create executor: %w", err)
+	}
+
+	sftpCmd.Stdin = sess
+	sftpCmd.Stdout = sess
+	sftpCmd.Stderr = sess.Stderr()
+
+	log.Tracef("starting SFTP with privilege dropping to user %s (UID=%d, GID=%d)", targetUser.Username, uid, gid)
+
+	if err := sftpCmd.Start(); err != nil {
+		return fmt.Errorf("starting SFTP executor: %w", err)
+	}
+
+	if err := sftpCmd.Wait(); err != nil {
+		var exitError *exec.ExitError
+		if errors.As(err, &exitError) {
+			log.Tracef("SFTP process exited with code %d", exitError.ExitCode())
+			return nil
+		}
+		return fmt.Errorf("exec: %w", err)
+	}
+
+	return nil
+}
+
+// createSftpExecutorCommand creates a command that spawns netbird ssh sftp for privilege dropping
+func (s *Server) createSftpExecutorCommand(sess ssh.Session, uid, gid uint32, groups []uint32, workingDir string) (*exec.Cmd, error) {
+	netbirdPath, err := os.Executable()
+	if err != nil {
+		return nil, err
+	}
+
+	args := []string{
+		"ssh", "sftp",
+		"--uid", strconv.FormatUint(uint64(uid), 10),
+		"--gid", strconv.FormatUint(uint64(gid), 10),
+		"--working-dir", workingDir,
+	}
+
+	for _, group := range groups {
+		args = append(args, "--groups", strconv.FormatUint(uint64(group), 10))
+	}
+
+	log.Tracef("creating SFTP executor command: %s %v", netbirdPath, args)
+	return exec.CommandContext(sess.Context(), netbirdPath, args...), nil
+}