Monorepo for Tangled
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import ( "context" "errors" "fmt" "io" "log/slog" "net/http" "net/url" "os" "os/exec" "strings" "syscall"
securejoin "github.com/cyphar/filepath-securejoin" "github.com/urfave/cli/v3" "tangled.org/core/knotserver/sandbox" "tangled.org/core/log")
func Command() *cli.Command { return &cli.Command{ Name: "guard", Usage: "role-based access control for git over ssh (not for manual use)", Action: Run, Flags: []cli.Flag{ &cli.StringFlag{ Name: "user", Usage: "allowed git user", Required: true, }, &cli.StringFlag{ Name: "git-dir", Usage: "base directory for git repos", Value: "/home/git", }, &cli.StringFlag{ Name: "log-path", Usage: "path to log file", Value: "/home/git/guard.log", }, &cli.StringFlag{ Name: "internal-api", Usage: "internal API endpoint", Value: "http://localhost:5444", }, &cli.StringFlag{ Name: "motd-file", Usage: "path to message of the day file", Value: "/home/git/motd", }, &cli.BoolFlag{ Name: "secure-mode", Usage: "isolate git subprocesses to their own repository directory", }, }, }}
func Run(ctx context.Context, cmd *cli.Command) error { l := log.FromContext(ctx)
incomingUser := cmd.String("user") gitDir := cmd.String("git-dir") logPath := cmd.String("log-path") endpoint := cmd.String("internal-api") motdFile := cmd.String("motd-file") secureMode := cmd.Bool("secure-mode")
logFile, err := os.OpenFile(logPath, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644) if err != nil { l.Error("failed to open log file", "error", err) return err } else { fileHandler := slog.NewJSONHandler(logFile, &slog.HandlerOptions{Level: slog.LevelInfo}) l = slog.New(fileHandler) }
var clientIP string if connInfo := os.Getenv("SSH_CONNECTION"); connInfo != "" { parts := strings.Fields(connInfo) if len(parts) > 0 { clientIP = parts[0] } }
if incomingUser == "" { l.Error("access denied: no user specified") fmt.Fprintln(os.Stderr, "access denied: no user specified") os.Exit(-1) }
sshCommand := os.Getenv("SSH_ORIGINAL_COMMAND")
l.Info("connection attempt", "user", incomingUser, "command", sshCommand, "client", clientIP)
// TODO: greet user with their resolved handle instead of did if sshCommand == "" { l.Info("access denied: no interactive shells", "user", incomingUser) fmt.Fprintf(os.Stderr, "Hi @%s! You've successfully authenticated.\n", incomingUser) os.Exit(-1) }
cmdParts := strings.Fields(sshCommand) if len(cmdParts) < 2 { l.Error("invalid command format", "command", sshCommand) fmt.Fprintln(os.Stderr, "invalid command format") os.Exit(-1) }
gitCommand := cmdParts[0] repoPath := cmdParts[1]
validCommands := map[string]bool{ "git-receive-pack": true, "git-upload-pack": true, "git-upload-archive": true, } if !validCommands[gitCommand] { l.Error("access denied: invalid git command", "command", gitCommand) fmt.Fprintln(os.Stderr, "access denied: invalid git command") return fmt.Errorf("access denied: invalid git command") }
// qualify repo path from internal server which holds the knot config qualifiedRepoPath, err := guardAndQualifyRepo(l, endpoint, incomingUser, repoPath, gitCommand) if err != nil { l.Error("failed to run guard", "err", err) fmt.Fprintln(os.Stderr, err) os.Exit(1) }
fullPath, _ := securejoin.SecureJoin(gitDir, qualifiedRepoPath)
l.Info("processing command", "user", incomingUser, "command", gitCommand, "repo", repoPath, "fullPath", fullPath, "client", clientIP)
var motdReader io.Reader if reader, err := os.Open(motdFile); err != nil { if !errors.Is(err, os.ErrNotExist) { l.Error("failed to read motd file", "error", err) } motdReader = strings.NewReader("Welcome to this knot!\n") } else { motdReader = reader } if gitCommand == "git-upload-pack" { io.WriteString(os.Stderr, "\x02") } io.Copy(os.Stderr, motdReader)
gitCmd := exec.Command(gitCommand, fullPath) gitCmd.Stdout = os.Stdout gitCmd.Stderr = os.Stderr gitCmd.Stdin = os.Stdin gitCmd.Env = append(os.Environ(), fmt.Sprintf("GIT_USER_DID=%s", incomingUser), )
if secureMode { sb, warn := sandbox.New(func(repoPath string) (uint32, uint32, error) { return sandbox.LookupUIDForRepoPath(gitDir, repoPath) }) if warn != "" { l.Warn("secure-mode: sandbox degraded", "reason", warn) } else { l.Info("secure-mode: wrapping git command", "backend", sb.Name()) } wrapped, wrapErr := sb.Wrap(fullPath, gitCmd) if wrapErr != nil { l.Error("sandbox wrap failed", "error", wrapErr) } else { gitCmd = wrapped } }
if gitCmd.SysProcAttr == nil { gitCmd.SysProcAttr = &syscall.SysProcAttr{Setpgid: true} }
if err := gitCmd.Run(); err != nil { l.Error("command failed", "error", err) fmt.Fprintf(os.Stderr, "command failed: %v\n", err) return fmt.Errorf("command failed: %v", err) }
l.Info("command completed", "user", incomingUser, "command", gitCommand, "repo", repoPath, "success", true)
return nil}
// runs guardAndQualifyRepo logicfunc guardAndQualifyRepo(l *slog.Logger, endpoint, incomingUser, repo, gitCommand string) (string, error) { u, _ := url.Parse(endpoint + "/guard") q := u.Query() q.Add("user", incomingUser) q.Add("repo", repo) q.Add("gitCmd", gitCommand) u.RawQuery = q.Encode()
resp, err := http.Get(u.String()) if err != nil { return "", err } defer resp.Body.Close()
l.Info("Running guard", "url", u.String(), "status", resp.Status)
body, err := io.ReadAll(resp.Body) if err != nil { return "", err } text := string(body)
switch resp.StatusCode { case http.StatusOK: return text, nil case http.StatusForbidden: l.Error("access denied: user not allowed", "did", incomingUser, "reponame", text) return text, errors.New("access denied: user not allowed") default: return "", errors.New(text) }}