Secure prompt debug filesystem writes

2026-08-13 01:28:49 +00:00
parent a38d291f63
commit 44af91cadf
9 changed files with 421 additions and 107 deletions
--- a/internal/promptdebug/secure_directory_unix.go
+++ b/internal/promptdebug/secure_directory_unix.go
@@ -0,0 +1,196 @@
+//go:build unix
+
+package promptdebug
+
+import (
+	"crypto/rand"
+	"encoding/hex"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"golang.org/x/sys/unix"
+)
+
+// secureDirectory is an opened directory descriptor. Every operation remains
+// relative to that descriptor so later pathname swaps cannot redirect writes.
+type secureDirectory struct {
+	fd int
+}
+
+func openSecureDirectory(path string) (*secureDirectory, error) {
+	if !filepath.IsAbs(path) {
+		return nil, fmt.Errorf("directory must be absolute")
+	}
+	cleaned := filepath.Clean(path)
+	current, err := unix.Open(string(filepath.Separator), unix.O_RDONLY|unix.O_DIRECTORY|unix.O_CLOEXEC, 0)
+	if err != nil {
+		return nil, err
+	}
+	defer func() {
+		if current != -1 {
+			_ = unix.Close(current)
+		}
+	}()
+	for _, component := range strings.Split(strings.TrimPrefix(cleaned, string(filepath.Separator)), string(filepath.Separator)) {
+		if component == "" {
+			continue
+		}
+		next, created, err := openOrCreateSecureDirectory(current, component)
+		if err != nil {
+			return nil, err
+		}
+		if created {
+			if err := unix.Fchmod(next, debugDirectoryMode); err != nil {
+				_ = unix.Close(next)
+				return nil, err
+			}
+		}
+		_ = unix.Close(current)
+		current = next
+	}
+	if err := unix.Fchmod(current, debugDirectoryMode); err != nil {
+		return nil, err
+	}
+	result := &secureDirectory{fd: current}
+	current = -1
+	return result, nil
+}
+
+func (directory *secureDirectory) Close() error {
+	if directory == nil || directory.fd < 0 {
+		return nil
+	}
+	fd := directory.fd
+	directory.fd = -1
+	return unix.Close(fd)
+}
+
+func (directory *secureDirectory) openDirectory(components ...string) (*secureDirectory, error) {
+	if directory == nil || directory.fd < 0 {
+		return nil, fmt.Errorf("prompt debug directory is closed")
+	}
+	current := directory.fd
+	owned := false
+	defer func() {
+		if owned {
+			_ = unix.Close(current)
+		}
+	}()
+	for _, component := range components {
+		if err := validateSecureDirectoryName(component); err != nil {
+			return nil, err
+		}
+		next, _, err := openOrCreateSecureDirectory(current, component)
+		if err != nil {
+			return nil, err
+		}
+		if err := unix.Fchmod(next, debugDirectoryMode); err != nil {
+			_ = unix.Close(next)
+			return nil, err
+		}
+		if owned {
+			_ = unix.Close(current)
+		}
+		current = next
+		owned = true
+	}
+	result := &secureDirectory{fd: current}
+	owned = false
+	return result, nil
+}
+
+func openOrCreateSecureDirectory(parent int, name string) (int, bool, error) {
+	fd, err := unix.Openat(parent, name, unix.O_RDONLY|unix.O_DIRECTORY|unix.O_NOFOLLOW|unix.O_CLOEXEC, 0)
+	if err == nil {
+		return fd, false, nil
+	}
+	if !errors.Is(err, unix.ENOENT) {
+		return -1, false, fmt.Errorf("open secure directory %q: %w", name, err)
+	}
+	if err := unix.Mkdirat(parent, name, debugDirectoryMode); err != nil && !errors.Is(err, unix.EEXIST) {
+		return -1, false, fmt.Errorf("create secure directory %q: %w", name, err)
+	}
+	fd, err = unix.Openat(parent, name, unix.O_RDONLY|unix.O_DIRECTORY|unix.O_NOFOLLOW|unix.O_CLOEXEC, 0)
+	if err != nil {
+		return -1, false, fmt.Errorf("open created secure directory %q: %w", name, err)
+	}
+	return fd, true, nil
+}
+
+func validateSecureDirectoryName(name string) error {
+	if name == "" || name == "." || name == ".." || strings.ContainsRune(name, filepath.Separator) {
+		return fmt.Errorf("prompt debug directory component %q is invalid", name)
+	}
+	return nil
+}
+
+func (directory *secureDirectory) writeJSON(name string, value any) error {
+	if directory == nil || directory.fd < 0 {
+		return fmt.Errorf("prompt debug directory is closed")
+	}
+	if err := validateSecureDirectoryName(name); err != nil {
+		return err
+	}
+	data, err := json.MarshalIndent(value, "", "  ")
+	if err != nil {
+		return fmt.Errorf("marshal prompt debug artifact: %w", err)
+	}
+	var information unix.Stat_t
+	if err := unix.Fstatat(directory.fd, name, &information, unix.AT_SYMLINK_NOFOLLOW); err == nil {
+		if information.Mode&unix.S_IFMT != unix.S_IFREG {
+			return fmt.Errorf("prompt debug file %q is not a regular file", name)
+		}
+	} else if !errors.Is(err, unix.ENOENT) {
+		return fmt.Errorf("inspect prompt debug file %q: %w", name, err)
+	}
+	temporaryName, temporary, err := directory.createTemporaryFile(name)
+	if err != nil {
+		return err
+	}
+	defer func() {
+		if temporary != nil {
+			_ = temporary.Close()
+		}
+		_ = unix.Unlinkat(directory.fd, temporaryName, 0)
+	}()
+	if err := temporary.Chmod(debugFileMode); err != nil {
+		return err
+	}
+	if written, err := temporary.Write(data); err != nil {
+		return err
+	} else if written != len(data) {
+		return io.ErrShortWrite
+	}
+	if err := temporary.Close(); err != nil {
+		return err
+	}
+	temporary = nil
+	if err := unix.Renameat(directory.fd, temporaryName, directory.fd, name); err != nil {
+		return fmt.Errorf("replace prompt debug file %q: %w", name, err)
+	}
+	return nil
+}
+
+func (directory *secureDirectory) createTemporaryFile(name string) (string, *os.File, error) {
+	for attempt := 0; attempt < 16; attempt++ {
+		random := make([]byte, 12)
+		if _, err := rand.Read(random); err != nil {
+			return "", nil, err
+		}
+		temporaryName := "." + name + "." + hex.EncodeToString(random) + ".tmp"
+		fd, err := unix.Openat(directory.fd, temporaryName, unix.O_WRONLY|unix.O_CREAT|unix.O_EXCL|unix.O_CLOEXEC|unix.O_NOFOLLOW, debugFileMode)
+		if errors.Is(err, unix.EEXIST) {
+			continue
+		}
+		if err != nil {
+			return "", nil, fmt.Errorf("create temporary prompt debug file: %w", err)
+		}
+		return temporaryName, os.NewFile(uintptr(fd), temporaryName), nil
+	}
+	return "", nil, fmt.Errorf("create temporary prompt debug file: too many name collisions")
+}