fetch_ml/internal/audit/audit.go

package audit

import (
	"bufio"
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"fmt"
	"os"
	"path/filepath"
	"strings"
	"sync"
	"time"

	"github.com/jfraeys/fetch_ml/internal/logging"
)

// EventType represents the type of audit event
type EventType string

const (
	EventAuthAttempt       EventType = "authentication_attempt"
	EventAuthSuccess       EventType = "authentication_success"
	EventAuthFailure       EventType = "authentication_failure"
	EventJobQueued         EventType = "job_queued"
	EventJobStarted        EventType = "job_started"
	EventJobCompleted      EventType = "job_completed"
	EventJobFailed         EventType = "job_failed"
	EventJupyterStart      EventType = "jupyter_start"
	EventJupyterStop       EventType = "jupyter_stop"
	EventExperimentCreated EventType = "experiment_created"
	EventExperimentDeleted EventType = "experiment_deleted"

	// HIPAA-specific file access events
	EventFileRead      EventType = "file_read"
	EventFileWrite     EventType = "file_write"
	EventFileDelete    EventType = "file_delete"
	EventDatasetAccess EventType = "dataset_access"
)

// Event represents an audit log event with integrity chain.
// SECURITY NOTE: Metadata uses map[string]any which relies on Go 1.20+'s
// guaranteed stable JSON key ordering for hash determinism. If you need to
// hash events externally, ensure the same ordering is used, or exclude
// Metadata from the hashed portion.
type Event struct {
	Timestamp   time.Time      `json:"timestamp"`
	Metadata    map[string]any `json:"metadata,omitempty"`
	EventType   EventType      `json:"event_type"`
	UserID      string         `json:"user_id,omitempty"`
	IPAddress   string         `json:"ip_address,omitempty"`
	Resource    string         `json:"resource,omitempty"`
	Action      string         `json:"action,omitempty"`
	ErrorMsg    string         `json:"error,omitempty"`
	PrevHash    string         `json:"prev_hash,omitempty"`
	EventHash   string         `json:"event_hash,omitempty"`
	SequenceNum int64          `json:"sequence_num,omitempty"`
	Success     bool           `json:"success"`
}

// Logger handles audit logging with integrity chain
type Logger struct {
	file        *os.File
	logger      *logging.Logger
	filePath    string
	lastHash    string
	sequenceNum int64
	mu          sync.Mutex
	enabled     bool
}

// NewLogger creates a new audit logger with secure path validation.
// It validates the filePath for path traversal, symlink attacks, and ensures
// it stays within the base directory (/var/lib/fetch_ml/audit).
func NewLogger(enabled bool, filePath string, logger *logging.Logger) (*Logger, error) {
	return NewLoggerWithBase(enabled, filePath, logger, "/var/lib/fetch_ml/audit")
}

// NewLoggerWithBase creates a new audit logger with a configurable base directory.
// This is useful for testing. For production, use NewLogger which uses the default base.
func NewLoggerWithBase(enabled bool, filePath string, logger *logging.Logger, baseDir string) (*Logger, error) {
	al := &Logger{
		enabled: enabled,
		logger:  logger,
	}

	if !enabled || filePath == "" {
		return al, nil
	}

	// Use secure path validation
	fullPath, err := validateAndSecurePath(filePath, baseDir)
	if err != nil {
		return nil, fmt.Errorf("invalid audit log path: %w", err)
	}

	// Check if file is a symlink (security check)
	if err := checkFileNotSymlink(fullPath); err != nil {
		return nil, fmt.Errorf("audit log security check failed: %w", err)
	}

	if err := os.MkdirAll(filepath.Dir(fullPath), 0o700); err != nil {
		return nil, fmt.Errorf("failed to create audit directory: %w", err)
	}

	file, err := os.OpenFile(fullPath, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0o600)
	if err != nil {
		return nil, fmt.Errorf("failed to open audit log file: %w", err)
	}

	al.file = file
	al.filePath = fullPath

	// Restore chain state from existing log to prevent integrity break on restart
	if err := al.resumeFromFile(); err != nil {
		file.Close()
		return nil, fmt.Errorf("failed to resume audit chain: %w", err)
	}

	return al, nil
}

// Log logs an audit event with integrity chain
func (al *Logger) Log(event Event) {
	if !al.enabled {
		return
	}

	event.Timestamp = time.Now().UTC()

	al.mu.Lock()
	defer al.mu.Unlock()

	// Set sequence number and previous hash for integrity chain
	al.sequenceNum++
	event.SequenceNum = al.sequenceNum
	event.PrevHash = al.lastHash

	// Calculate hash of this event for tamper evidence
	event.EventHash = al.CalculateEventHash(event)
	al.lastHash = event.EventHash

	// Marshal to JSON
	data, err := json.Marshal(event)
	if err != nil {
		if al.logger != nil {
			al.logger.Error("failed to marshal audit event", "error", err)
		}
		return
	}

	// Write to file if configured
	if al.file != nil {
		_, err = al.file.Write(append(data, '\n'))
		if err != nil && al.logger != nil {
			al.logger.Error("failed to write audit event", "error", err)
		}
		// fsync ensures data is flushed to disk before updating hash in memory.
		// Critical for crash safety: prevents chain inconsistency if system
		// crashes after hash advance but before write completion.
		if err == nil {
			if syncErr := al.file.Sync(); syncErr != nil && al.logger != nil {
				al.logger.Error("failed to sync audit log", "error", syncErr)
			}
		}
	}

	hashPreview := event.EventHash
	if len(hashPreview) > 16 {
		hashPreview = hashPreview[:16]
	}

	// Also log via structured logger
	if al.logger != nil {
		al.logger.Info("audit_event",
			"event_type", event.EventType,
			"user_id", event.UserID,
			"resource", event.Resource,
			"success", event.Success,
			"seq", event.SequenceNum,
			"hash", hashPreview,
		)
	}
}

// CalculateEventHash computes SHA-256 hash of event data for integrity chain
// Exported for testing purposes
func (al *Logger) CalculateEventHash(event Event) string {
	eventCopy := event
	eventCopy.EventHash = "" // keep PrevHash for chaining

	data, err := json.Marshal(eventCopy)
	if err != nil {
		fallback := fmt.Sprintf(
			"%s:%s:%d:%s",
			event.Timestamp.UTC().Format(time.RFC3339Nano),
			event.EventType,
			event.SequenceNum,
			event.PrevHash,
		)
		data = []byte(fallback)
	}

	hash := sha256.Sum256(data)
	return hex.EncodeToString(hash[:])
}

// LogFileAccess logs a file access operation (HIPAA requirement)
func (al *Logger) LogFileAccess(
	eventType EventType,
	userID, filePath, ipAddr string,
	success bool,
	errMsg string,
) {
	var action string

	switch eventType {
	case EventFileRead:
		action = "read"
	case EventFileWrite:
		action = "write"
	case EventFileDelete:
		action = "delete"
	case EventDatasetAccess:
		action = "dataset_access"
	default:
		// Defensive: prevent silent misclassification
		if al.logger != nil {
			al.logger.Error(
				"invalid file access event type",
				"event_type", eventType,
			)
		}
		return
	}

	al.Log(Event{
		EventType: eventType,
		UserID:    userID,
		IPAddress: ipAddr,
		Resource:  filePath,
		Action:    action,
		Success:   success,
		ErrorMsg:  errMsg,
	})
}

// VerifyChain checks the integrity of the audit log chain.
// The events slice must be provided in ascending sequence order.
// Returns the first sequence number where tampering is detected, or -1 if valid.
func (al *Logger) VerifyChain(events []Event) (tamperedSeq int, err error) {
	if len(events) == 0 {
		return -1, nil
	}

	var expectedPrevHash string

	for i, event := range events {
		// Enforce strict sequence ordering (events must be sorted by SequenceNum)
		if event.SequenceNum != int64(i+1) {
			return int(event.SequenceNum), fmt.Errorf(
				"sequence mismatch: expected %d, got %d",
				i+1, event.SequenceNum,
			)
		}

		if i == 0 {
			if event.PrevHash != "" {
				return int(event.SequenceNum), fmt.Errorf(
					"first event must have empty prev_hash",
				)
			}
			// Explicit check: first event must have SequenceNum == 1
			if event.SequenceNum != 1 {
				return int(event.SequenceNum), fmt.Errorf(
					"first event must have sequence_num=1, got %d",
					event.SequenceNum,
				)
			}
		} else {
			if event.PrevHash != expectedPrevHash {
				return int(event.SequenceNum), fmt.Errorf(
					"chain break at sequence %d",
					event.SequenceNum,
				)
			}
		}

		expectedHash := al.CalculateEventHash(event)
		if event.EventHash != expectedHash {
			return int(event.SequenceNum), fmt.Errorf(
				"hash mismatch at sequence %d",
				event.SequenceNum,
			)
		}

		expectedPrevHash = event.EventHash
	}

	return -1, nil
}

// LogAuthAttempt logs an authentication attempt
func (al *Logger) LogAuthAttempt(userID, ipAddr string, success bool, errMsg string) {
	eventType := EventAuthSuccess
	if !success {
		eventType = EventAuthFailure
	}

	al.Log(Event{
		EventType: eventType,
		UserID:    userID,
		IPAddress: ipAddr,
		Success:   success,
		ErrorMsg:  errMsg,
	})
}

// LogJobOperation logs a job-related operation
func (al *Logger) LogJobOperation(
	eventType EventType,
	userID, jobID, ipAddr string,
	success bool,
	errMsg string,
) {
	al.Log(Event{
		EventType: eventType,
		UserID:    userID,
		IPAddress: ipAddr,
		Resource:  jobID,
		Action:    "job_operation",
		Success:   success,
		ErrorMsg:  errMsg,
	})
}

// LogJupyterOperation logs a Jupyter service operation
func (al *Logger) LogJupyterOperation(
	eventType EventType,
	userID, serviceID, ipAddr string,
	success bool,
	errMsg string,
) {
	al.Log(Event{
		EventType: eventType,
		UserID:    userID,
		IPAddress: ipAddr,
		Resource:  serviceID,
		Action:    "jupyter_operation",
		Success:   success,
		ErrorMsg:  errMsg,
	})
}

// Close closes the audit logger
func (al *Logger) Close() error {
	al.mu.Lock()
	defer al.mu.Unlock()

	if al.file != nil {
		return al.file.Close()
	}
	return nil
}

// resumeFromFile reads the last entry from the audit log file and restores
// the chain state (sequenceNum and lastHash) to prevent chain reset on restart.
// This is critical for tamper-evident logging integrity.
func (al *Logger) resumeFromFile() error {
	if al.file == nil {
		return nil
	}

	// Open file for reading to get the last entry
	file, err := os.Open(al.filePath)
	if err != nil {
		return fmt.Errorf("failed to open audit log for resume: %w", err)
	}
	defer file.Close()

	var lastEvent Event
	scanner := bufio.NewScanner(file)
	lineNum := 0

	for scanner.Scan() {
		lineNum++
		line := scanner.Text()
		if line == "" {
			continue
		}

		var event Event
		if err := json.Unmarshal([]byte(line), &event); err != nil {
			// Corrupted line - log but continue
			if al.logger != nil {
				al.logger.Warn("corrupted audit log entry during resume",
					"line", lineNum,
					"error", err)
			}
			continue
		}
		lastEvent = event
	}

	if err := scanner.Err(); err != nil {
		return fmt.Errorf("error reading audit log during resume: %w", err)
	}

	// Restore chain state from last valid event
	if lastEvent.SequenceNum > 0 {
		al.sequenceNum = lastEvent.SequenceNum
		al.lastHash = lastEvent.EventHash
		if al.logger != nil {
			al.logger.Info("audit chain resumed",
				"sequence", al.sequenceNum,
				"hash_preview", truncateHash(al.lastHash, 16))
		}
	}

	return nil
}

// truncateHash returns a truncated hash string for logging (safe preview)
func truncateHash(hash string, maxLen int) string {
	if len(hash) <= maxLen {
		return hash
	}
	return hash[:maxLen]
}

// validateAndSecurePath validates a file path for security issues.
// It checks for path traversal, symlinks, and ensures the path stays within baseDir.
func validateAndSecurePath(filePath, baseDir string) (string, error) {
	// Reject absolute paths
	if filepath.IsAbs(filePath) {
		return "", fmt.Errorf("absolute paths not allowed: %s", filePath)
	}

	// Clean the path to resolve any . or .. components
	cleanPath := filepath.Clean(filePath)

	// Check for path traversal attempts after cleaning
	// If the path starts with .., it's trying to escape
	if strings.HasPrefix(cleanPath, "..") {
		return "", fmt.Errorf("path traversal attempt detected: %s", filePath)
	}

	// Resolve base directory symlinks (critical for security)
	resolvedBase, err := filepath.EvalSymlinks(baseDir)
	if err != nil {
		// Base may not exist yet, use as-is but this is less secure
		resolvedBase = baseDir
	}

	// Construct full path
	fullPath := filepath.Join(resolvedBase, cleanPath)

	// Resolve any symlinks in the full path
	resolvedPath, err := filepath.EvalSymlinks(fullPath)
	if err != nil {
		// File doesn't exist yet - check parent directory
		parent := filepath.Dir(fullPath)
		resolvedParent, err := filepath.EvalSymlinks(parent)
		if err != nil {
			// Parent doesn't exist - validate the path itself
			// Check that the path stays within base directory
			if !strings.HasPrefix(fullPath, resolvedBase+string(os.PathSeparator)) &&
				fullPath != resolvedBase {
				return "", fmt.Errorf("path escapes base directory: %s", filePath)
			}
			resolvedPath = fullPath
		} else {
			// Parent resolved - verify it's still within base
			if !strings.HasPrefix(resolvedParent, resolvedBase) {
				return "", fmt.Errorf("parent directory escapes base: %s", filePath)
			}
			// Reconstruct path with resolved parent
			base := filepath.Base(fullPath)
			resolvedPath = filepath.Join(resolvedParent, base)
		}
	}

	// Final verification: resolved path must be within base directory
	if !strings.HasPrefix(resolvedPath, resolvedBase+string(os.PathSeparator)) &&
		resolvedPath != resolvedBase {
		return "", fmt.Errorf("path escapes base directory after symlink resolution: %s", filePath)
	}

	return resolvedPath, nil
}

// checkFileNotSymlink verifies that the given path is not a symlink
func checkFileNotSymlink(path string) error {
	info, err := os.Lstat(path)
	if err != nil {
		if os.IsNotExist(err) {
			return nil // File doesn't exist, can't be a symlink
		}
		return fmt.Errorf("failed to stat file: %w", err)
	}

	if info.Mode()&os.ModeSymlink != 0 {
		return fmt.Errorf("file is a symlink: %s", path)
	}

	return nil
}