feat: add defaultDenyRead mode for strict filesystem isolation (#24)

internal/config/config.go

@@ -37,6 +37,8 @@ type NetworkConfig struct {
 
 // FilesystemConfig defines filesystem restrictions.
 type FilesystemConfig struct {
+	DefaultDenyRead bool     `json:"defaultDenyRead,omitempty"` // If true, deny reads by default except system paths and AllowRead
+	AllowRead       []string `json:"allowRead"`                 // Paths to allow reading (used when DefaultDenyRead is true)
 	DenyRead        []string `json:"denyRead"`
 	AllowWrite      []string `json:"allowWrite"`
 	DenyWrite       []string `json:"denyWrite"`
@@ -179,6 +181,9 @@ func (c *Config) Validate() error {
 		}
 	}
 
+	if slices.Contains(c.Filesystem.AllowRead, "") {
+		return errors.New("filesystem.allowRead contains empty path")
+	}
 	if slices.Contains(c.Filesystem.DenyRead, "") {
 		return errors.New("filesystem.denyRead contains empty path")
 	}
@@ -427,7 +432,11 @@ func Merge(base, override *Config) *Config {
 		},
 
 		Filesystem: FilesystemConfig{
+			// Boolean fields: true if either enables it
+			DefaultDenyRead: base.Filesystem.DefaultDenyRead || override.Filesystem.DefaultDenyRead,
+
 			// Append slices
+			AllowRead:  mergeStrings(base.Filesystem.AllowRead, override.Filesystem.AllowRead),
 			DenyRead:   mergeStrings(base.Filesystem.DenyRead, override.Filesystem.DenyRead),
 			AllowWrite: mergeStrings(base.Filesystem.AllowWrite, override.Filesystem.AllowWrite),
 			DenyWrite:  mergeStrings(base.Filesystem.DenyWrite, override.Filesystem.DenyWrite),

internal/config/config_test.go

@@ -111,6 +111,15 @@ func TestConfigValidate(t *testing.T) {
 			},
 			wantErr: true,
 		},
+		{
+			name: "empty allowRead path",
+			config: Config{
+				Filesystem: FilesystemConfig{
+					AllowRead: []string{""},
+				},
+			},
+			wantErr: true,
+		},
 		{
 			name: "empty denyRead path",
 			config: Config{
@@ -453,6 +462,50 @@ func TestMerge(t *testing.T) {
 		}
 	})
 
+	t.Run("merge defaultDenyRead and allowRead", func(t *testing.T) {
+		base := &Config{
+			Filesystem: FilesystemConfig{
+				DefaultDenyRead: true,
+				AllowRead:       []string{"/home/user/project"},
+			},
+		}
+		override := &Config{
+			Filesystem: FilesystemConfig{
+				AllowRead: []string{"/home/user/other"},
+			},
+		}
+		result := Merge(base, override)
+
+		if !result.Filesystem.DefaultDenyRead {
+			t.Error("expected DefaultDenyRead to be true (from base)")
+		}
+		if len(result.Filesystem.AllowRead) != 2 {
+			t.Errorf("expected 2 allowRead paths, got %d: %v", len(result.Filesystem.AllowRead), result.Filesystem.AllowRead)
+		}
+	})
+
+	t.Run("merge defaultDenyRead from override", func(t *testing.T) {
+		base := &Config{
+			Filesystem: FilesystemConfig{
+				DefaultDenyRead: false,
+			},
+		}
+		override := &Config{
+			Filesystem: FilesystemConfig{
+				DefaultDenyRead: true,
+				AllowRead:       []string{"/home/user/project"},
+			},
+		}
+		result := Merge(base, override)
+
+		if !result.Filesystem.DefaultDenyRead {
+			t.Error("expected DefaultDenyRead to be true (from override)")
+		}
+		if len(result.Filesystem.AllowRead) != 1 {
+			t.Errorf("expected 1 allowRead path, got %d", len(result.Filesystem.AllowRead))
+		}
+	})
+
 	t.Run("override ports", func(t *testing.T) {
 		base := &Config{
 			Network: NetworkConfig{

internal/sandbox/dangerous.go

@@ -53,6 +53,102 @@ func GetDefaultWritePaths() []string {
 	return paths
 }
 
+// GetDefaultReadablePaths returns paths that should remain readable when defaultDenyRead is enabled.
+// These are essential system paths needed for most programs to run.
+//
+// Note on user tooling paths: Version managers like nvm, pyenv, etc. require read access to their
+// entire installation directories (not just bin/) because runtimes need to load libraries and
+// modules from these paths. For example, Node.js needs to read ~/.nvm/versions/.../lib/ to load
+// globally installed packages. This is a trade-off between functionality and strict isolation.
+// Users who need tighter control can use denyRead to block specific subpaths within these directories.
+func GetDefaultReadablePaths() []string {
+	home, _ := os.UserHomeDir()
+
+	paths := []string{
+		// Core system paths
+		"/bin",
+		"/sbin",
+		"/usr",
+		"/lib",
+		"/lib64",
+
+		// System configuration (needed for DNS, SSL, locale, etc.)
+		"/etc",
+
+		// Proc filesystem (needed for process info)
+		"/proc",
+
+		// Sys filesystem (needed for system info)
+		"/sys",
+
+		// Device nodes
+		"/dev",
+
+		// macOS specific
+		"/System",
+		"/Library",
+		"/Applications",
+		"/private/etc",
+		"/private/var/db",
+		"/private/var/run",
+
+		// Linux distributions may have these
+		"/opt",
+		"/run",
+
+		// Temp directories (needed for many operations)
+		"/tmp",
+		"/private/tmp",
+
+		// Common package manager paths
+		"/usr/local",
+		"/opt/homebrew",
+		"/nix",
+		"/snap",
+	}
+
+	// User-installed tooling paths. These version managers and language runtimes need
+	// read access to their full directories (not just bin/) to function properly.
+	// Runtimes load libraries, modules, and configs from within these directories.
+	if home != "" {
+		paths = append(paths,
+			// Node.js version managers (need lib/ for global packages)
+			filepath.Join(home, ".nvm"),
+			filepath.Join(home, ".fnm"),
+			filepath.Join(home, ".volta"),
+			filepath.Join(home, ".n"),
+
+			// Python version managers (need lib/ for installed packages)
+			filepath.Join(home, ".pyenv"),
+			filepath.Join(home, ".local/pipx"),
+
+			// Ruby version managers (need lib/ for gems)
+			filepath.Join(home, ".rbenv"),
+			filepath.Join(home, ".rvm"),
+
+			// Rust (bin only - cargo doesn't need full .cargo for execution)
+			filepath.Join(home, ".cargo/bin"),
+			filepath.Join(home, ".rustup"),
+
+			// Go (bin only)
+			filepath.Join(home, "go/bin"),
+			filepath.Join(home, ".go"),
+
+			// User local binaries (bin only)
+			filepath.Join(home, ".local/bin"),
+			filepath.Join(home, "bin"),
+
+			// Bun (bin only)
+			filepath.Join(home, ".bun/bin"),
+
+			// Deno (bin only)
+			filepath.Join(home, ".deno/bin"),
+		)
+	}
+
+	return paths
+}
+
 // GetMandatoryDenyPatterns returns glob patterns for paths that must always be protected.
 func GetMandatoryDenyPatterns(cwd string, allowGitConfig bool) []string {
 	var patterns []string

internal/sandbox/linux.go

@@ -355,8 +355,52 @@ func WrapCommandLinuxWithOptions(cfg *config.Config, command string, bridge *Lin
 		}
 	}
 
-	// Start with read-only root filesystem (default deny writes)
+	defaultDenyRead := cfg != nil && cfg.Filesystem.DefaultDenyRead
+
+	if defaultDenyRead {
+		// In defaultDenyRead mode, we only bind essential system paths read-only
+		// and user-specified allowRead paths. Everything else is inaccessible.
+		if opts.Debug {
+			fmt.Fprintf(os.Stderr, "[fence:linux] DefaultDenyRead mode enabled - binding only essential system paths\n")
+		}
+
+		// Bind essential system paths read-only
+		// Skip /dev, /proc, /tmp as they're mounted with special options below
+		for _, systemPath := range GetDefaultReadablePaths() {
+			if systemPath == "/dev" || systemPath == "/proc" || systemPath == "/tmp" ||
+				systemPath == "/private/tmp" {
+				continue
+			}
+			if fileExists(systemPath) {
+				bwrapArgs = append(bwrapArgs, "--ro-bind", systemPath, systemPath)
+			}
+		}
+
+		// Bind user-specified allowRead paths
+		if cfg != nil && cfg.Filesystem.AllowRead != nil {
+			boundPaths := make(map[string]bool)
+
+			expandedPaths := ExpandGlobPatterns(cfg.Filesystem.AllowRead)
+			for _, p := range expandedPaths {
+				if fileExists(p) && !strings.HasPrefix(p, "/dev/") && !strings.HasPrefix(p, "/proc/") && !boundPaths[p] {
+					boundPaths[p] = true
+					bwrapArgs = append(bwrapArgs, "--ro-bind", p, p)
+				}
+			}
+			// Add non-glob paths
+			for _, p := range cfg.Filesystem.AllowRead {
+				normalized := NormalizePath(p)
+				if !ContainsGlobChars(normalized) && fileExists(normalized) &&
+					!strings.HasPrefix(normalized, "/dev/") && !strings.HasPrefix(normalized, "/proc/") && !boundPaths[normalized] {
+					boundPaths[normalized] = true
+					bwrapArgs = append(bwrapArgs, "--ro-bind", normalized, normalized)
+				}
+			}
+		}
+	} else {
+		// Default mode: bind entire root filesystem read-only
 		bwrapArgs = append(bwrapArgs, "--ro-bind", "/", "/")
+	}
 
 	// Mount special filesystems
 	// Use --dev-bind for /dev instead of --dev to preserve host device permissions

internal/sandbox/macos.go

@@ -35,6 +35,8 @@ type MacOSSandboxParams struct {
 	AllowAllUnixSockets     bool
 	AllowLocalBinding       bool
 	AllowLocalOutbound      bool
+	DefaultDenyRead         bool
+	ReadAllowPaths          []string
 	ReadDenyPaths           []string
 	WriteAllowPaths         []string
 	WriteDenyPaths          []string
@@ -143,13 +145,54 @@ func getTmpdirParent() []string {
 }
 
 // generateReadRules generates filesystem read rules for the sandbox profile.
-func generateReadRules(denyPaths []string, logTag string) []string {
+func generateReadRules(defaultDenyRead bool, allowPaths, denyPaths []string, logTag string) []string {
 	var rules []string
 
+	if defaultDenyRead {
+		// When defaultDenyRead is enabled:
+		// 1. Allow file-read-metadata globally (needed for directory traversal, stat, etc.)
+		// 2. Allow file-read-data only for system paths + user-specified allowRead paths
+		// This lets programs see what files exist but not read their contents.
+
+		// Allow metadata operations globally (stat, readdir, etc.) and root dir (for path resolution)
+		rules = append(rules, "(allow file-read-metadata)")
+		rules = append(rules, `(allow file-read-data (literal "/"))`)
+
+		// Allow reading data from essential system paths
+		for _, systemPath := range GetDefaultReadablePaths() {
+			rules = append(rules,
+				"(allow file-read-data",
+				fmt.Sprintf("  (subpath %s))", escapePath(systemPath)),
+			)
+		}
+
+		// Allow reading data from user-specified paths
+		for _, pathPattern := range allowPaths {
+			normalized := NormalizePath(pathPattern)
+
+			if ContainsGlobChars(normalized) {
+				regex := GlobToRegex(normalized)
+				rules = append(rules,
+					"(allow file-read-data",
+					fmt.Sprintf("  (regex %s))", escapePath(regex)),
+				)
+			} else {
+				rules = append(rules,
+					"(allow file-read-data",
+					fmt.Sprintf("  (subpath %s))", escapePath(normalized)),
+				)
+			}
+		}
+	} else {
 		// Allow all reads by default
 		rules = append(rules, "(allow file-read*)")
+	}
 
-	// Deny specific paths
+	// In both modes, deny specific paths (denyRead takes precedence).
+	// Note: We use file-read* (not file-read-data) so denied paths are fully hidden.
+	// In defaultDenyRead mode, this overrides the global file-read-metadata allow,
+	// meaning denied paths can't even be listed or stat'd - more restrictive than
+	// default mode where denied paths are still visible but unreadable.
 	for _, pathPattern := range denyPaths {
 		normalized := NormalizePath(pathPattern)
 
@@ -494,7 +537,7 @@ func GenerateSandboxProfile(params MacOSSandboxParams) string {
 
 	// Read rules
 	profile.WriteString("; File read\n")
-	for _, rule := range generateReadRules(params.ReadDenyPaths, logTag) {
+	for _, rule := range generateReadRules(params.DefaultDenyRead, params.ReadAllowPaths, params.ReadDenyPaths, logTag) {
 		profile.WriteString(rule + "\n")
 	}
 	profile.WriteString("\n")
@@ -566,6 +609,8 @@ func WrapCommandMacOS(cfg *config.Config, command string, httpPort, socksPort in
 		AllowAllUnixSockets:     cfg.Network.AllowAllUnixSockets,
 		AllowLocalBinding:       allowLocalBinding,
 		AllowLocalOutbound:      allowLocalOutbound,
+		DefaultDenyRead:         cfg.Filesystem.DefaultDenyRead,
+		ReadAllowPaths:          cfg.Filesystem.AllowRead,
 		ReadDenyPaths:           cfg.Filesystem.DenyRead,
 		WriteAllowPaths:         allowPaths,
 		WriteDenyPaths:          cfg.Filesystem.DenyWrite,

internal/sandbox/macos_test.go

@@ -115,6 +115,8 @@ func buildMacOSParamsForTest(cfg *config.Config) MacOSSandboxParams {
 		AllowAllUnixSockets:     cfg.Network.AllowAllUnixSockets,
 		AllowLocalBinding:       allowLocalBinding,
 		AllowLocalOutbound:      allowLocalOutbound,
+		DefaultDenyRead:         cfg.Filesystem.DefaultDenyRead,
+		ReadAllowPaths:          cfg.Filesystem.AllowRead,
 		ReadDenyPaths:           cfg.Filesystem.DenyRead,
 		WriteAllowPaths:         allowPaths,
 		WriteDenyPaths:          cfg.Filesystem.DenyWrite,
@@ -177,6 +179,89 @@ func TestMacOS_ProfileNetworkSection(t *testing.T) {
 	}
 }
 
+// TestMacOS_DefaultDenyRead verifies that the defaultDenyRead option properly restricts filesystem reads.
+func TestMacOS_DefaultDenyRead(t *testing.T) {
+	tests := []struct {
+		name                      string
+		defaultDenyRead           bool
+		allowRead                 []string
+		wantContainsBlanketAllow  bool
+		wantContainsMetadataAllow bool
+		wantContainsSystemAllows  bool
+		wantContainsUserAllowRead bool
+	}{
+		{
+			name:                      "default mode - blanket allow read",
+			defaultDenyRead:           false,
+			allowRead:                 nil,
+			wantContainsBlanketAllow:  true,
+			wantContainsMetadataAllow: false, // No separate metadata allow needed
+			wantContainsSystemAllows:  false, // No need for explicit system allows
+			wantContainsUserAllowRead: false,
+		},
+		{
+			name:                      "defaultDenyRead enabled - metadata allow, system data allows",
+			defaultDenyRead:           true,
+			allowRead:                 nil,
+			wantContainsBlanketAllow:  false,
+			wantContainsMetadataAllow: true, // Should have file-read-metadata for traversal
+			wantContainsSystemAllows:  true, // Should have explicit system path allows
+			wantContainsUserAllowRead: false,
+		},
+		{
+			name:                      "defaultDenyRead with allowRead paths",
+			defaultDenyRead:           true,
+			allowRead:                 []string{"/home/user/project"},
+			wantContainsBlanketAllow:  false,
+			wantContainsMetadataAllow: true,
+			wantContainsSystemAllows:  true,
+			wantContainsUserAllowRead: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			params := MacOSSandboxParams{
+				Command:         "echo test",
+				HTTPProxyPort:   8080,
+				SOCKSProxyPort:  1080,
+				DefaultDenyRead: tt.defaultDenyRead,
+				ReadAllowPaths:  tt.allowRead,
+			}
+
+			profile := GenerateSandboxProfile(params)
+
+			// Check for blanket "(allow file-read*)" without path restrictions
+			// This appears at the start of read rules section in default mode
+			hasBlanketAllow := strings.Contains(profile, "(allow file-read*)\n")
+			if hasBlanketAllow != tt.wantContainsBlanketAllow {
+				t.Errorf("blanket file-read allow = %v, want %v", hasBlanketAllow, tt.wantContainsBlanketAllow)
+			}
+
+			// Check for file-read-metadata allow (for directory traversal in defaultDenyRead mode)
+			hasMetadataAllow := strings.Contains(profile, "(allow file-read-metadata)")
+			if hasMetadataAllow != tt.wantContainsMetadataAllow {
+				t.Errorf("file-read-metadata allow = %v, want %v", hasMetadataAllow, tt.wantContainsMetadataAllow)
+			}
+
+			// Check for system path allows (e.g., /usr, /bin) - should use file-read-data in strict mode
+			hasSystemAllows := strings.Contains(profile, `(subpath "/usr")`) ||
+				strings.Contains(profile, `(subpath "/bin")`)
+			if hasSystemAllows != tt.wantContainsSystemAllows {
+				t.Errorf("system path allows = %v, want %v\nProfile:\n%s", hasSystemAllows, tt.wantContainsSystemAllows, profile)
+			}
+
+			// Check for user-specified allowRead paths
+			if tt.wantContainsUserAllowRead && len(tt.allowRead) > 0 {
+				hasUserAllow := strings.Contains(profile, tt.allowRead[0])
+				if !hasUserAllow {
+					t.Errorf("user allowRead path %q not found in profile", tt.allowRead[0])
+				}
+			}
+		})
+	}
+}
+
 // TestExpandMacOSTmpPaths verifies that /tmp and /private/tmp paths are properly mirrored.
 func TestExpandMacOSTmpPaths(t *testing.T) {
 	tests := []struct {

internal/templates/code-strict.json

@@ -0,0 +1,29 @@
+{
+  "extends": "code",
+  "filesystem": {
+    // Deny reads by default, only system paths and allowRead are accessible
+    "defaultDenyRead": true,
+    "allowRead": [
+      // Current working directory
+      ".",
+
+      // macOS preferences (needed by many apps)
+      "~/Library/Preferences",
+
+      // AI coding tool configs (need to read their own settings)
+      "~/.claude",
+      "~/.claude.json",
+      "~/.codex",
+      "~/.cursor",
+      "~/.opencode",
+      "~/.gemini",
+      "~/.factory",
+
+      // XDG config directory
+      "~/.config",
+
+      // Cache directories (some tools read from cache)
+      "~/.cache"
+    ]
+  }
+}

internal/templates/templates.go

@@ -43,6 +43,7 @@ var templateDescriptions = map[string]string{
 	"git-readonly":      "Blocks destructive commands like git push, rm -rf, etc.",
 	"code":              "Production-ready config for AI coding agents (Claude Code, Codex, Copilot, etc.)",
 	"code-relaxed":      "Like 'code' but allows direct network for apps that ignore HTTP_PROXY (cursor-agent, opencode)",
+	"code-strict":       "Like 'code' but denies reads by default; only allows reading the current project directory and essential system paths",
 }
 
 // List returns all available template names sorted alphabetically.

internal/templates/templates_test.go

@@ -126,6 +126,63 @@ func TestCodeTemplate(t *testing.T) {
 	}
 }
 
+func TestCodeStrictTemplate(t *testing.T) {
+	cfg, err := Load("code-strict")
+	if err != nil {
+		t.Fatalf("failed to load code-strict template: %v", err)
+	}
+
+	// Should inherit AllowPty from code template
+	if !cfg.AllowPty {
+		t.Error("code-strict should inherit AllowPty=true from code")
+	}
+
+	// Should have defaultDenyRead enabled
+	if !cfg.Filesystem.DefaultDenyRead {
+		t.Error("code-strict should have DefaultDenyRead=true")
+	}
+
+	// Should have allowRead with current directory
+	if len(cfg.Filesystem.AllowRead) == 0 {
+		t.Error("code-strict should have allowRead paths")
+	}
+	hasCurrentDir := false
+	for _, path := range cfg.Filesystem.AllowRead {
+		if path == "." {
+			hasCurrentDir = true
+			break
+		}
+	}
+	if !hasCurrentDir {
+		t.Error("code-strict should allow reading current directory")
+	}
+
+	// Should inherit allowWrite from code
+	if len(cfg.Filesystem.AllowWrite) == 0 {
+		t.Error("code-strict should inherit allowWrite from code")
+	}
+
+	// Should inherit denyWrite from code
+	if len(cfg.Filesystem.DenyWrite) == 0 {
+		t.Error("code-strict should inherit denyWrite from code")
+	}
+
+	// Should inherit allowed domains from code
+	if len(cfg.Network.AllowedDomains) == 0 {
+		t.Error("code-strict should inherit allowed domains from code")
+	}
+
+	// Should inherit denied commands from code
+	if len(cfg.Command.Deny) == 0 {
+		t.Error("code-strict should inherit denied commands from code")
+	}
+
+	// Extends should be cleared after resolution
+	if cfg.Extends != "" {
+		t.Error("extends should be cleared after loading")
+	}
+}
+
 func TestCodeRelaxedTemplate(t *testing.T) {
 	cfg, err := Load("code-relaxed")
 	if err != nil {