helper/bwrap: implement overlayfs builder

Signed-off-by: Ophestra <cat@gensokyo.uk>

fst/config.go

@@ -136,11 +136,17 @@ func (s *SandboxConfig) Bwrap(os linux.System) (*bwrap.Config, error) {
 	}
 
 	conf := (&bwrap.Config{
-		Net:           s.Net,
-		UserNS:        s.UserNS,
-		Hostname:      s.Hostname,
-		Clearenv:      true,
-		SetEnv:        s.Env,
+		Net:      s.Net,
+		UserNS:   s.UserNS,
+		Hostname: s.Hostname,
+		Clearenv: true,
+		SetEnv:   s.Env,
+
+		/* this is only 4 KiB of memory on a 64-bit system,
+		permissive defaults on NixOS results in around 100 entries
+		so this capacity should eliminate copies for most setups */
+		Filesystem: make([]bwrap.FSBuilder, 0, 256),
+
 		NewSession:    !s.NoNewSession,
 		DieWithParent: true,
 		AsInit:        true,

fst/info.go

@@ -0,0 +1,5 @@
+package fst
+
+type Info struct {
+	User int `json:"user"`
+}

helper/bwrap/arg.static.awkward.go

@@ -4,10 +4,20 @@ const (
 	Tmpfs = iota
 	Dir
 	Symlink
+
+	OverlaySrc
+	Overlay
+	TmpOverlay
+	ROOverlay
 )
 
 var awkwardArgs = [...]string{
 	Tmpfs:   "--tmpfs",
 	Dir:     "--dir",
 	Symlink: "--symlink",
+
+	OverlaySrc: "--overlay-src",
+	Overlay:    "--overlay",
+	TmpOverlay: "--tmp-overlay",
+	ROOverlay:  "--ro-overlay",
 }

helper/bwrap/config.go

@@ -52,7 +52,7 @@ type Config struct {
 	LockFile []string `json:"lock_file,omitempty"`
 
 	// ordered filesystem args
-	Filesystem []FSBuilder
+	Filesystem []FSBuilder `json:"filesystem,omitempty"`
 
 	// change permissions (must already exist)
 	// (--chmod OCTAL PATH)
@@ -78,6 +78,8 @@ type Config struct {
 	    --userns FD                  Use this user namespace (cannot combine with --unshare-user)
 	    --userns2 FD                 After setup switch to this user namespace, only useful with --userns
 	    --pidns FD                   Use this pid namespace (as parent namespace if using --unshare-pid)
+	    --bind-fd FD DEST            Bind open directory or path fd on DEST
+	    --ro-bind-fd FD DEST         Bind open directory or path fd read-only on DEST
 	    --exec-label LABEL           Exec label for the sandbox
 	    --file-label LABEL           File label for temporary sandbox content
 	    --file FD DEST               Copy from FD to destination DEST
@@ -178,6 +180,74 @@ func (t *TmpfsConfig) Append(args *[]string) {
 	*args = append(*args, awkwardArgs[Tmpfs], t.Dir)
 }
 
+type OverlayConfig struct {
+	/*
+		read files from SRC in the following overlay
+		(--overlay-src SRC)
+	*/
+	Src []string `json:"src,omitempty"`
+
+	/*
+		mount overlayfs on DEST, with RWSRC as the host path for writes and
+		WORKDIR an empty directory on the same filesystem as RWSRC
+		(--overlay RWSRC WORKDIR DEST)
+
+		if nil, mount overlayfs on DEST, with writes going to an invisible tmpfs
+		(--tmp-overlay DEST)
+
+		if either strings are empty, mount overlayfs read-only on DEST
+		(--ro-overlay DEST)
+	*/
+	Persist *[2]string `json:"persist,omitempty"`
+
+	/*
+		--overlay RWSRC WORKDIR DEST
+
+		--tmp-overlay DEST
+
+		--ro-overlay DEST
+	*/
+	Dest string `json:"dest"`
+}
+
+func (o *OverlayConfig) Path() string {
+	return o.Dest
+}
+
+func (o *OverlayConfig) Len() int {
+	// (--tmp-overlay DEST) or (--ro-overlay DEST)
+	p := 2
+	// (--overlay RWSRC WORKDIR DEST)
+	if o.Persist != nil && o.Persist[0] != "" && o.Persist[1] != "" {
+		p = 4
+	}
+
+	return p + len(o.Src)*2
+}
+
+func (o *OverlayConfig) Append(args *[]string) {
+	// --overlay-src SRC
+	for _, src := range o.Src {
+		*args = append(*args, awkwardArgs[OverlaySrc], src)
+	}
+
+	if o.Persist != nil {
+		if o.Persist[0] != "" && o.Persist[1] != "" {
+			// --overlay RWSRC WORKDIR
+			*args = append(*args, awkwardArgs[Overlay], o.Persist[0], o.Persist[1])
+		} else {
+			// --ro-overlay
+			*args = append(*args, awkwardArgs[ROOverlay])
+		}
+	} else {
+		// --tmp-overlay
+		*args = append(*args, awkwardArgs[TmpOverlay])
+	}
+
+	// DEST
+	*args = append(*args, o.Dest)
+}
+
 type SymlinkConfig [2]string
 
 func (s SymlinkConfig) Path() string {

helper/bwrap/config.set.go

@@ -96,6 +96,31 @@ func (c *Config) Tmpfs(dest string, size int, perm ...os.FileMode) *Config {
 	return c
 }
 
+// Overlay mount overlayfs on DEST, with writes going to an invisible tmpfs
+// (--tmp-overlay DEST)
+func (c *Config) Overlay(dest string, src ...string) *Config {
+	c.Filesystem = append(c.Filesystem, &OverlayConfig{Src: src, Dest: dest})
+	return c
+}
+
+// Join mount overlayfs read-only on DEST
+// (--ro-overlay DEST)
+func (c *Config) Join(dest string, src ...string) *Config {
+	c.Filesystem = append(c.Filesystem, &OverlayConfig{Src: src, Dest: dest, Persist: new([2]string)})
+	return c
+}
+
+// Persist mount overlayfs on DEST, with RWSRC as the host path for writes and
+// WORKDIR an empty directory on the same filesystem as RWSRC
+// (--overlay RWSRC WORKDIR DEST)
+func (c *Config) Persist(dest, rwsrc, workdir string, src ...string) *Config {
+	if rwsrc == "" || workdir == "" {
+		panic("persist called without required paths")
+	}
+	c.Filesystem = append(c.Filesystem, &OverlayConfig{Src: src, Dest: dest, Persist: &[2]string{rwsrc, workdir}})
+	return c
+}
+
 // Mqueue mount new mqueue in sandbox
 // (--mqueue DEST)
 func (c *Config) Mqueue(dest string) *Config {

helper/bwrap/config_test.go

@@ -1,19 +1,40 @@
-package bwrap
+package bwrap_test
 
 import (
 	"slices"
 	"testing"
+
+	"git.gensokyo.uk/security/fortify/helper/bwrap"
 )
 
 func TestConfig_Args(t *testing.T) {
 	testCases := []struct {
 		name string
-		conf *Config
+		conf *bwrap.Config
 		want []string
 	}{
+		{
+			name: "overlayfs",
+			conf: (new(bwrap.Config)).
+				Overlay("/etc", "/etc").
+				Join("/.fortify/bin", "/bin", "/usr/bin", "/usr/local/bin").
+				Persist("/nix", "/data/data/org.chromium.Chromium/overlay/rwsrc", "/data/data/org.chromium.Chromium/workdir", "/data/app/org.chromium.Chromium/nix"),
+			want: []string{
+				"--unshare-all", "--unshare-user",
+				"--disable-userns", "--assert-userns-disabled",
+				// Overlay("/etc", "/etc")
+				"--overlay-src", "/etc", "--tmp-overlay", "/etc",
+				// Join("/.fortify/bin", "/bin", "/usr/bin", "/usr/local/bin")
+				"--overlay-src", "/bin", "--overlay-src", "/usr/bin",
+				"--overlay-src", "/usr/local/bin", "--ro-overlay", "/.fortify/bin",
+				// Persist("/nix", "/data/data/org.chromium.Chromium/overlay/rwsrc", "/data/data/org.chromium.Chromium/workdir", "/data/app/org.chromium.Chromium/nix")
+				"--overlay-src", "/data/app/org.chromium.Chromium/nix",
+				"--overlay", "/data/data/org.chromium.Chromium/overlay/rwsrc", "/data/data/org.chromium.Chromium/workdir", "/nix",
+			},
+		},
 		{
 			name: "xdg-dbus-proxy constraint sample",
-			conf: (&Config{
+			conf: (&bwrap.Config{
 				Unshare:       nil,
 				UserNS:        false,
 				Clearenv:      true,
@@ -71,7 +92,7 @@ func TestConfig_Args(t *testing.T) {
 		},
 		{
 			name: "fortify permissive default nixos",
-			conf: (&Config{
+			conf: (&bwrap.Config{
 				Unshare:  nil,
 				Net:      true,
 				UserNS:   true,

internal/linux/std.go

@@ -1,6 +1,7 @@
 package linux
 
 import (
+	"errors"
 	"io"
 	"io/fs"
 	"os"
@@ -8,6 +9,7 @@ import (
 	"os/user"
 	"strconv"
 	"sync"
+	"syscall"
 
 	"git.gensokyo.uk/security/fortify/internal"
 	"git.gensokyo.uk/security/fortify/internal/fmsg"
@@ -79,9 +81,15 @@ func (s *Std) Uid(aid int) (int, error) {
 		cmd.Stderr = os.Stderr // pass through fatal messages
 		cmd.Env = []string{"FORTIFY_APP_ID=" + strconv.Itoa(aid)}
 		cmd.Dir = "/"
-		var p []byte
+		var (
+			p         []byte
+			exitError *exec.ExitError
+		)
+
 		if p, u.err = cmd.Output(); u.err == nil {
 			u.uid, u.err = strconv.Atoi(string(p))
+		} else if errors.As(u.err, &exitError) && exitError != nil && exitError.ExitCode() == 1 {
+			u.err = syscall.EACCES
 		}
 		return u.uid, u.err
 	}

main.go

@@ -4,6 +4,7 @@ import (
 	_ "embed"
 	"flag"
 	"fmt"
+	"os"
 	"os/user"
 	"strconv"
 	"strings"
@@ -16,7 +17,6 @@ import (
 	"git.gensokyo.uk/security/fortify/internal/app"
 	"git.gensokyo.uk/security/fortify/internal/fmsg"
 	"git.gensokyo.uk/security/fortify/internal/linux"
-	"git.gensokyo.uk/security/fortify/internal/state"
 	"git.gensokyo.uk/security/fortify/internal/system"
 )
 
@@ -33,7 +33,7 @@ func init() {
 	flag.BoolVar(&flagJSON, "json", false, "Format output in JSON when applicable")
 }
 
-var os = new(linux.Std)
+var sys linux.System = new(linux.Std)
 
 type gl []string
 
@@ -65,7 +65,7 @@ func main() {
 		fmt.Println("Usage:\tfortify [-v] [--json] COMMAND [OPTIONS]")
 		fmt.Println()
 		fmt.Println("Commands:")
-		w := tabwriter.NewWriter(os.Stdout(), 0, 1, 4, ' ', 0)
+		w := tabwriter.NewWriter(os.Stdout, 0, 1, 4, ' ', 0)
 		commands := [][2]string{
 			{"app", "Launch app defined by the specified config file"},
 			{"run", "Configure and start a permissive default sandbox"},
@@ -128,24 +128,20 @@ func main() {
 		// Ignore errors; set is set for ExitOnError.
 		_ = set.Parse(args[1:])
 
-		var (
-			config   *fst.Config
-			instance *state.State
-			name     string
-		)
-
-		if len(set.Args()) != 1 {
+		switch len(set.Args()) {
+		case 0: // system
+			printShowSystem(short)
+		case 1: // instance
+			name := set.Args()[0]
+			config, instance := tryShort(name)
+			if config == nil {
+				config = tryPath(name)
+			}
+			printShowInstance(instance, config, short)
+		default:
 			fmsg.Fatal("show requires 1 argument")
-		} else {
-			name = set.Args()[0]
-			config, instance = tryShort(name)
 		}
 
-		if config == nil {
-			config = tryPath(name)
-		}
-
-		printShow(instance, config, short)
 		fmsg.Exit(0)
 	case "app": // launch app from configuration file
 		if len(args) < 2 {
@@ -211,7 +207,7 @@ func main() {
 			passwdOnce sync.Once
 			passwdFunc = func() {
 				var us string
-				if uid, err := os.Uid(aid); err != nil {
+				if uid, err := sys.Uid(aid); err != nil {
 					fmsg.Fatalf("cannot obtain uid from fsu: %v", err)
 				} else {
 					us = strconv.Itoa(uid)
@@ -292,7 +288,7 @@ func main() {
 }
 
 func runApp(config *fst.Config) {
-	a, err := app.New(os)
+	a, err := app.New(sys)
 	if err != nil {
 		fmsg.Fatalf("cannot create app: %s\n", err)
 	} else if err = a.Seal(config); err != nil {

parse.go

@@ -4,7 +4,7 @@ import (
 	"encoding/json"
 	"errors"
 	"io"
-	direct "os"
+	"os"
 	"strconv"
 	"strings"
 	"syscall"
@@ -38,7 +38,7 @@ func tryPath(name string) (config *fst.Config) {
 			}()
 		}
 	} else {
-		r = direct.Stdin
+		r = os.Stdin
 	}
 
 	if err := json.NewDecoder(r).Decode(&config); err != nil {
@@ -61,7 +61,7 @@ func tryFd(name string) io.ReadCloser {
 			}
 			fmsg.Fatalf("cannot get fd %d: %v", fd, errno)
 		}
-		return direct.NewFile(fd, strconv.Itoa(v))
+		return os.NewFile(fd, strconv.Itoa(v))
 	}
 }
 
@@ -85,7 +85,7 @@ func tryShort(name string) (config *fst.Config, instance *state.State) {
 	if likePrefix && len(name) >= 8 {
 		fmsg.VPrintln("argument looks like prefix")
 
-		s := state.NewMulti(os.Paths().RunDirPath)
+		s := state.NewMulti(sys.Paths().RunDirPath)
 		if entries, err := state.Join(s); err != nil {
 			fmsg.Printf("cannot join store: %v", err)
 			// drop to fetch from file

print.go

@@ -16,14 +16,37 @@ import (
 	"git.gensokyo.uk/security/fortify/internal/state"
 )
 
-func printShow(instance *state.State, config *fst.Config, short bool) {
-	if flagJSON {
-		v := any(config)
-		if instance != nil {
-			v = instance
-		}
+func printShowSystem(short bool) {
+	info := new(fst.Info)
 
-		printJSON(v)
+	// get fid by querying uid of aid 0
+	if uid, err := sys.Uid(0); err != nil {
+		fmsg.Fatalf("cannot obtain uid from fsu: %v", err)
+	} else {
+		info.User = (uid / 10000) - 100
+	}
+
+	if flagJSON {
+		printJSON(info)
+		return
+	}
+
+	w := tabwriter.NewWriter(direct.Stdout, 0, 1, 4, ' ', 0)
+
+	fmt.Fprintf(w, "User:\t%d\n", info.User)
+
+	if err := w.Flush(); err != nil {
+		fmsg.Fatalf("cannot flush tabwriter: %v", err)
+	}
+}
+
+func printShowInstance(instance *state.State, config *fst.Config, short bool) {
+	if flagJSON {
+		if instance != nil {
+			printJSON(instance)
+		} else {
+			printJSON(config)
+		}
 		return
 	}
 
@@ -171,7 +194,7 @@ func printPs(short bool) {
 	now := time.Now().UTC()
 
 	var entries state.Entries
-	s := state.NewMulti(os.Paths().RunDirPath)
+	s := state.NewMulti(sys.Paths().RunDirPath)
 	if e, err := state.Join(s); err != nil {
 		fmsg.Fatalf("cannot join store: %v", err)
 	} else {

test.nix

@@ -19,11 +19,19 @@ nixosTest {
   nodes.machine =
     { lib, pkgs, ... }:
     {
-      users.users.alice = {
-        isNormalUser = true;
-        description = "Alice Foobar";
-        password = "foobar";
-        uid = 1000;
+      users.users = {
+        alice = {
+          isNormalUser = true;
+          description = "Alice Foobar";
+          password = "foobar";
+          uid = 1000;
+        };
+        untrusted = {
+          isNormalUser = true;
+          description = "Untrusted user";
+          password = "foobar";
+          uid = 1001;
+        };
       };
 
       home-manager.users.alice.home.stateVersion = "24.11";
@@ -198,6 +206,9 @@ nixosTest {
     machine.wait_for_file("/run/user/1000/wayland-1")
     machine.wait_for_file("/tmp/sway-ipc.sock")
 
+    # Deny unmapped uid:
+    print(machine.fail("sudo -u untrusted -i ${self.packages.${system}.fortify}/bin/fortify -v run"))
+
     # Create fortify uid 0 state directory:
     machine.succeed("install -dm 0755 -o u0_a0 -g users /var/lib/fortify/u0")