helper/bwrap: implement file copy flags

These are significantly more efficient and less error-prone than mounting an external tmpfile. This should also reduce attack surface as the resulting files are private to its specific sandbox. Signed-off-by: Ophestra <cat@gensokyo.uk>
2025-02-15 03:12:28 +09:00 · 2025-02-15 03:12:28 +09:00 · 72b0160aad
commit 72b0160aad
parent ea8d1c07df
4 changed files with 124 additions and 35 deletions
--- a/helper/bwrap/builder.go
+++ b/helper/bwrap/builder.go
@ -1,6 +1,8 @@
 package bwrap

-import "os"
+import (
+	"os"
+)

 /*
 Bind binds mount src on host to dest in sandbox.
@ -61,6 +63,29 @@ func (c *Config) Bind(src, dest string, opts ...bool) *Config {
 	}
 }

+// Write copy from FD to destination DEST
+// (--file FD DEST)
+func (c *Config) Write(dest string, payload []byte) *Config {
+	c.Filesystem = append(c.Filesystem, &DataConfig{Dest: dest, Data: payload, Type: DataWrite})
+	return c
+}
+
+/*
+CopyBind copy from FD to file which is readonly bind-mounted on DEST
+(--ro-bind-data FD DEST)
+
+CopyBind(dest, payload, true) copy from FD to file which is bind-mounted on DEST
+(--bind-data FD DEST)
+*/
+func (c *Config) CopyBind(dest string, payload []byte, opts ...bool) *Config {
+	t := DataROBind
+	if len(opts) > 0 && opts[0] {
+		t = DataBind
+	}
+	c.Filesystem = append(c.Filesystem, &DataConfig{Dest: dest, Data: payload, Type: t})
+	return c
+}
+
 // Dir create dir in sandbox
 // (--dir DEST)
 func (c *Config) Dir(dest string) *Config {
--- a/helper/bwrap/config.go
+++ b/helper/bwrap/config.go
@ -71,9 +71,6 @@ type Config struct {
 	    --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
-	    --bind-data FD DEST          Copy from FD to file which is bind-mounted on DEST
-	    --ro-bind-data FD DEST       Copy from FD to file which is readonly bind-mounted on DEST
 	    --add-seccomp-fd FD          Load and use seccomp rules from FD (repeatable)
 	    --block-fd FD                Block on FD until some data to read is available
 	    --userns-block-fd FD         Block on FD until the user namespace is ready
--- a/helper/bwrap/config_test.go
+++ b/helper/bwrap/config_test.go
@ -21,15 +21,14 @@ func TestConfig_Args(t *testing.T) {
 		want []string
 	}{
 		{
-			name: "bind",
-			conf: (new(bwrap.Config)).
+			"bind", (new(bwrap.Config)).
 				Bind("/etc", "/.fortify/etc").
 				Bind("/etc", "/.fortify/etc", true).
 				Bind("/run", "/.fortify/run", false, true).
 				Bind("/sys/devices", "/.fortify/sys/devices", true, true).
 				Bind("/dev/dri", "/.fortify/dev/dri", false, true, true).
 				Bind("/dev/dri", "/.fortify/dev/dri", true, true, true),
-			want: []string{
+			[]string{
 				"--unshare-all", "--unshare-user",
 				"--disable-userns", "--assert-userns-disabled",
 				// Bind("/etc", "/.fortify/etc")
@ -47,14 +46,13 @@ func TestConfig_Args(t *testing.T) {
 			},
 		},
 		{
-			name: "dir remount-ro proc dev mqueue",
-			conf: (new(bwrap.Config)).
+			"dir remount-ro proc dev mqueue", (new(bwrap.Config)).
 				Dir("/.fortify").
 				RemountRO("/home").
 				Procfs("/proc").
 				DevTmpfs("/dev").
 				Mqueue("/dev/mqueue"),
-			want: []string{
+			[]string{
 				"--unshare-all", "--unshare-user",
 				"--disable-userns", "--assert-userns-disabled",
 				// Dir("/.fortify")
@ -70,11 +68,10 @@ func TestConfig_Args(t *testing.T) {
 			},
 		},
 		{
-			name: "tmpfs",
-			conf: (new(bwrap.Config)).
+			"tmpfs", (new(bwrap.Config)).
 				Tmpfs("/run/user", 8192).
 				Tmpfs("/run/dbus", 8192, 0755),
-			want: []string{
+			[]string{
 				"--unshare-all", "--unshare-user",
 				"--disable-userns", "--assert-userns-disabled",
 				// Tmpfs("/run/user", 8192)
@ -84,11 +81,10 @@ func TestConfig_Args(t *testing.T) {
 			},
 		},
 		{
-			name: "symlink",
-			conf: (new(bwrap.Config)).
+			"symlink", (new(bwrap.Config)).
 				Symlink("/.fortify/sbin/init", "/sbin/init").
 				Symlink("/.fortify/sbin/init", "/sbin/init", 0755),
-			want: []string{
+			[]string{
 				"--unshare-all", "--unshare-user",
 				"--disable-userns", "--assert-userns-disabled",
 				// Symlink("/.fortify/sbin/init", "/sbin/init")
@ -98,12 +94,11 @@ func TestConfig_Args(t *testing.T) {
 			},
 		},
 		{
-			name: "overlayfs",
-			conf: (new(bwrap.Config)).
+			"overlayfs", (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{
+			[]string{
 				"--unshare-all", "--unshare-user",
 				"--disable-userns", "--assert-userns-disabled",
 				// Overlay("/etc", "/etc")
@ -117,8 +112,23 @@ func TestConfig_Args(t *testing.T) {
 			},
 		},
 		{
-			name: "unshare",
-			conf: &bwrap.Config{Unshare: &bwrap.UnshareConfig{
+			"copy", (new(bwrap.Config)).
+				Write("/.fortify/version", make([]byte, 8)).
+				CopyBind("/etc/group", make([]byte, 8)).
+				CopyBind("/etc/passwd", make([]byte, 8), true),
+			[]string{
+				"--unshare-all", "--unshare-user",
+				"--disable-userns", "--assert-userns-disabled",
+				// Write("/.fortify/version", make([]byte, 8))
+				"--file", "3", "/.fortify/version",
+				// CopyBind("/etc/group", make([]byte, 8))
+				"--ro-bind-data", "4", "/etc/group",
+				// CopyBind("/etc/passwd", make([]byte, 8), true)
+				"--bind-data", "5", "/etc/passwd",
+			},
+		},
+		{
+			"unshare", &bwrap.Config{Unshare: &bwrap.UnshareConfig{
 				User:   false,
 				IPC:    false,
 				PID:    false,
@ -126,14 +136,13 @@ func TestConfig_Args(t *testing.T) {
 				UTS:    false,
 				CGroup: false,
 			}},
-			want: []string{"--disable-userns", "--assert-userns-disabled"},
+			[]string{"--disable-userns", "--assert-userns-disabled"},
 		},
 		{
-			name: "uid gid sync",
-			conf: (new(bwrap.Config)).
+			"uid gid sync", (new(bwrap.Config)).
 				SetUID(1971).
 				SetGID(100),
-			want: []string{
+			[]string{
 				"--unshare-all", "--unshare-user",
 				"--disable-userns", "--assert-userns-disabled",
 				// SetUID(1971)
@ -143,8 +152,7 @@ func TestConfig_Args(t *testing.T) {
 			},
 		},
 		{
-			name: "hostname chdir setenv unsetenv lockfile chmod syscall",
-			conf: &bwrap.Config{
+			"hostname chdir setenv unsetenv lockfile chmod syscall", &bwrap.Config{
 				Hostname: "fortify",
 				Chdir:    "/.fortify",
 				SetEnv:   map[string]string{"FORTIFY_INIT": "/.fortify/sbin/init"},
@ -153,7 +161,7 @@ func TestConfig_Args(t *testing.T) {
 				Syscall:  new(bwrap.SyscallPolicy),
 				Chmod:    map[string]os.FileMode{"/.fortify/sbin/init": 0755},
 			},
-			want: []string{
+			[]string{
 				"--unshare-all", "--unshare-user",
 				"--disable-userns", "--assert-userns-disabled",
 				// Hostname: "fortify"
@ -175,8 +183,7 @@ func TestConfig_Args(t *testing.T) {
 		},

 		{
-			name: "xdg-dbus-proxy constraint sample",
-			conf: (&bwrap.Config{Clearenv: true, DieWithParent: true}).
+			"xdg-dbus-proxy constraint sample", (&bwrap.Config{Clearenv: true, DieWithParent: true}).
 				Symlink("usr/bin", "/bin").
 				Symlink("var/home", "/home").
 				Symlink("usr/lib", "/lib").
@ -199,7 +206,7 @@ func TestConfig_Args(t *testing.T) {
 				Bind("/sysroot", "/sysroot").
 				Bind("/usr", "/usr").
 				Bind("/etc", "/etc"),
-			want: []string{
+			[]string{
 				"--unshare-all", "--unshare-user",
 				"--disable-userns", "--assert-userns-disabled",
 				"--clearenv", "--die-with-parent",
--- a/helper/bwrap/sequential.go
+++ b/helper/bwrap/sequential.go
@ -2,14 +2,19 @@ package bwrap

 import (
 	"encoding/gob"
+	"fmt"
+	"io"
 	"os"
 	"strconv"
+
+	"git.gensokyo.uk/security/fortify/helper/proc"
 )

 func init() {
 	gob.Register(new(PermConfig[SymlinkConfig]))
 	gob.Register(new(PermConfig[*TmpfsConfig]))
 	gob.Register(new(OverlayConfig))
+	gob.Register(new(DataConfig))
 }

 type PositionalArg int
@ -44,6 +49,10 @@ const (

 	SyncFd
 	Seccomp
+
+	File
+	BindData
+	ROBindData
 )

 var positionalArgs = [...]string{
@ -74,6 +83,10 @@ var positionalArgs = [...]string{

 	SyncFd:  "--sync-fd",
 	Seccomp: "--seccomp",
+
+	File:       "--file",
+	BindData:   "--bind-data",
+	ROBindData: "--ro-bind-data",
 }

 type PermConfig[T FSBuilder] struct {
@ -202,12 +215,59 @@ func (s SymlinkConfig) Append(args *[]string) { *args = append(*args, Symlink.St

 type ChmodConfig map[string]os.FileMode

-func (c ChmodConfig) Len() int {
-	return len(c)
-}
-
+func (c ChmodConfig) Len() int { return len(c) }
 func (c ChmodConfig) Append(args *[]string) {
 	for path, mode := range c {
 		*args = append(*args, Chmod.String(), strconv.FormatInt(int64(mode), 8), path)
 	}
 }
+
+const (
+	DataWrite = iota
+	DataBind
+	DataROBind
+)
+
+type DataConfig struct {
+	Dest string `json:"dest"`
+	Data []byte `json:"data,omitempty"`
+	Type int    `json:"type"`
+	proc.File
+}
+
+func (d *DataConfig) Path() string { return d.Dest }
+func (d *DataConfig) Len() int {
+	if d == nil || d.Data == nil {
+		return 0
+	}
+	return 3
+}
+func (d *DataConfig) Init(fd uintptr, v **os.File) uintptr {
+	if d.File != nil {
+		panic("file initialised twice")
+	}
+	d.File = proc.NewWriterTo(d)
+	return d.File.Init(fd, v)
+}
+func (d *DataConfig) WriteTo(w io.Writer) (int64, error) {
+	n, err := w.Write(d.Data)
+	return int64(n), err
+}
+func (d *DataConfig) Append(args *[]string) {
+	if d == nil || d.Data == nil {
+		return
+	}
+	var a PositionalArg
+	switch d.Type {
+	case DataWrite:
+		a = File
+	case DataBind:
+		a = BindData
+	case DataROBind:
+		a = ROBindData
+	default:
+		panic(fmt.Sprintf("invalid type %d", a))
+	}
+
+	*args = append(*args, a.String(), strconv.Itoa(int(d.Fd())), d.Dest)
+}