6 files changed, 339 insertions, 0 deletions
diff --git a/internal/app/instance/common/container.go b/internal/app/instance/common/container.go
new file mode 100644
index 00000000..cc26c803
--- /dev/null
+++ b/internal/app/instance/common/container.go
@@ -0,0 +1,187 @@
+package common
+
+import (
+	"errors"
+	"fmt"
+	"io/fs"
+	"maps"
+	"path"
+	"slices"
+	"syscall"
+
+	"git.gensokyo.uk/security/fortify/dbus"
+	"git.gensokyo.uk/security/fortify/fst"
+	"git.gensokyo.uk/security/fortify/internal/sys"
+	"git.gensokyo.uk/security/fortify/sandbox"
+	"git.gensokyo.uk/security/fortify/sandbox/seccomp"
+)
+
+// NewContainer initialises [sandbox.Params] via [fst.SandboxConfig].
+// Note that remaining container setup must be queued by the caller.
+func NewContainer(s *fst.SandboxConfig, os sys.State, uid, gid *int) (*sandbox.Params, map[string]string, error) {
+	if s == nil {
+		return nil, nil, syscall.EBADE
+	}
+
+	container := &sandbox.Params{
+		Hostname: s.Hostname,
+		Ops:      new(sandbox.Ops),
+		Seccomp:  s.Seccomp,
+	}
+
+	if s.Multiarch {
+		container.Seccomp |= seccomp.FilterMultiarch
+	}
+
+	/* 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 */
+	*container.Ops = slices.Grow(*container.Ops, 1<<8)
+
+	if s.Devel {
+		container.Flags |= sandbox.FAllowDevel
+	}
+	if s.Userns {
+		container.Flags |= sandbox.FAllowUserns
+	}
+	if s.Net {
+		container.Flags |= sandbox.FAllowNet
+	}
+	if s.Tty {
+		container.Flags |= sandbox.FAllowTTY
+	}
+
+	if s.MapRealUID {
+		/* some programs fail to connect to dbus session running as a different uid
+		so this workaround is introduced to map priv-side caller uid in container */
+		container.Uid = os.Getuid()
+		*uid = container.Uid
+		container.Gid = os.Getgid()
+		*gid = container.Gid
+	} else {
+		*uid = sandbox.OverflowUid()
+		*gid = sandbox.OverflowGid()
+	}
+
+	container.
+		Proc("/proc").
+		Tmpfs(fst.Tmp, 1<<12, 0755)
+
+	if !s.Device {
+		container.Dev("/dev").Mqueue("/dev/mqueue")
+	} else {
+		container.Bind("/dev", "/dev", sandbox.BindWritable|sandbox.BindDevice)
+	}
+
+	/* retrieve paths and hide them if they're made available in the sandbox;
+	this feature tries to improve user experience of permissive defaults, and
+	to warn about issues in custom configuration; it is NOT a security feature
+	and should not be treated as such, ALWAYS be careful with what you bind */
+	var hidePaths []string
+	sc := os.Paths()
+	hidePaths = append(hidePaths, sc.RuntimePath, sc.SharePath)
+	_, systemBusAddr := dbus.Address()
+	if entries, err := dbus.Parse([]byte(systemBusAddr)); err != nil {
+		return nil, nil, err
+	} else {
+		// there is usually only one, do not preallocate
+		for _, entry := range entries {
+			if entry.Method != "unix" {
+				continue
+			}
+			for _, pair := range entry.Values {
+				if pair[0] == "path" {
+					if path.IsAbs(pair[1]) {
+						// get parent dir of socket
+						dir := path.Dir(pair[1])
+						if dir == "." || dir == "/" {
+							os.Printf("dbus socket %q is in an unusual location", pair[1])
+						}
+						hidePaths = append(hidePaths, dir)
+					} else {
+						os.Printf("dbus socket %q is not absolute", pair[1])
+					}
+				}
+			}
+		}
+	}
+	hidePathMatch := make([]bool, len(hidePaths))
+	for i := range hidePaths {
+		if err := evalSymlinks(os, &hidePaths[i]); err != nil {
+			return nil, nil, err
+		}
+	}
+
+	for _, c := range s.Filesystem {
+		if c == nil {
+			continue
+		}
+
+		if !path.IsAbs(c.Src) {
+			return nil, nil, fmt.Errorf("src path %q is not absolute", c.Src)
+		}
+
+		dest := c.Dst
+		if c.Dst == "" {
+			dest = c.Src
+		} else if !path.IsAbs(dest) {
+			return nil, nil, fmt.Errorf("dst path %q is not absolute", dest)
+		}
+
+		srcH := c.Src
+		if err := evalSymlinks(os, &srcH); err != nil {
+			return nil, nil, err
+		}
+
+		for i := range hidePaths {
+			// skip matched entries
+			if hidePathMatch[i] {
+				continue
+			}
+
+			if ok, err := deepContainsH(srcH, hidePaths[i]); err != nil {
+				return nil, nil, err
+			} else if ok {
+				hidePathMatch[i] = true
+				os.Printf("hiding paths from %q", c.Src)
+			}
+		}
+
+		var flags int
+		if c.Write {
+			flags |= sandbox.BindWritable
+		}
+		if c.Device {
+			flags |= sandbox.BindDevice | sandbox.BindWritable
+		}
+		if !c.Must {
+			flags |= sandbox.BindOptional
+		}
+		container.Bind(c.Src, dest, flags)
+	}
+
+	// cover matched paths
+	for i, ok := range hidePathMatch {
+		if ok {
+			container.Tmpfs(hidePaths[i], 1<<13, 0755)
+		}
+	}
+
+	for _, l := range s.Link {
+		container.Link(l[0], l[1])
+	}
+
+	return container, maps.Clone(s.Env), nil
+}
+
+func evalSymlinks(os sys.State, v *string) error {
+	if p, err := os.EvalSymlinks(*v); err != nil {
+		if !errors.Is(err, fs.ErrNotExist) {
+			return err
+		}
+		os.Printf("path %q does not yet exist", *v)
+	} else {
+		*v = p
+	}
+	return nil
+}
diff --git a/internal/app/instance/common/path.go b/internal/app/instance/common/path.go
new file mode 100644
index 00000000..ce40f4f1
--- /dev/null
+++ b/internal/app/instance/common/path.go
@@ -0,0 +1,11 @@
+package common
+
+import (
+	"path/filepath"
+	"strings"
+)
+
+func deepContainsH(basepath, targpath string) (bool, error) {
+	rel, err := filepath.Rel(basepath, targpath)
+	return err == nil && rel != ".." && !strings.HasPrefix(rel, string([]byte{'.', '.', filepath.Separator})), err
+}
diff --git a/internal/app/instance/common/path_test.go b/internal/app/instance/common/path_test.go
new file mode 100644
index 00000000..b14f24df
--- /dev/null
+++ b/internal/app/instance/common/path_test.go
@@ -0,0 +1,85 @@
+package common
+
+import (
+	"testing"
+)
+
+func TestDeepContainsH(t *testing.T) {
+	testCases := []struct {
+		name     string
+		basepath string
+		targpath string
+		want     bool
+		wantErr  bool
+	}{
+		{
+			name: "empty",
+			want: true,
+		},
+		{
+			name:     "equal abs",
+			basepath: "/run",
+			targpath: "/run",
+			want:     true,
+		},
+		{
+			name:     "equal rel",
+			basepath: "./run",
+			targpath: "run",
+			want:     true,
+		},
+		{
+			name:     "contains abs",
+			basepath: "/run",
+			targpath: "/run/dbus",
+			want:     true,
+		},
+		{
+			name:     "inverse contains abs",
+			basepath: "/run/dbus",
+			targpath: "/run",
+			want:     false,
+		},
+		{
+			name:     "contains rel",
+			basepath: "../run",
+			targpath: "../run/dbus",
+			want:     true,
+		},
+		{
+			name:     "inverse contains rel",
+			basepath: "../run/dbus",
+			targpath: "../run",
+			want:     false,
+		},
+		{
+			name:     "weird abs",
+			basepath: "/run/dbus",
+			targpath: "/run/dbus/../current-system",
+			want:     false,
+		},
+		{
+			name:     "weird rel",
+			basepath: "../run/dbus",
+			targpath: "../run/dbus/../current-system",
+			want:     false,
+		},
+
+		{
+			name:     "invalid mix",
+			basepath: "/run",
+			targpath: "./run",
+			wantErr:  true,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			if got, err := deepContainsH(tc.basepath, tc.targpath); (err != nil) != tc.wantErr {
+				t.Errorf("deepContainsH() error = %v, wantErr %v", err, tc.wantErr)
+			} else if got != tc.want {
+				t.Errorf("deepContainsH() = %v, want %v", got, tc.want)
+			}
+		})
+	}
+}
diff --git a/internal/app/instance/errors.go b/internal/app/instance/errors.go
new file mode 100644
index 00000000..b3331e3d
--- /dev/null
+++ b/internal/app/instance/errors.go
@@ -0,0 +1,17 @@
+package instance
+
+import (
+	"syscall"
+
+	"git.gensokyo.uk/security/fortify/internal/app"
+	"git.gensokyo.uk/security/fortify/internal/app/internal/setuid"
+)
+
+func PrintRunStateErr(whence int, rs *app.RunState, runErr error) (code int) {
+	switch whence {
+	case ISetuid:
+		return setuid.PrintRunStateErr(rs, runErr)
+	default:
+		panic(syscall.EINVAL)
+	}
+}
diff --git a/internal/app/instance/new.go b/internal/app/instance/new.go
new file mode 100644
index 00000000..cb5e8b27
--- /dev/null
+++ b/internal/app/instance/new.go
@@ -0,0 +1,33 @@
+// Package instance exposes cross-package implementation details and provides constructors for builtin implementations.
+package instance
+
+import (
+	"context"
+	"log"
+	"syscall"
+
+	"git.gensokyo.uk/security/fortify/internal/app"
+	"git.gensokyo.uk/security/fortify/internal/app/internal/setuid"
+	"git.gensokyo.uk/security/fortify/internal/sys"
+)
+
+const (
+	ISetuid = iota
+)
+
+func New(whence int, ctx context.Context, os sys.State) (app.App, error) {
+	switch whence {
+	case ISetuid:
+		return setuid.New(ctx, os)
+	default:
+		return nil, syscall.EINVAL
+	}
+}
+
+func MustNew(whence int, ctx context.Context, os sys.State) app.App {
+	a, err := New(whence, ctx, os)
+	if err != nil {
+		log.Fatalf("cannot create app: %v", err)
+	}
+	return a
+}
diff --git a/internal/app/instance/shim.go b/internal/app/instance/shim.go
new file mode 100644
index 00000000..bc497ad3
--- /dev/null
+++ b/internal/app/instance/shim.go
@@ -0,0 +1,6 @@
+package instance
+
+import "git.gensokyo.uk/security/fortify/internal/app/internal/setuid"
+
+// ShimMain is the main function of the shim process and runs as the unconstrained target user.
+func ShimMain() { setuid.ShimMain() }