Add override_stat xattr support for virtiofs mounts

internal/xattr/walk.go

@@ -0,0 +1,65 @@
+// SPDX-FileCopyrightText: Copyright 2025 Stacklok, Inc.
+// SPDX-License-Identifier: Apache-2.0
+
+//go:build darwin || linux
+
+package xattr
+
+import (
+	"fmt"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// SetOverrideStatTree walks root and sets user.containers.override_stat
+// on every file and directory. Each entry's real mode (from Lstat) is
+// preserved in the xattr value. Symlinks are skipped — they cannot carry
+// user.* xattrs on Linux, and skipping them prevents setting xattrs
+// outside the mount boundary via symlink traversal.
+//
+// The root path is resolved via [filepath.EvalSymlinks] before walking,
+// and every visited entry is verified to remain under the resolved root.
+//
+// Errors on individual entries are logged at debug level and skipped.
+// Returns an error only if the root itself cannot be accessed.
+//
+// On platforms other than macOS and Linux a no-op stub is provided.
+func SetOverrideStatTree(root string, uid, gid int) error {
+	if _, err := os.Lstat(root); err != nil {
+		return fmt.Errorf("access root %s: %w", root, err)
+	}
+
+	realRoot, err := filepath.EvalSymlinks(root)
+	if err != nil {
+		return fmt.Errorf("resolve root: %w", err)
+	}
+	realRoot = filepath.Clean(realRoot)
+	rootPrefix := realRoot + string(filepath.Separator)
+
+	return filepath.WalkDir(realRoot, func(path string, d fs.DirEntry, err error) error {
+		if err != nil {
+			return nil // best-effort, skip inaccessible entries
+		}
+		// Skip symlinks: prevents setting xattrs outside mount boundary,
+		// and Linux rejects user.* xattrs on symlinks anyway.
+		if d.Type()&fs.ModeSymlink != 0 {
+			return nil
+		}
+		// Boundary check: verify path stays under resolved root.
+		cleanPath := filepath.Clean(path)
+		if cleanPath != realRoot && !strings.HasPrefix(cleanPath, rootPrefix) {
+			if d.IsDir() {
+				return fs.SkipDir
+			}
+			return nil
+		}
+		info, err := d.Info()
+		if err != nil {
+			return nil
+		}
+		SetOverrideStat(path, uid, gid, info.Mode())
+		return nil
+	})
+}

internal/xattr/walk_other.go

@@ -0,0 +1,9 @@
+// SPDX-FileCopyrightText: Copyright 2025 Stacklok, Inc.
+// SPDX-License-Identifier: Apache-2.0
+
+//go:build !darwin && !linux
+
+package xattr
+
+// SetOverrideStatTree is a no-op on platforms without xattr support.
+func SetOverrideStatTree(_ string, _, _ int) error { return nil }

internal/xattr/walk_test.go

@@ -0,0 +1,148 @@
+// SPDX-FileCopyrightText: Copyright 2025 Stacklok, Inc.
+// SPDX-License-Identifier: Apache-2.0
+
+//go:build darwin || linux
+
+package xattr
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"golang.org/x/sys/unix"
+)
+
+func TestSetOverrideStatTree_NestedTree(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+	sub1 := filepath.Join(root, "a")
+	sub2 := filepath.Join(root, "a", "b")
+	require.NoError(t, os.MkdirAll(sub2, 0o755))
+
+	// Create a regular file — it should also get the xattr.
+	filePath := filepath.Join(sub1, "file.txt")
+	require.NoError(t, os.WriteFile(filePath, []byte("hi"), 0o644))
+
+	require.NoError(t, SetOverrideStatTree(root, 1000, 1000))
+
+	// All directories should have the xattr set.
+	for _, dir := range []string{root, sub1, sub2} {
+		val := readXattrOpt(t, dir)
+		assert.Contains(t, val, "1000:1000:", "dir %s should have override xattr", dir)
+	}
+
+	// Regular files should also have the xattr set.
+	val := readXattrOpt(t, filePath)
+	assert.Contains(t, val, "1000:1000:", "file should have override xattr")
+}
+
+func TestSetOverrideStatTree_SymlinkToExternalDir(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+	external := t.TempDir()
+	externalSub := filepath.Join(external, "secret")
+	require.NoError(t, os.Mkdir(externalSub, 0o755))
+
+	// Create a symlink inside root pointing to an external directory.
+	require.NoError(t, os.Symlink(external, filepath.Join(root, "escape")))
+
+	require.NoError(t, SetOverrideStatTree(root, 1000, 1000))
+
+	// The external directory must NOT have the xattr set.
+	_, err := unix.Lgetxattr(external, overrideKey, make([]byte, 256))
+	assert.Error(t, err, "external dir should not have override xattr")
+	_, err = unix.Lgetxattr(externalSub, overrideKey, make([]byte, 256))
+	assert.Error(t, err, "external subdir should not have override xattr")
+}
+
+func TestSetOverrideStatTree_SymlinkToFile(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+	target := filepath.Join(root, "real.txt")
+	require.NoError(t, os.WriteFile(target, []byte("data"), 0o644))
+	require.NoError(t, os.Symlink(target, filepath.Join(root, "link.txt")))
+
+	require.NoError(t, SetOverrideStatTree(root, 1000, 1000))
+
+	// The real file gets the xattr (it's a regular file under root).
+	val := readXattrOpt(t, target)
+	assert.Contains(t, val, "1000:1000:", "real file should have override xattr")
+
+	// The symlink itself should NOT have the xattr.
+	link := filepath.Join(root, "link.txt")
+	_, err := unix.Lgetxattr(link, overrideKey, make([]byte, 256))
+	assert.Error(t, err, "symlink should not have override xattr")
+}
+
+func TestSetOverrideStatTree_InaccessibleRoot(t *testing.T) {
+	t.Parallel()
+
+	err := SetOverrideStatTree("/nonexistent/path/xattr-test", 1000, 1000)
+	assert.Error(t, err, "should fail on inaccessible root")
+}
+
+func TestSetOverrideStatTree_EmptyDir(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+	require.NoError(t, SetOverrideStatTree(root, 1000, 1000))
+
+	// Root dir itself should have the xattr.
+	val := readXattrOpt(t, root)
+	assert.Contains(t, val, "1000:1000:", "root dir should have override xattr")
+}
+
+func TestSetOverrideStatTree_RootIsSymlink(t *testing.T) {
+	t.Parallel()
+
+	real := t.TempDir()
+	sub := filepath.Join(real, "child")
+	require.NoError(t, os.Mkdir(sub, 0o755))
+
+	// Create a symlink that points to real. The walk should resolve it
+	// and set xattrs on the real directory tree.
+	link := filepath.Join(t.TempDir(), "link")
+	require.NoError(t, os.Symlink(real, link))
+
+	require.NoError(t, SetOverrideStatTree(link, 1000, 1000))
+
+	val := readXattrOpt(t, real)
+	assert.Contains(t, val, "1000:1000:", "resolved root should have override xattr")
+	val = readXattrOpt(t, sub)
+	assert.Contains(t, val, "1000:1000:", "child dir should have override xattr")
+}
+
+func TestSetOverrideStatTree_DifferentUIDGID(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+	filePath := filepath.Join(root, "file.txt")
+	require.NoError(t, os.WriteFile(filePath, []byte("data"), 0o644))
+
+	// Use different UID and GID to verify both are written independently.
+	require.NoError(t, SetOverrideStatTree(root, 1000, 2000))
+
+	val := readXattrOpt(t, root)
+	assert.Contains(t, val, "1000:2000:", "dir should have uid=1000 gid=2000")
+
+	val = readXattrOpt(t, filePath)
+	assert.Contains(t, val, "1000:2000:", "file should have uid=1000 gid=2000")
+}
+
+// readXattrOpt reads the override_stat xattr and returns its value, or
+// empty string if the xattr is not set.
+func readXattrOpt(t *testing.T, path string) string {
+	t.Helper()
+	buf := make([]byte, 256)
+	n, err := unix.Lgetxattr(path, overrideKey, buf)
+	if err != nil {
+		return ""
+	}
+	return string(buf[:n])
+}

microvm.go

@@ -37,6 +37,7 @@ import (
 	"github.com/stacklok/go-microvm/hypervisor"
 	"github.com/stacklok/go-microvm/hypervisor/libkrun"
 	"github.com/stacklok/go-microvm/image"
+	"github.com/stacklok/go-microvm/internal/xattr"
 	"github.com/stacklok/go-microvm/net/firewall"
 	"github.com/stacklok/go-microvm/net/hosted"
 	rootfspkg "github.com/stacklok/go-microvm/rootfs"
@@ -235,6 +236,32 @@ func Run(ctx context.Context, imageRef string, opts ...Option) (*VM, error) {
 		span.End()
 	}
 
+	// 5b. Validate and set override_stat xattrs on virtiofs mount entries so
+	// the guest sees correct ownership (macOS + Linux; no-op on other platforms).
+	for _, m := range cfg.virtioFS {
+		if m.OverrideUID < 0 || m.OverrideGID < 0 {
+			return nil, fmt.Errorf("virtiofs mount %q: OverrideUID/OverrideGID must be non-negative", m.Tag)
+		}
+		if m.OverrideUID == 0 && m.OverrideGID > 0 {
+			return nil, fmt.Errorf("virtiofs mount %q: OverrideGID set without OverrideUID", m.Tag)
+		}
+	}
+	_, xattrSpan := tracer.Start(ctx, "microvm.VirtioFSOverrideStat")
+	for _, m := range cfg.virtioFS {
+		if m.OverrideUID > 0 && !m.ReadOnly {
+			gid := m.OverrideGID
+			if gid <= 0 {
+				gid = m.OverrideUID
+			}
+			if err := xattr.SetOverrideStatTree(m.HostPath, m.OverrideUID, gid); err != nil {
+				xattrSpan.RecordError(err)
+				slog.Warn("failed to set override_stat on virtiofs mount",
+					"tag", m.Tag, "path", m.HostPath, "error", err)
+			}
+		}
+	}
+	xattrSpan.End()
+
 	// 6. Start VM via backend.
 	_, vmSpawnSpan := tracer.Start(ctx, "microvm.VMSpawn")
 	slog.Debug("starting VM")

options.go

@@ -47,6 +47,18 @@ type VirtioFSMount struct {
 	// support host-side read-only virtiofs. A compromised guest kernel
 	// could bypass this restriction.
 	ReadOnly bool
+	// OverrideUID, when > 0, causes go-microvm to set the
+	// user.containers.override_stat xattr on every file and directory under
+	// HostPath before the VM starts. This makes libkrun's virtiofs FUSE
+	// server report the given UID/GID to the guest instead of the real
+	// host values. Symlinks are skipped for safety.
+	// A zero value means "no override." Since 0 is the zero value for int,
+	// overriding to UID 0 (root) is not supported through this field.
+	// Ignored for ReadOnly mounts.
+	OverrideUID int
+	// OverrideGID sets the group ID for the override_stat xattr.
+	// When 0 and OverrideUID > 0, defaults to OverrideUID.
+	OverrideGID int
 }
 
 // EgressPolicy restricts outbound VM traffic to specific DNS hostnames.