Refuse a placement or mount at the machine's own trees, and use a found directory as found

A module's places setting could point a directory anywhere, with an owner it
named, and the engine chowned and chmodded whatever it was pointed at as root,
through a link too; its accesses setting could mount any path into a container
(hq issue 339). The engine now refuses, as a failed resource with its reason,
a directory, access or mount source that is or holds one of the machine's
roots, anything placed in the kernel's, the boot loader's, root's or the
engine's own trees, a non-root directory below /etc, /usr or /run, an access
there, another account's directory below a home, and a container mounting
anything it refused, judged with every link resolved. A directory whose last
component is a link is refused, and one found here that the mesh did not make
keeps its owner and mode and is never removed.

internal/apply/apply.go

@@ -75,6 +75,8 @@ type Outcome struct {
 	groups []string
 	// unpacked is, for an archive, what it put on the machine (novox/hq issue 162).
 	unpacked *store.Unpacked
+	// asFound is, for a directory, that it is used as it was found (novox/hq issue 339).
+	asFound bool
 	// reads is, for a container, the digest of each file it was created reading, by path — so
 	// the next apply can say which one changed (novox/hq 04-ISSUES/103).
 	reads map[string]string
@@ -460,8 +462,17 @@ func ApplyMindingWindows(
 	// does not answer during a maintenance window waits for the window instead of failing.
 	in := inputs{declares: map[string]string{}, known: &known, windows: windows, away: newStoreAway(windows, log),
 		groups: wanted}
+	// Where nothing is placed or mounted from, judged before anything is applied (novox/hq issue 339), and
+	// which declared directories were already here: one this apply makes is the mesh's, one found may not be.
+	in.refused = refusedPlaces(d.Resources)
+	in.dirsBefore = map[string]bool{}
 	for _, resource := range d.Resources {
 		in.declares[resource.Identity()] = declaredDigest(resource)
+		if dir, ok := resource.(*declaration.Directory); ok {
+			if _, err := os.Lstat(dir.Path); err == nil {
+				in.dirsBefore[filepath.Clean(dir.Path)] = true
+			}
+		}
 	}
 
 	// Everything is attempted, and every failure is reported.
@@ -725,6 +736,7 @@ func ApplyMindingWindows(
 			Linger:    outcome.linger,
 			Groups:    outcome.groups,
 			Unpacked:  outcome.unpacked,
+			AsFound:   outcome.asFound,
 			Holds:     holds(resource),
 		})
 		if outcome.found != nil {
@@ -918,9 +930,21 @@ type Unseal func(sealed string) ([]byte, error)
 func applyOne(ctx context.Context, sys system.System, r declaration.Resource, run Runner,
 	changed map[string]bool, in inputs, previous store.Applied,
 	unseal Unseal, keepFound Keep) (Outcome, error) {
+	// Nothing is placed in or mounted from the machine's own trees, whoever asked (novox/hq issue 339): a
+	// refusal is this resource failing, with its reason, in the node's report.
+	switch res := r.(type) {
+	case *declaration.Directory, *declaration.Access:
+		if err := in.refused[filepath.Clean(r.Target())]; err != nil {
+			return begin(r), err
+		}
+	case *declaration.Container:
+		if err := refuseMounts(res, in.refused); err != nil {
+			return begin(r), err
+		}
+	}
 	switch res := r.(type) {
 	case *declaration.Directory:
-		return applyDirectory(res)
+		return applyDirectory(res, previous, in.dirsBefore[filepath.Clean(res.Path)])
 	case *declaration.File:
 		return applyFile(res, previous, unseal, keepFound)
 	case *declaration.Service:
@@ -965,21 +989,56 @@ func modeOf(spec string, fallback os.FileMode) (os.FileMode, error) {
 	return os.FileMode(parsed), nil
 }
 
-func applyDirectory(r *declaration.Directory) (Outcome, error) {
+// applyDirectory makes a directory what was declared.
+//
+// **The path is not followed** (novox/hq issue 339): a directory whose last component is a link is refused,
+// because a chmod and a chown through it would change wherever the link points — /etc, as root.
+//
+// **A directory found here, that the mesh did not make, is used as found** (novox/hq issue 339): its owner
+// and mode are left, and the outcome says what was declared and what was found. Changing them would be root
+// handing a directory it never made — wherever a declaration pointed it — to whatever account was named. A
+// directory is the mesh's when its record says the mesh applied it at this path before (a record from before
+// this rule counts, which is every directory on a running machine), when this apply made it, or when it
+// already has the declared owner and mode, so a person who sets them by hand at the machine hands it over.
+func applyDirectory(r *declaration.Directory, previous store.Applied, wasBefore bool) (Outcome, error) {
 	out := begin(r)
 	mode, err := modeOf(r.Mode, 0o755)
 	if err != nil {
 		return out, err
 	}
 
-	before, err := os.Stat(r.Path)
+	before, err := os.Lstat(r.Path)
 	existed := err == nil
 	if err != nil && !errors.Is(err, os.ErrNotExist) {
 		return out, err
 	}
+	if existed && before.Mode()&os.ModeSymlink != 0 {
+		return out, &PlacementRefusedError{Path: r.Path, Why: "it is a symbolic link, and the node-engine " +
+			"follows no link to a directory it owns"}
+	}
 	if existed && !before.IsDir() {
 		return out, fmt.Errorf("%s exists and is not a directory", r.Path)
 	}
+	if existed && wasBefore {
+		ours := previous.Target != "" && filepath.Clean(previous.Target) == filepath.Clean(r.Path) && !previous.AsFound
+		if !ours {
+			owned, err := ownedBy(r.Path, r.Owner)
+			if err != nil {
+				return out, err
+			}
+			if !owned || before.Mode().Perm() != mode.Perm() {
+				out.Action, out.asFound = "unchanged", true
+				owner := r.Owner
+				if owner == "" {
+					owner = "root"
+				}
+				out.Detail = fmt.Sprintf("found here before the mesh and used as found: its owner and mode %o are "+
+					"left, though %s and %o were declared (novox/hq issue 339); set them by hand to hand it to the mesh",
+					before.Mode().Perm(), owner, mode.Perm())
+				return out, nil
+			}
+		}
+	}
 
 	if !existed {
 		if err := makeDirs(r.Path, mode, r.Owner); err != nil {
@@ -1623,6 +1682,10 @@ func remove(ctx context.Context, sys system.System, a store.Applied, run Runner,
 		// This is the host's own line, applied to the one shape where getting it wrong is not
 		// recoverable: it removes what it made and leaves what it merely configured. An empty
 		// directory is what it made. A full one is not.
+		if a.AsFound {
+			// Found here before the mesh, and never the mesh's (novox/hq issue 339).
+			return "forgotten", "found here before the mesh and used as found: left as it is", nil
+		}
 		entries, err := os.ReadDir(a.Target)
 		if errors.Is(err, os.ErrNotExist) {
 			return "forgotten", "no longer there", nil
@@ -1850,6 +1913,12 @@ type inputs struct {
 	// groups is every group the declaration asks an account to be in, and by which resources
 	// (novox/hq ADR 0252): a group one user resource stops asking for stays while another asks.
 	groups Wanted
+	// refused is every directory and access the engine will not place, by path, with why (novox/hq issue 339):
+	// each fails, and a container mounting one fails with it.
+	refused map[string]error
+	// dirsBefore is every declared directory that was on the machine when this apply began (novox/hq issue
+	// 339): one the apply itself made — a file's parent — is the mesh's, one that was there may not be.
+	dirsBefore map[string]bool
 }
 
 // fileDigest is what a file the container reads holds, by digest.

internal/apply/placement_guard.go

@@ -0,0 +1,300 @@
+package apply
+
+// Where the node-engine places nothing and mounts nothing, whoever asks (novox/hq issue 339).
+//
+// A directory names its path, and the controller resolves part of that path from what was set for the
+// module: its `places` setting moves a directory anywhere, with an owner it names, and its `accesses` setting
+// says which of the machine's paths are mounted into its container. The engine runs as root, so a path it
+// accepts blindly is a path anyone who could change those settings hands to any account: a directory at /etc
+// owned by a caller's account gives it /etc, and an access at / mounts the machine's root into a container.
+// The controller refuses both where a setting is made; the engine refuses them again where it applies,
+// because a guard in one place is a guard one change away from gone. Whatever the declaration says:
+//
+//  1. **No directory, access or mount source is one of the machine's own roots, or holds one**: /, /etc,
+//     /usr, /var, /var/lib, /home, /run and the rest of protectedRoots. Modules place directories BELOW /etc
+//     or /var/lib, never the root itself; owning one is owning everything in it.
+//  2. **Nothing is placed in /proc, /sys, /dev, /boot or /root, nor in the node-engine's own trees** (its
+//     state, its identity, its installed builds) but by its own module; nothing is mounted from /boot, /root or
+//     the engine's trees. A mount of a kernel file, a device or the clock is the plumbing systemPath names.
+//  3. **A directory below /etc, /usr or /run is root's.** The machine's configuration and programs are read
+//     as root's word; a directory there owned by another account is that account writing root's word. An
+//     access is never there at all: the operator's data is not the machine's configuration.
+//  4. **Below a person's or an agent's home, a directory is that account's.** Root's or another account's
+//     directory there is one the account does not control in a tree whose every parent it does. A home
+//     itself may be a module's directory (a backup repository kept as an account's home is one), and as
+//     every directory the mesh did not make, it is used as found: never chowned or chmodded (applyDirectory).
+//  5. **A mount source that is a refused directory or a refused access is refused with it**: the container
+//     would otherwise bind the very path the engine would not place, and the runtime creates a missing one
+//     as root.
+//
+// Each is a failed resource with its reason in the node's report; nothing is touched. Paths are judged as
+// declared and again with every link in them resolved, so a link at /srv/x pointing at /etc places nothing.
+
+import (
+	"bufio"
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strconv"
+	"strings"
+
+	"github.com/novox/mesh-host/internal/declaration"
+)
+
+// protectedRoots are paths no directory, access or mount source may be, nor hold.
+var protectedRoots = []string{"/", "/bin", "/boot", "/dev", "/etc", "/home", "/lib", "/lib32", "/lib64",
+	"/media", "/mnt", "/opt", "/proc", "/root", "/run", "/sbin", "/srv", "/sys", "/tmp", "/usr", "/usr/bin",
+	"/usr/lib", "/usr/lib64", "/usr/local", "/usr/local/bin", "/usr/local/lib", "/usr/local/sbin", "/usr/sbin",
+	"/usr/share", "/var", "/var/cache", "/var/lib", "/var/lib/mesh", "/var/log", "/var/run", "/var/tmp",
+	"/var/spool"}
+
+// forbiddenBelow are trees nothing is placed in or mounted from: the kernel's, the boot loader's and root's
+// home. engineTrees are the node-engine's own, which only its own module places in.
+var (
+	forbiddenBelow = []string{"/proc", "/sys", "/dev", "/boot", "/root"}
+	// forbiddenBelowMount are the trees no container mounts from: a mount of the kernel's files and devices is
+	// the plumbing a container may need (systemPath); root's home and the boot loader's are no plumbing.
+	forbiddenBelowMount = []string{"/boot", "/root"}
+	engineTrees         = []string{"/var/lib/mesh-host", "/usr/lib/nox-mesh-host"}
+	// rootsOnly are trees a directory below is root's, and an access is never in.
+	rootsOnly = []string{"/etc", "/usr", "/run", "/var/run"}
+)
+
+// engineModule is the module whose resources may place in the engine's own trees.
+const engineModule = "mesh-host"
+
+// passwdFile is the user database homes are read from. A variable so a test names its own.
+var passwdFile = "/etc/passwd"
+
+// PlacementRefusedError is a resource the engine will not place, or mount, where it says.
+type PlacementRefusedError struct {
+	Path, Why string
+}
+
+func (e *PlacementRefusedError) Error() string {
+	return fmt.Sprintf("%s is not placed: %s (novox/hq issue 339); nothing was touched", e.Path, e.Why)
+}
+
+// homeAccount is a person's or an agent's account and its home.
+type homeAccount struct {
+	Name string
+	UID  int
+}
+
+// accountsOfHomes is each person's or agent's home and the account it belongs to, from the user database: an
+// account with a uid of 1000 or more, or a home under /home. A variable so a test names its own.
+var accountsOfHomes = func() map[string]homeAccount {
+	f, err := os.Open(passwdFile)
+	if err != nil {
+		return nil
+	}
+	defer f.Close()
+	out := map[string]homeAccount{}
+	sc := bufio.NewScanner(f)
+	for sc.Scan() {
+		fields := strings.Split(sc.Text(), ":")
+		if len(fields) < 6 {
+			continue
+		}
+		uid, err := strconv.Atoi(fields[2])
+		if err != nil {
+			continue
+		}
+		home := filepath.Clean(fields[5])
+		if home == "/" || home == "." || home == "" || home == "/nonexistent" {
+			continue
+		}
+		if (uid >= 1000 && uid != 65534) || strings.HasPrefix(home, "/home/") {
+			out[home] = homeAccount{Name: fields[0], UID: uid}
+		}
+	}
+	return out
+}
+
+// below says whether path is strictly below dir.
+func below(path, dir string) bool {
+	if dir == "/" {
+		return path != "/"
+	}
+	return strings.HasPrefix(path, dir+"/")
+}
+
+// atOrBelow says whether path is dir or below it.
+func atOrBelow(path, dir string) bool { return path == dir || below(path, dir) }
+
+// resolved is a path with every link in it followed, as far as the path exists, and the rest as declared.
+func resolved(path string) string {
+	rest := ""
+	for p := path; ; p = filepath.Dir(p) {
+		if real, err := filepath.EvalSymlinks(p); err == nil {
+			return filepath.Clean(filepath.Join(real, rest))
+		}
+		if filepath.Dir(p) == p {
+			return path
+		}
+		rest = filepath.Join(filepath.Base(p), rest)
+	}
+}
+
+// ownedByAccount says whether a declared owner is that account: by name, or by its uid ("1001", "1001:1001").
+func ownedByAccount(owner string, a homeAccount) bool {
+	if owner == a.Name {
+		return true
+	}
+	user, _, _ := strings.Cut(owner, ":")
+	if uid, err := strconv.Atoi(user); err == nil {
+		return uid == a.UID
+	}
+	return false
+}
+
+// rootOwner says whether a declared owner is root: none, "root", or uid 0.
+func rootOwner(owner string) bool {
+	if owner == "" || owner == "root" {
+		return true
+	}
+	user, _, _ := strings.Cut(owner, ":")
+	return user == "0"
+}
+
+// what a guarded path is, for the words of a refusal.
+type placing int
+
+const (
+	placingDirectory placing = iota
+	placingAccess
+	placingMount
+)
+
+// refusePath says why a path is not placed or mounted; nil when it may be. module is the resource's module,
+// owner a directory's declared owner.
+func refusePath(path string, kind placing, module, owner string) error {
+	clean := filepath.Clean(path)
+	if !filepath.IsAbs(clean) {
+		return nil // the declaration refuses a relative path already; a named volume is not a path
+	}
+	for _, p := range []string{clean, resolved(clean)} {
+		if err := refuseOne(p, kind, module, owner); err != nil {
+			if p != clean {
+				err.Why = fmt.Sprintf("through a link, it is %s, and %s", p, err.Why)
+				err.Path = clean
+			}
+			return err
+		}
+	}
+	return nil
+}
+
+func refuseOne(path string, kind placing, module, owner string) *PlacementRefusedError {
+	for _, root := range protectedRoots {
+		if path == root || below(root, path) {
+			return &PlacementRefusedError{Path: path, Why: root + " is one of the machine's own directories, " +
+				"and owning or mounting it would be owning or mounting everything in it"}
+		}
+	}
+	trees := forbiddenBelow
+	if kind == placingMount {
+		// A mount is the machine's plumbing as often as a module's data — the clock, a kernel file, /dev/null
+		// (systemPath) — and what a setting can mount at all is a directory or an access, refused above it.
+		trees = forbiddenBelowMount
+	}
+	for _, tree := range trees {
+		if atOrBelow(path, tree) {
+			return &PlacementRefusedError{Path: path, Why: "nothing is placed in or mounted from " + tree}
+		}
+	}
+	if module != engineModule {
+		for _, tree := range engineTrees {
+			if atOrBelow(path, tree) {
+				return &PlacementRefusedError{Path: path, Why: tree + " is the node-engine's own, placed in by " +
+					"its own module alone"}
+			}
+		}
+	}
+	for _, tree := range rootsOnly {
+		if !below(path, tree) {
+			continue
+		}
+		switch {
+		case kind == placingAccess:
+			return &PlacementRefusedError{Path: path, Why: "an access is the operator's data, and " + tree +
+				" is the machine's own"}
+		case kind == placingDirectory && !rootOwner(owner):
+			return &PlacementRefusedError{Path: path, Why: fmt.Sprintf("a directory below %s is root's, and this "+
+				"one is declared %s's", tree, owner)}
+		}
+	}
+	if kind != placingDirectory {
+		return nil
+	}
+	homes := accountsOfHomes()
+	var deepest string
+	for home := range homes {
+		if below(path, home) && len(home) > len(deepest) {
+			deepest = home
+		}
+	}
+	if deepest != "" {
+		if a := homes[deepest]; !ownedByAccount(owner, a) {
+			if owner == "" {
+				owner = "root"
+			}
+			return &PlacementRefusedError{Path: path, Why: fmt.Sprintf("it is below %s's home and declared %s's; "+
+				"below a home only that account's directories are placed", a.Name, owner)}
+		}
+	}
+	return nil
+}
+
+// moduleOfID is the module a resource id names, or "".
+func moduleOfID(id string) string {
+	module, _ := moduleOf(id)
+	return module
+}
+
+// refusedPlaces judges every directory and access of a declaration before anything is applied, and answers
+// each refusal by path: what is refused is refused again as a container's mount source.
+func refusedPlaces(resources []declaration.Resource) map[string]error {
+	out := map[string]error{}
+	for _, r := range resources {
+		var err error
+		switch res := r.(type) {
+		case *declaration.Directory:
+			err = refusePath(res.Path, placingDirectory, moduleOfID(res.ID), res.Owner)
+		case *declaration.Access:
+			err = refusePath(res.Path, placingAccess, moduleOfID(res.ID), "")
+		default:
+			continue
+		}
+		if err != nil {
+			out[filepath.Clean(r.Target())] = err
+		}
+	}
+	return out
+}
+
+// refuseMounts says why a container's mounts are not made; nil when they may be.
+func refuseMounts(c *declaration.Container, refused map[string]error) error {
+	for _, v := range c.Volumes {
+		src := mountSource(v)
+		if !strings.HasPrefix(src, "/") {
+			continue // a named volume, which the runtime keeps in its own tree
+		}
+		src = filepath.Clean(src)
+		for path, why := range refused {
+			if atOrBelow(src, path) {
+				var refusal *PlacementRefusedError
+				if errors.As(why, &refusal) {
+					return &PlacementRefusedError{Path: src, Why: "it is mounted from " + path +
+						", which is refused: " + refusal.Why}
+				}
+				return why
+			}
+		}
+		if err := refusePath(src, placingMount, moduleOfID(c.ID), ""); err != nil {
+			return err
+		}
+	}
+	return nil
+}

internal/apply/placement_guard_test.go

@@ -0,0 +1,195 @@
+package apply
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"github.com/novox/mesh-host/internal/store"
+)
+
+// Where the node-engine places nothing and mounts nothing, whoever asks (novox/hq issue 339). A module's
+// `places` setting moved a directory anywhere, with an owner it named, and the engine — root — made it so: a
+// directory at /etc owned by a caller's account handed it /etc. Each test here is a declaration the controller
+// might send; the engine refuses it whatever the controller decided.
+
+func applyOnce(t *testing.T, resources string, run Runner, state store.State) (Report, store.State, error) {
+	t.Helper()
+	if run == nil {
+		run = noServices
+	}
+	return Apply(context.Background(), archHost(t), declare(t, resources), state, store.OriginDeclared, run, nil, nil)
+}
+
+func refusedFor339(t *testing.T, what string, err error) {
+	t.Helper()
+	if err == nil || !strings.Contains(err.Error(), "issue 339") || !strings.Contains(err.Error(), "nothing was touched") {
+		t.Fatalf("%s was not refused as the machine's own: %v", what, err)
+	}
+}
+
+func TestNoDirectoryIsPlacedAtTheMachinesOwn(t *testing.T) {
+	for _, path := range []string{"/", "/etc", "/etc/", "/usr", "/var", "/var/lib", "/home", "/run", "/boot", "/root",
+		"/var/lib/mesh-host", "/var/lib/mesh-host/identity", "/proc/1/task", "/sys/kernel", "/dev/shm", "/root/.ssh",
+		"/srv/../etc"} {
+		_, _, err := applyOnce(t, `{"id":"notes.data","type":"directory","path":"`+path+`","mode":"0755"}`, nil, store.State{})
+		refusedFor339(t, path, err)
+	}
+	// Below /etc, /usr or /run, a directory is root's: the machine's configuration is read as root's word.
+	_, _, err := applyOnce(t, `{"id":"notes.data","type":"directory","path":"/etc/sudoers.d","owner":"1001:1001"}`,
+		nil, store.State{})
+	refusedFor339(t, "/etc/sudoers.d owned by another account", err)
+	_, _, err = applyOnce(t, `{"id":"notes.data","type":"directory","path":"/etc/systemd/system/getty@.service.d",`+
+		`"owner":"1001:1001"}`, nil, store.State{})
+	refusedFor339(t, "a drop-in directory owned by another account", err)
+}
+
+func TestNoAccessIsAtTheMachinesOwn(t *testing.T) {
+	for _, path := range []string{"/", "/etc", "/etc/ssl", "/usr", "/var/lib", "/home", "/root", "/proc",
+		"/var/lib/mesh-host", "/run/docker"} {
+		_, _, err := applyOnce(t, `{"id":"notes.media","type":"access","path":"`+path+`"}`, nil, store.State{})
+		refusedFor339(t, "an access at "+path, err)
+	}
+}
+
+// A container is never handed the machine's own as a mount, nor a directory or access the engine refused.
+func TestNoContainerMountsTheMachinesOwn(t *testing.T) {
+	ran := false
+	run := func(ctx context.Context, name string, args ...string) (string, error) {
+		switch {
+		case args[0] == "info":
+			return "27.0\n", nil
+		case args[0] == "container":
+			return "", os.ErrNotExist
+		case args[0] == "run":
+			ran = true
+			return "deadbeef\n", nil
+		}
+		return "", nil
+	}
+	for _, mount := range []string{"/:/host", "/etc:/host-etc", "/var/lib:/data", "/home:/homes:ro",
+		"/var/lib/mesh-host:/state", "/proc:/p", "/dev:/d", "/root/.ssh:/keys", "/boot/efi:/efi"} {
+		ran = false
+		_, _, err := Apply(context.Background(), archHost(t), parseTrusted(t, `{"declaration":1,"resources":[
+		  {"id":"notes.server","type":"container","name":"notes","image":"`+pinned+`","volumes":["`+mount+`"]}
+		]}`), store.State{}, store.OriginCarried, run, nil, nil)
+		refusedFor339(t, "a container mounting "+mount, err)
+		if ran {
+			t.Fatalf("a container mounting %s was run", mount)
+		}
+	}
+}
+
+// The path is not followed: a link where a directory is declared, or a link on the way to it, is judged where
+// it leads.
+func TestALinkIsNotFollowedToTheMachinesOwn(t *testing.T) {
+	dir := t.TempDir()
+	target := filepath.Join(dir, "target")
+	if err := os.Mkdir(target, 0o700); err != nil {
+		t.Fatal(err)
+	}
+	link := filepath.Join(dir, "link")
+	if err := os.Symlink(target, link); err != nil {
+		t.Fatal(err)
+	}
+	_, _, err := applyOnce(t, `{"id":"notes.data","type":"directory","path":"`+link+`","mode":"0755"}`, nil, store.State{})
+	refusedFor339(t, "a directory that is a link", err)
+	if info, _ := os.Stat(target); info.Mode().Perm() != 0o700 {
+		t.Fatalf("the link's target was changed to %o", info.Mode().Perm())
+	}
+	// A link on the way: <dir>/via → /usr, so <dir>/via/share is /usr/share.
+	via := filepath.Join(dir, "via")
+	if err := os.Symlink("/usr", via); err != nil {
+		t.Fatal(err)
+	}
+	_, _, err = applyOnce(t, `{"id":"notes.data","type":"directory","path":"`+via+`/share"}`, nil, store.State{})
+	refusedFor339(t, "a directory reached through a link to /usr", err)
+	_, _, err = applyOnce(t, `{"id":"notes.media","type":"access","path":"`+via+`"}`, nil, store.State{})
+	refusedFor339(t, "an access reached through a link to /usr", err)
+}
+
+// A directory found here, that the mesh did not make, keeps its owner and mode; one the mesh made keeps the
+// declared mode, as before.
+func TestAFoundDirectoryIsUsedAsFound(t *testing.T) {
+	dir := t.TempDir()
+	found := filepath.Join(dir, "found")
+	if err := os.Mkdir(found, 0o700); err != nil {
+		t.Fatal(err)
+	}
+	report, state, err := applyOnce(t, `{"id":"notes.data","type":"directory","path":"`+found+`","mode":"0755"}`,
+		nil, store.State{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if info, _ := os.Stat(found); info.Mode().Perm() != 0o700 {
+		t.Fatalf("a directory the mesh did not make was chmodded to %o", info.Mode().Perm())
+	}
+	if len(report.Outcomes) != 1 || !strings.Contains(report.Outcomes[0].Detail, "used as found") {
+		t.Fatalf("the report does not say it was used as found: %+v", report.Outcomes)
+	}
+	if a, _ := state.Find("notes.data"); !a.AsFound {
+		t.Fatal("the record does not say the directory was found")
+	}
+	// Still found at the next apply, and never removed: undeclared, it is forgotten and left.
+	if _, state, err = applyOnce(t, `{"id":"notes.data","type":"directory","path":"`+found+`","mode":"0755"}`,
+		nil, state); err != nil {
+		t.Fatal(err)
+	}
+	if info, _ := os.Stat(found); info.Mode().Perm() != 0o700 {
+		t.Fatalf("a found directory was chmodded at the next apply to %o", info.Mode().Perm())
+	}
+	if _, _, err = applyOnce(t, `{"id":"notes.other","type":"directory","path":"`+filepath.Join(dir, "other")+`"}`,
+		nil, state); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := os.Stat(found); err != nil {
+		t.Fatalf("a found directory was removed: %v", err)
+	}
+
+	// The mesh's own: made by it, its mode is maintained.
+	made := filepath.Join(dir, "made")
+	_, state, err = applyOnce(t, `{"id":"notes.made","type":"directory","path":"`+made+`","mode":"0755"}`, nil, store.State{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Chmod(made, 0o700); err != nil {
+		t.Fatal(err)
+	}
+	if _, state, err = applyOnce(t, `{"id":"notes.made","type":"directory","path":"`+made+`","mode":"0755"}`,
+		nil, state); err != nil {
+		t.Fatal(err)
+	}
+	if info, _ := os.Stat(made); info.Mode().Perm() != 0o755 {
+		t.Fatalf("the mesh's own directory was not given its mode back: %o", info.Mode().Perm())
+	}
+	// Moved — a placement changed — onto a directory that was already there: that one is found, not the mesh's.
+	if _, _, err = applyOnce(t, `{"id":"notes.made","type":"directory","path":"`+found+`","mode":"0755"}`,
+		nil, state); err != nil {
+		t.Fatal(err)
+	}
+	if info, _ := os.Stat(found); info.Mode().Perm() != 0o700 {
+		t.Fatalf("a placement moved onto a found directory chmodded it to %o", info.Mode().Perm())
+	}
+}
+
+// The module's own directories keep being placed: below /var/lib, below /etc as root's, a container's mount of
+// the machine's docker socket.
+func TestAModulesOwnPlacesAreStillPlaced(t *testing.T) {
+	for _, path := range []string{"/var/lib/notes/data", "/etc/systemd/logind.conf.d", "/var/run/fail2ban",
+		"/srv/notes", "/mnt/plex/config", "/storage/media", "/var/lib/mesh/registry"} {
+		if err := refusePath(path, placingDirectory, "notes", ""); err != nil {
+			t.Errorf("%s: %v", path, err)
+		}
+	}
+	for _, path := range []string{"/var/lib/minio-store", "/var/run/docker.sock", "/services/ollama", "/storage/media",
+		"/etc/localtime", "/proc/cpuinfo", "/dev/null", "/dev/dri"} {
+		if err := refusePath(path, placingMount, "notes", ""); err != nil {
+			t.Errorf("a mount of %s: %v", path, err)
+		}
+	}
+	if err := refusePath("/var/lib/notes/data", placingDirectory, "notes", "1001:2000"); err != nil {
+		t.Errorf("an owner below /var/lib: %v", err)
+	}
+}

internal/apply/placement_homes_test.go

@@ -0,0 +1,67 @@
+package apply
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strconv"
+	"testing"
+
+	"github.com/novox/mesh-host/internal/store"
+)
+
+// Below a person's or an agent's home, a directory is that account's (novox/hq issue 339): root's, or another
+// account's, there is one the account does not control in a tree whose every parent it does — and a container
+// mounting one the engine refused is refused with it.
+func TestBelowAHomeOnlyThatAccountsDirectoryIsPlaced(t *testing.T) {
+	dir := t.TempDir()
+	home := filepath.Join(dir, "home", "alice")
+	if err := os.MkdirAll(home, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	me := os.Getuid()
+	passwd := filepath.Join(dir, "passwd")
+	if err := os.WriteFile(passwd, fmt.Appendf(nil, "root:x:0:0::/root:/bin/sh\nalice:x:%d:%d::%s:/bin/sh\n",
+		me, os.Getgid(), home), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	before := passwdFile
+	passwdFile = passwd
+	t.Cleanup(func() { passwdFile = before })
+
+	_, _, err := applyOnce(t, `{"id":"notes.data","type":"directory","path":"`+home+`/.config/autostart"}`, nil, store.State{})
+	refusedFor339(t, "root's directory below alice's home", err)
+	_, _, err = applyOnce(t, `{"id":"notes.data","type":"directory","path":"`+home+`/.config/autostart",`+
+		`"owner":"`+strconv.Itoa(me+1)+`"}`, nil, store.State{})
+	refusedFor339(t, "another account's directory below alice's home", err)
+	if _, err := os.Stat(filepath.Join(home, ".config")); err == nil {
+		t.Fatal("a refused directory's parent was made")
+	}
+	// Alice's own is placed.
+	if _, _, err := applyOnce(t, `{"id":"notes.data","type":"directory","path":"`+home+`/.config/notes",`+
+		`"owner":"`+strconv.Itoa(me)+":"+strconv.Itoa(os.Getgid())+`"}`, nil, store.State{}); err != nil {
+		t.Fatalf("alice's own directory: %v", err)
+	}
+	// A container mounting a refused directory is refused with it, and not run.
+	ran := false
+	run := func(ctx context.Context, name string, args ...string) (string, error) {
+		switch args[0] {
+		case "info":
+			return "27.0\n", nil
+		case "container":
+			return "", os.ErrNotExist
+		case "run":
+			ran = true
+		}
+		return "", nil
+	}
+	_, _, err = Apply(context.Background(), archHost(t), parseTrusted(t, `{"declaration":1,"resources":[
+	  {"id":"notes.data","type":"directory","path":"`+home+`/.ssh"},
+	  {"id":"notes.server","type":"container","name":"notes","image":"`+pinned+`","volumes":["`+home+`/.ssh:/keys"]}
+	]}`), store.State{}, store.OriginCarried, run, nil, nil)
+	refusedFor339(t, "a container mounting a refused directory", err)
+	if ran {
+		t.Fatal("a container mounting a refused directory was run")
+	}
+}

internal/store/store.go

@@ -131,6 +131,11 @@ type Applied struct {
 	// anything else in the directory, and leaves it in place.
 	Unpacked *Unpacked `json:"unpacked,omitempty"`
 
+	// AsFound is, for a directory, that it was there before the mesh first applied it and was not the
+	// declared owner and mode (novox/hq issue 339): the mesh uses it as found, never changing its owner or
+	// mode, and never removes it. Absent on a record from before: such a directory is the mesh's.
+	AsFound bool `json:"as_found,omitempty"`
+
 	// Reads is, for a container, the digest of each file it was created reading — its env-files
 	// and the files mounted into it — by path (novox/hq 04-ISSUES/103).
 	//