agent v0.36.4: serialize AttachDrive/DetachDrive (no double-bind TOCTOU race)

A GuestBinder mutex prevents a concurrent reconnect + periodic reconcile from
both passing isHostMountpoint and double-binding a stable path.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -3,6 +3,12 @@
 All notable changes to **felhom-agent** are recorded here. Update on every code
 change that gets pushed.
 
+## v0.36.4 — serialize AttachDrive/DetachDrive (no double-bind race) (2026-06-15)
+
+A mutex on GuestBinder serializes AttachDrive/DetachDrive so a controller-triggered reconnect and the
+agent`s periodic reconcile can no longer both pass the isHostMountpoint check and double-bind the same
+stable path (a TOCTOU race observed live as 2 stacked binds during rapid eject/reconnect).
+
 ## v0.36.3 — DetachDrive loop-umounts stacked binds (2026-06-15)
 
 DetachDrive now umounts ALL stacked binds at a stable path (bounded loop), not just one layer — so an

cmd/felhom-agent/main.go

@@ -44,7 +44,7 @@ import (
 
 // version is the agent version. Overridable at build time with
 // -ldflags "-X main.version=<v>"; defaults to the in-repo CHANGELOG version.
-var version = "0.36.3"
+var version = "0.36.4"
 
 // runGuestHook is the PVE pre-start hook body (`felhom-agent guest-hook <vmid> <phase>`). On the
 // pre-start phase it creates placeholder dirs for any absent bind-mount source so the guest always boots

internal/localapi/guestbind.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"log/slog"
 	"strconv"
+	"sync"
 
 	"gitea.dooplex.hu/admin/felhom-agent/internal/proxmox"
 )
@@ -35,6 +36,10 @@ const felhomDataNS = "felhom-data"
 type GuestBinder struct {
 	runner proxmox.Runner
 	logger *slog.Logger
+	// mountMu serializes AttachDrive/DetachDrive so a concurrent reconnect (controller-triggered) and the
+	// agent's periodic reconcile can't both pass the isHostMountpoint check and double-bind the same
+	// stable path (a TOCTOU race — observed live as 2 stacked binds).
+	mountMu sync.Mutex
 }
 
 // NewGuestBinder builds a binder over the given root-CLI runner.

internal/localapi/intermediary.go

@@ -151,6 +151,8 @@ func (b *GuestBinder) installSharedParentUnit(ctx context.Context) error {
 // (umount + mount) to fire a fresh propagation event into the current guest namespace. Idempotent when
 // the guest already sees it.
 func (b *GuestBinder) AttachDrive(ctx context.Context, vmid int, where string) (string, error) {
+	b.mountMu.Lock() // serialize vs a concurrent DetachDrive/AttachDrive (no double-bind TOCTOU)
+	defer b.mountMu.Unlock()
 	stable := StablePathForRaw(where)
 	if stable == "" {
 		return "", fmt.Errorf("guest-attach: %q is not a /mnt/<name> mount", where)
@@ -289,6 +291,8 @@ func hostBtime() string {
 // leaving the bare HOST-ROOT-owned stable dir → fail-closed (the guest can't write to it even as root,
 // since host uid 0 is unmapped). No pct, no reboot. Idempotent: a non-mountpoint is a no-op.
 func (b *GuestBinder) DetachDrive(ctx context.Context, where string) error {
+	b.mountMu.Lock() // serialize vs a concurrent AttachDrive (so detach can't race a re-bind)
+	defer b.mountMu.Unlock()
 	stable := StablePathForRaw(where)
 	if stable == "" {
 		return fmt.Errorf("guest-detach: %q is not a /mnt/<name> mount", where)