fix(ci): stop image-build /dev bind from wiping host /dev

A killed ubuntu system-image build can leave `mount --bind /dev` under
its build dir. The e2e Clean Workspace step then ran
`sudo rm -rf "$GITHUB_WORKSPACE"/*` as root, traversed the leaked bind,
and deleted the host's /dev (incl. /dev/null), wedging the runner.

- e2e.yml: umount anything under the workspace (deepest-first) before
  deleting, and pass `rm --one-file-system` so rm can never cross a mount
  boundary into host /dev. Hard backstop, applied to all 3 Clean Workspace
  steps.
- build-system-image.sh: recursive lazy unmount in cleanup() detaches the
  chroot binds even when busy, and mark the build subtree rprivate so the
  /dev bind can't propagate into the host namespace.

.github/workflows/e2e.yml

@@ -70,7 +70,15 @@ jobs:
           true
 
       - name: Clean Workspace
-        run: sudo rm -rf "$GITHUB_WORKSPACE"/*
+        run: |
+          # Detach any leaked mounts under the workspace before deleting. A
+          # killed image build can leave a `mount --bind /dev` under its build
+          # dir; if `rm -rf` traverses it as root it wipes the host's /dev
+          # (deletes /dev/null and wedges the box). umount deepest-first, then
+          # forbid rm from crossing any mount boundary as a hard backstop.
+          awk -v w="$GITHUB_WORKSPACE" '$2 ~ "^"w {print $2}' /proc/mounts \
+            | sort -r | xargs -r -I{} sudo umount -l -R {} || true
+          sudo rm -rf --one-file-system "$GITHUB_WORKSPACE"/*
 
       - uses: actions/create-github-app-token@v3
         id: app-token
@@ -385,7 +393,15 @@ jobs:
           true
 
       - name: Clean Workspace
-        run: sudo rm -rf "$GITHUB_WORKSPACE"/*
+        run: |
+          # Detach any leaked mounts under the workspace before deleting. A
+          # killed image build can leave a `mount --bind /dev` under its build
+          # dir; if `rm -rf` traverses it as root it wipes the host's /dev
+          # (deletes /dev/null and wedges the box). umount deepest-first, then
+          # forbid rm from crossing any mount boundary as a hard backstop.
+          awk -v w="$GITHUB_WORKSPACE" '$2 ~ "^"w {print $2}' /proc/mounts \
+            | sort -r | xargs -r -I{} sudo umount -l -R {} || true
+          sudo rm -rf --one-file-system "$GITHUB_WORKSPACE"/*
 
       - uses: actions/create-github-app-token@v3
         id: app-token
@@ -793,7 +809,15 @@ jobs:
           true
 
       - name: Clean Workspace
-        run: sudo rm -rf "$GITHUB_WORKSPACE"/*
+        run: |
+          # Detach any leaked mounts under the workspace before deleting. A
+          # killed image build can leave a `mount --bind /dev` under its build
+          # dir; if `rm -rf` traverses it as root it wipes the host's /dev
+          # (deletes /dev/null and wedges the box). umount deepest-first, then
+          # forbid rm from crossing any mount boundary as a hard backstop.
+          awk -v w="$GITHUB_WORKSPACE" '$2 ~ "^"w {print $2}' /proc/mounts \
+            | sort -r | xargs -r -I{} sudo umount -l -R {} || true
+          sudo rm -rf --one-file-system "$GITHUB_WORKSPACE"/*
 
       - uses: actions/create-github-app-token@v3
         id: app-token

scripts/build-system-image.sh

@@ -98,12 +98,13 @@ esac
 
 cleanup() {
     echo "Cleaning up..."
-    # Ubuntu chroot has bind mounts that must be released first
-    sudo umount "${MOUNT_DIR}/dev/pts" 2>/dev/null || true
-    sudo umount "${MOUNT_DIR}/dev"     2>/dev/null || true
-    sudo umount "${MOUNT_DIR}/sys"     2>/dev/null || true
-    sudo umount "${MOUNT_DIR}/proc"    2>/dev/null || true
-    sudo umount "${MOUNT_DIR}"         2>/dev/null || true
+    # Recursive lazy unmount detaches the chroot binds (/dev, /dev/pts, /sys,
+    # /proc) and the root mount from the path hierarchy at once, even if a
+    # straggler still holds one busy. This MUST succeed: a leaked /dev bind left
+    # under the build dir would be traversed by a later `rm -rf` and wipe the
+    # host's /dev (deletes /dev/null, wedges the box). -R recurses children,
+    # -l detaches busy mounts immediately so the path no longer crosses /dev.
+    sudo umount -R -l "${MOUNT_DIR}" 2>/dev/null || true
     sudo qemu-nbd --disconnect "${NBD_DEV}" 2>/dev/null || true
     exec 9>&- 2>/dev/null || true  # release nbd lock if held
     echo "Done."
@@ -288,6 +289,10 @@ fi
 # Step 4: Mount and customize
 echo "Mounting root filesystem..."
 sudo mount "${ROOT_PART}" "$MOUNT_DIR"
+# Mark the build-dir subtree private so the chroot's /dev bind below cannot
+# propagate into the host mount namespace (a propagated bind is what turned a
+# leaked mount into a host /dev wipe on the CI runners).
+sudo mount --make-rprivate "$MOUNT_DIR" 2>/dev/null || true
 
 # Set up resolv.conf for DNS inside chroot.
 # Ubuntu cloud images symlink /etc/resolv.conf → /run/systemd/resolve/stub-resolv.conf