[PATCH 8/8] host/rootfs: run filesystem daemons as non-root

Alyssa Ross
2025-12-10T12:48:47Z

We'd like these to be non-root, but xdg-document-portal in
particular still needs to be root within its namespace so it can mount
a fuse filesystem. We therefore map the fs user in the host namespace
to root in the new namespace, and pass through every non-root user so
non-root users (e.g. for xdg-desktop-portal-spectrum) are still usable
within the namespace.

Signed-off-by: Alyssa Ross <hi@alyssa.is>
---
.../image/etc/s6-linux-init/run-image/etc/group | 1 +
.../image/etc/s6-linux-init/run-image/etc/passwd | 1 +
.../vm-services/template/data/service/dbus/run | 6 +++++-
.../template/data/service/vhost-user-fs/run | 7 ++++++-
.../service/xdg-desktop-portal-spectrum-host/run | 6 ++++++
host/rootfs/image/usr/bin/create-vm-dependencies | 13 +++++++++----
host/rootfs/image/usr/bin/run-flatpak | 8 ++++++--
7 files changed, 34 insertions(+), 8 deletions(-)

diff --git a/host/rootfs/image/etc/s6-linux-init/run-image/etc/group b/host/rootfs/image/etc/s6-linux-init/run-image/etc/group
index 019f5525..6e894d93 100644
--- a/host/rootfs/image/etc/s6-linux-init/run-image/etc/group
+++ b/host/rootfs/image/etc/s6-linux-init/run-image/etc/group
@@ -14,3 +14,4 @@ cdrom:x:12:
tape:x:13:
kvm:x:14:
wayland:x:15:wayland
+fs:x:1000:
diff --git a/host/rootfs/image/etc/s6-linux-init/run-image/etc/passwd b/host/rootfs/image/etc/s6-linux-init/run-image/etc/passwd
index 50def56d..dc104ec1 100644
--- a/host/rootfs/image/etc/s6-linux-init/run-image/etc/passwd
+++ b/host/rootfs/image/etc/s6-linux-init/run-image/etc/passwd
@@ -1,2 +1,3 @@
root:x:0:0:System administrator:/:/bin/sh
wayland:x:15:15:Wayland compositor:/:/bin/nologin
+fs:x:1000:1000:Spectrum files:/:/bin/nologin
diff --git a/host/rootfs/image/etc/s6-linux-init/run-image/service/vm-services/template/data/service/dbus/run b/host/rootfs/image/etc/s6-linux-init/run-image/service/vm-services/template/data/service/dbus/run
index 20f1daff..7330ab4c 100755
--- a/host/rootfs/image/etc/s6-linux-init/run-image/service/vm-services/template/data/service/dbus/run
+++ b/host/rootfs/image/etc/s6-linux-init/run-image/service/vm-services/template/data/service/dbus/run
@@ -14,8 +14,12 @@ s6-ipcserver-socketbinder -B /run/portal-bus/${VM}
fdmove -c 3 0
redirfd -r 0 /dev/null

+s6-envuidgid fs
+s6-applyuidgid -Uzu 0
getcwd -E dir
-nsenter --mount=/run/vm/by-id/${VM}/mount
+nsenter --preserve-credentials -S0
+ --mount=/run/vm/by-id/${VM}/mount
+ --user=/run/vm/by-id/${VM}/user

unshare --cgroup --ipc --net --uts

diff --git a/host/rootfs/image/etc/s6-linux-init/run-image/service/vm-services/template/data/service/vhost-user-fs/run b/host/rootfs/image/etc/s6-linux-init/run-image/service/vm-services/template/data/service/vhost-user-fs/run
index 116570c3..525940d1 100755
--- a/host/rootfs/image/etc/s6-linux-init/run-image/service/vm-services/template/data/service/vhost-user-fs/run
+++ b/host/rootfs/image/etc/s6-linux-init/run-image/service/vm-services/template/data/service/vhost-user-fs/run
@@ -10,9 +10,14 @@ redirfd -r 0 /dev/null

export TMPDIR /run

+s6-envuidgid fs
+s6-applyuidgid -Uzu 0
importas -i VM VM
+nsenter --preserve-credentials -S0
+ --mount=/run/vm/by-id/${VM}/mount
+ --user=/run/vm/by-id/${VM}/user

-nsenter --mount=/run/vm/by-id/${VM}/mount
+# Show the guest files owned by uid/gid 1000.
unshare -U --map-user 1000 --map-group 1000 --uts --ipc --cgroup

virtiofsd --fd 3 --shared-dir /run/fs/${VM}
diff --git a/host/rootfs/image/etc/s6-linux-init/run-image/service/vm-services/template/data/service/xdg-desktop-portal-spectrum-host/run b/host/rootfs/image/etc/s6-linux-init/run-image/service/vm-services/template/data/service/xdg-desktop-portal-spectrum-host/run
index b83d23dd..cb2195d1 100755
--- a/host/rootfs/image/etc/s6-linux-init/run-image/service/vm-services/template/data/service/xdg-desktop-portal-spectrum-host/run
+++ b/host/rootfs/image/etc/s6-linux-init/run-image/service/vm-services/template/data/service/xdg-desktop-portal-spectrum-host/run
@@ -13,6 +13,12 @@ s6-ipcserver-socketbinder -a 0700 /run/vsock/${VM}/vsock_219
if { fdmove 1 3 echo }
fdclose 3

+s6-envuidgid fs
+s6-applyuidgid -Uzu 0
+nsenter --preserve-credentials -S0
+ --mount=/run/vm/by-id/${VM}/mount
+ --user=/run/vm/by-id/${VM}/user
+
s6-setuidgid xdp-spectrum-${VM}

xdg-desktop-portal-spectrum-host
diff --git a/host/rootfs/image/usr/bin/create-vm-dependencies b/host/rootfs/image/usr/bin/create-vm-dependencies
index 344e7778..6f9d0a60 100755
--- a/host/rootfs/image/usr/bin/create-vm-dependencies
+++ b/host/rootfs/image/usr/bin/create-vm-dependencies
@@ -14,16 +14,21 @@ if {
}

if {
- unshare --propagation=slave
- --map-users all
- --map-groups all
+ redirfd -r 3 /run/vm/by-id/${1}/config
+
+ s6-envuidgid fs
+ s6-applyuidgid -Uzu 0
+
+ unshare -S0 --propagation=slave
+ --map-users 0:1000:1 --map-users 1:1:999 --map-users 1001:1001:4294966294
+ --map-groups 0:1000:1 --map-groups 1:1:999 --map-groups 1001:1001:4294966294
--mount=/run/vm/by-id/${1}/mount
--user=/run/vm/by-id/${1}/user

# The VM should not be able to write directly into a tmpfs, and the host
# should be able to assume there are no untrusted symlinks there, but there
# can be writable block-based bind mounted subdirectories.
- if { mount --make-shared --rbind -o nofail /run/vm/by-id/${1}/config/fs /run/fs/${1}/config }
+ if { mount --make-shared --rbind -o nofail /proc/self/fd/3/fs /run/fs/${1}/config }
if { mount --rbind -o ro /run/fs/${1} /run/fs/${1} }

if { mount --make-shared --rbind /run/doc/${1} /run/doc/${1} }
diff --git a/host/rootfs/image/usr/bin/run-flatpak b/host/rootfs/image/usr/bin/run-flatpak
index f9179819..695df21f 100755
--- a/host/rootfs/image/usr/bin/run-flatpak
+++ b/host/rootfs/image/usr/bin/run-flatpak
@@ -10,7 +10,7 @@ if { useradd -P /run -Urd / -s /bin/nologin xdp-spectrum-${id} }
if {
elgetpositionals

- if { mkdir -p /run/configs/${id}/fs }
+ if { install -do fs /run/configs/${id}/fs }

if {
ln -s /usr/lib/spectrum/img/appvm/blk /usr/lib/spectrum/img/appvm/vmlinux
@@ -22,7 +22,11 @@ if {
if { create-vm-dependencies $id }

if {
- nsenter --mount=${dir}/mount
+ s6-envuidgid fs
+ s6-applyuidgid -Uzu 0
+ nsenter --preserve-credentials -S0
+ --mount=/run/vm/by-id/${id}/mount
+ --user=/run/vm/by-id/${id}/user
cd /run/fs/${id}/config
if { redirfd -w 1 type echo flatpak }
mount-flatpak $@
--
2.51.0