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

Demi Marie Obenour
2025-12-10T18:29:21Z

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On 12/10/25 07:47, Alyssa Ross wrote:
> 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:

Would it be better to run each VM's daemons as dedicated users?

> 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 }

Why is this -o nofail? Also, file descriptor 3 should be closed afterwards.

> 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 $@

--
Sincerely,
Demi Marie Obenour (she/her/hers)

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