Re: [PATCH v2 1/4] host/rootfs: Sandbox crosvm

Alyssa Ross
2025-12-03T12:43:58Z

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Demi Marie Obenour <demiobenour@gmail.com> writes:

> This means that a breach of crosvm is not guaranteed to be fatal.
>
> The Wayland socket is still only accessible by root, so crosvm must run
> as root. The known container escape via /proc/self/exe is blocked by
> bwrap being on a read-only filesystem. Container escapes via /proc are
> blocked by remounting /proc read-only. Crosvm does not have
> CAP_SYS_ADMIN so it cannot change mounts.
>
> The two remaining steps are:
>
> - Run crosvm as an unprivileged user.
> - Enable seccomp to block most system calls.
>
> The latter should be done from within crosvm itself.
>
> Signed-off-by: Demi Marie Obenour <demiobenour@gmail.com>
> ---
> host/rootfs/default.nix | 4 ++--
> .../template/data/service/vhost-user-gpu/run | 24 +++++++++++++++++++++-
> 2 files changed, 25 insertions(+), 3 deletions(-)
>
> diff --git a/host/rootfs/default.nix b/host/rootfs/default.nix
> index ca2084f26d58be5e0e1695634e125032c50f82b2..4716bb7298515b2940cad09bb55e42c196ce7ebc 100644
> --- a/host/rootfs/default.nix
> +++ b/host/rootfs/default.nix
> @@ -10,7 +10,7 @@ pkgsMusl.callPackage (
>
> { spectrum-host-tools, spectrum-router
> , lib, stdenvNoCC, nixos, runCommand, writeClosure, erofs-utils, s6-rc
> -, btrfs-progs, busybox, cloud-hypervisor, cosmic-files, crosvm
> +, btrfs-progs, bubblewrap, busybox, cloud-hypervisor, cosmic-files, crosvm
> , cryptsetup, dejavu_fonts, dbus, execline, foot, fuse3, iproute2
> , inotify-tools, jq, kmod, mdevd, mesa, mount-flatpak, s6
> , s6-linux-init, socat, systemd, util-linuxMinimal, virtiofsd
> @@ -25,7 +25,7 @@ let
> trivial;
>
> packages = [
> - btrfs-progs cloud-hypervisor cosmic-files crosvm cryptsetup dbus
> + btrfs-progs bubblewrap cloud-hypervisor cosmic-files crosvm cryptsetup dbus
> execline fuse3 inotify-tools iproute2 jq kmod mdevd mount-flatpak s6
> s6-linux-init s6-rc socat spectrum-host-tools spectrum-router
> util-linuxMinimal virtiofsd xdg-desktop-portal-spectrum-host
> diff --git a/host/rootfs/image/etc/s6-linux-init/run-image/service/vm-services/template/data/service/vhost-user-gpu/run b/host/rootfs/image/etc/s6-linux-init/run-image/service/vm-services/template/data/service/vhost-user-gpu/run
> index 0b4f6a00bc7aed0e721454d584d3bcd47fb18e2a..9b5dfad91944bd2c6c8994f387ab91394c68c1df 100755
> --- a/host/rootfs/image/etc/s6-linux-init/run-image/service/vm-services/template/data/service/vhost-user-gpu/run
> +++ b/host/rootfs/image/etc/s6-linux-init/run-image/service/vm-services/template/data/service/vhost-user-gpu/run
> @@ -1,10 +1,32 @@
> #!/bin/execlineb -P
> # SPDX-License-Identifier: EUPL-1.2+
> # SPDX-FileCopyrightText: 2025 Alyssa Ross <hi@alyssa.is>
> +# SPDX-FileCopyrightText: 2025 Demi Marie Obenour <demiobenour@gmail.com>

You add a copyright line here, but not in subsequent patches. Is that
on purpose?

> s6-ipcserver -1a 0700 -C 1 -b 1 env/crosvm.sock
>
> -crosvm --no-syslog device gpu
> +bwrap
> + --unshare-all
> + --unshare-user

--unshare-all doesn't imply --unshare-user?

> + --bind /run/user/0/wayland-1 /run/user/0/wayland-1
> + --ro-bind /usr /usr
> + --ro-bind /lib /lib
> + --tmpfs /tmp
> + --dev /dev
> + --tmpfs /dev/shm
> + --ro-bind /nix /nix
> + --disable-userns
> + --proc /proc
> + --remount-ro /proc
> + --ro-bind /dev/null /proc/timer_list
> + --tmpfs /proc/scsi
> + --remount-ro /proc/scsi
> + --ro-bind /dev/null /proc/kcore
> + --ro-bind /dev/null /proc/sysrq-trigger
> + --tmpfs /proc/acpi
> + --remount-ro /proc/acpi
> + --
> + crosvm --no-syslog device gpu

No indent necessary here. This is a chain-loading program like many
others we use in execline scripts. We don't indent for those or the
rightwards drift would be ridiculous!

> --fd 0
> --wayland-sock /run/user/0/wayland-1
> --params "{\"context-types\":\"cross-domain\"}"

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