[PATCH v4 1/6] host/rootfs: Sandbox crosvm

[Demi Marie Obenour <demiobenour@gmail.com>]

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       | 29 ++++++++++++++++++++++
 2 files changed, 31 insertions(+), 2 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..19d5a61388e6f49c7f722814d6de47227b02da01 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
@@ -1,9 +1,38 @@
 #!/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>
 
 s6-ipcserver -1a 0700 -C 1 -b 1 env/crosvm.sock
 
+bwrap
+  --unshare-all
+  # --unshare-all only implies --unshare-user-try.
+  # Make this more than a "try".
+  --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
+  --ro-bind /proc/sys /proc/sys
+  --tmpfs /proc/scsi
+  --remount-ro /proc/scsi
+  --tmpfs /proc/acpi
+  --remount-ro /proc/acpi
+  --tmpfs /proc/fs
+  --remount-ro /proc/fs
+  --tmpfs /proc/irq
+  --remount-ro /proc/irq
+  --ro-bind /dev/null /proc/timer_list
+  --ro-bind /dev/null /proc/kcore
+  --ro-bind /dev/null /proc/kallsyms
+  --ro-bind /dev/null /proc/sysrq-trigger
+  --
 crosvm --no-syslog device gpu
   --fd 0
   --wayland-sock /run/user/0/wayland-1

-- 
2.52.0