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

Demi Marie Obenour
2025-12-03T12:49:04Z

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On 12/3/25 07:43, Alyssa Ross wrote:
> 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?

No.

>> 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?

It implies --unshare-user-try, but I want it to fail if it can't
create a user namespace.

>> + --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!

Should I indent the parameters above it?

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

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

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