Re: [PATCH v2 2/2] binfmt_misc: enable sandboxed mounts

[Jan Kiszka <jan.kiszka@siemens.com>]

On 26.12.21 14:31, Serge E. Hallyn wrote:
> On Thu, Dec 16, 2021 at 12:26:59PM +0100, Christian Brauner wrote:
>> From: Christian Brauner <christian.brauner@ubuntu.com>
>>
>> Enable unprivileged sandboxes to create their own binfmt_misc mounts.
>> This is based on Laurent's work in [1] but has been significantly
>> reworked to fix various issues we identified in earlier versions.
>>
>> While binfmt_misc can currently only be mounted in the initial user
>> namespace, binary types registered in this binfmt_misc instance are
>> available to all sandboxes (Either by having them installed in the
>> sandbox or by registering the binary type with the F flag causing the
>> interpreter to be opened right away). So binfmt_misc binary types are
>> already delegated to sandboxes implicitly.
>>
>> However, while a sandbox has access to all registered binary types in
>> binfmt_misc a sandbox cannot currently register its own binary types
>> in binfmt_misc. This has prevented various use-cases some of which were
>> already outlined in [1] but we have a range of issues associated with
>> this (cf. [3]-[5] below which are just a small sample).
>>
>> Extend binfmt_misc to be mountable in non-initial user namespaces.
>> Similar to other filesystem such as nfsd, mqueue, and sunrpc we use
>> keyed superblock management. The key determines whether we need to
>> create a new superblock or can reuse an already existing one. We use the
>> user namespace of the mount as key. This means a new binfmt_misc
>> superblock is created once per user namespace creation. Subsequent
>> mounts of binfmt_misc in the same user namespace will mount the same
>> binfmt_misc instance. We explicitly do not create a new binfmt_misc
>> superblock on every binfmt_misc mount as the semantics for
>> load_misc_binary() line up with the keying model. This also allows us to
>> retrieve the relevant binfmt_misc instance based on the caller's user
>> namespace which can be done in a simple (bounded to 32 levels) loop.
>>
>> Similar to the current binfmt_misc semantics allowing access to the
>> binary types in the initial binfmt_misc instance we do allow sandboxes
>> access to their parent's binfmt_misc mounts if they do not have created
>> a separate binfmt_misc instance.
>>
>> Overall, this will unblock the use-cases mentioned below and in general
>> will also allow to support and harden execution of another
>> architecture's binaries in tight sandboxes. For instance, using the
>> unshare binary it possible to start a chroot of another architecture and
>> configure the binfmt_misc interpreter without being root to run the
>> binaries in this chroot and without requiring the host to modify its
>> binary type handlers.
>>
>> Henning had already posted a few experiments in the cover letter at [1].
>> But here's an additional example where an unprivileged container
>> registers qemu-user-static binary handlers for various binary types in
>> its separate binfmt_misc mount and is then seamlessly able to start
>> containers with a different architecture without affecting the host:
>>
>> root    [lxc monitor] /var/snap/lxd/common/lxd/containers f1
>> 1000000  \_ /sbin/init
>> 1000000      \_ /lib/systemd/systemd-journald
>> 1000000      \_ /lib/systemd/systemd-udevd
>> 1000100      \_ /lib/systemd/systemd-networkd
>> 1000101      \_ /lib/systemd/systemd-resolved
>> 1000000      \_ /usr/sbin/cron -f
>> 1000103      \_ /usr/bin/dbus-daemon --system --address=systemd: --nofork --nopidfile --systemd-activation --syslog-only
>> 1000000      \_ /usr/bin/python3 /usr/bin/networkd-dispatcher --run-startup-triggers
>> 1000104      \_ /usr/sbin/rsyslogd -n -iNONE
>> 1000000      \_ /lib/systemd/systemd-logind
>> 1000000      \_ /sbin/agetty -o -p -- \u --noclear --keep-baud console 115200,38400,9600 vt220
>> 1000107      \_ dnsmasq --conf-file=/dev/null -u lxc-dnsmasq --strict-order --bind-interfaces --pid-file=/run/lxc/dnsmasq.pid --liste
>> 1000000      \_ [lxc monitor] /var/lib/lxc f1-s390x
>> 1100000          \_ /usr/bin/qemu-s390x-static /sbin/init
>> 1100000              \_ /usr/bin/qemu-s390x-static /lib/systemd/systemd-journald
>> 1100000              \_ /usr/bin/qemu-s390x-static /usr/sbin/cron -f
>> 1100103              \_ /usr/bin/qemu-s390x-static /usr/bin/dbus-daemon --system --address=systemd: --nofork --nopidfile --systemd-ac
>> 1100000              \_ /usr/bin/qemu-s390x-static /usr/bin/python3 /usr/bin/networkd-dispatcher --run-startup-triggers
>> 1100104              \_ /usr/bin/qemu-s390x-static /usr/sbin/rsyslogd -n -iNONE
>> 1100000              \_ /usr/bin/qemu-s390x-static /lib/systemd/systemd-logind
>> 1100000              \_ /usr/bin/qemu-s390x-static /sbin/agetty -o -p -- \u --noclear --keep-baud console 115200,38400,9600 vt220
>> 1100000              \_ /usr/bin/qemu-s390x-static /sbin/agetty -o -p -- \u --noclear --keep-baud pts/0 115200,38400,9600 vt220
>> 1100000              \_ /usr/bin/qemu-s390x-static /sbin/agetty -o -p -- \u --noclear --keep-baud pts/1 115200,38400,9600 vt220
>> 1100000              \_ /usr/bin/qemu-s390x-static /sbin/agetty -o -p -- \u --noclear --keep-baud pts/2 115200,38400,9600 vt220
>> 1100000              \_ /usr/bin/qemu-s390x-static /sbin/agetty -o -p -- \u --noclear --keep-baud pts/3 115200,38400,9600 vt220
>> 1100000              \_ /usr/bin/qemu-s390x-static /lib/systemd/systemd-udevd
>>
>> [1]: https://lore.kernel.org/all/20191216091220.465626-1-laurent@vivier.eu
>> [2]: https://discuss.linuxcontainers.org/t/binfmt-misc-permission-denied
>> [3]: https://discuss.linuxcontainers.org/t/lxd-binfmt-support-for-qemu-static-interpreters
>> [4]: https://discuss.linuxcontainers.org/t/3-1-0-binfmt-support-service-in-unprivileged-guest-requires-write-access-on-hosts-proc-sys-fs-binfmt-misc
>> [5]: https://discuss.linuxcontainers.org/t/qemu-user-static-not-working-4-11
>>
>> Link: https://lore.kernel.org/r/20191216091220.465626-2-laurent@vivier.eu (origin)
>> Link: https://lore.kernel.org/r/20211028103114.2849140-2-brauner@kernel.org (v1)
>> Cc: Sargun Dhillon <sargun@sargun.me>
>> Cc: Serge Hallyn <serge@hallyn.com>
> 
> (one typo below)
> 
> Acked-by: Serge Hallyn <serge@hallyn.com>
> 

What happened to this afterwards? Any remaining issues?

Jan

-- 
Siemens AG, Technology
Competence Center Embedded Linux