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

Jan Kiszka
2023-02-28T06:20:40Z

On 02.06.22 12:41, Christian Brauner wrote:
> On Tue, May 31, 2022 at 12:24:37PM -0700, Kees Cook wrote:
>> On Mon, May 30, 2022 at 10:13:58AM +0200, Christian Brauner wrote:
>>> On Sun, May 29, 2022 at 09:35:40PM +0200, Jan Kiszka wrote:
>>>> 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?
>>>
>>> Not that we know. I plan to queue this up for 5.20.
>>
>> Hello!
>>
>> Thanks for the thread-ping -- I hadn't had a chance to read through this
>> before, but since it's touching binfmt, it popped up on my radar. :)
>>
>> I like it overall, though I'd rather see it split up more (there's
>> some refactoring built into the patches that would be nice to split out
>> just to make review easier), but since others have already reviewed it,
>> that's probably overkill.
>>
>> I'd really like to see some self-tests for this, though. Especially
>
> Yeah, I had started writing them but decoupled the upstreaming. Imho,
> you can start queueing this up. I'd like this to have very long exposure
> in -next. I'll follow up with selftests in the next weeks. (I'm out for
> a conference this week.)
>
>> around the cred logic changes and the namespace fallback logic. I'd like
>> to explicitly document and test what the expectations are around the
>> mounts, etc.
>>
>> Finally, I'd prefer this went via the execve tree.
>
> I mentioned this yesterday to you but just so there's a paper trail:
> The series and this iteration preceeds the maintainer entry. That's the
> only reason this originally wasn't aimed at that tree when the series
> was sent. You've been in Cc from the start though. :)
> I'd like to step up and maintain the binfmt_misc fs going forward. There
> are other tweaks it could use.
>

Did anything happen after this? I'm not finding traced in lkml at least.

Jan

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