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

[Kees Cook]

On Wed, Jul 12, 2023 at 01:54:48PM -0500, Serge E. Hallyn wrote:
> On Tue, Feb 28, 2023 at 07:20:28AM +0100, Jan Kiszka wrote:
> > 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@xxxxxxxxxx>
> > >>>>>>
> > >>>>>> 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@xxxxxxxxx
> > >>>>>> [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@xxxxxxxxx (origin)
> > >>>>>> Link: https://lore.kernel.org/r/20211028103114.2849140-2-brauner@xxxxxxxxxx (v1)
> > >>>>>> Cc: Sargun Dhillon <sargun@xxxxxxxxx>
> > >>>>>> Cc: Serge Hallyn <serge@xxxxxxxxxx>
> > >>>>>
> > >>>>> (one typo below)
> > >>>>>
> > >>>>> Acked-by: Serge Hallyn <serge@xxxxxxxxxx>
> > >>>>>
> > >>>>
> > >>>> 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
> 
> Looking at https://lore.kernel.org/all/20230630-hufen-herzallerliebst-fde8e7aecba0@brauner/
> looks like Christian was going to ping Kees about taking
> https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git/log/?h=vfs.binfmt_misc

Ah yeah! I forgot all about this series. I can pull this into the execve
tree now for max linux-next testing.

-- 
Kees Cook