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:

quoted

On 26.12.21 14:31, Serge E. Hallyn wrote:

quoted

On Thu, Dec 16, 2021 at 12:26:59PM +0100, Christian Brauner wrote:

quoted

From: Christian Brauner <redacted>

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 (local)
[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 (local) (origin)
Link: https://lore.kernel.org/r/20211028103114.2849140-2-brauner@kernel.org (local) (v1)
Cc: Sargun Dhillon <redacted>
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
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.

-Kees

-- 
Kees Cook