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 -- Siemens AG, Technology Competence Center Embedded Linux
