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
