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.
