On Thu, Nov 04, 2021 at 11:30:00PM -0500, Serge Hallyn wrote: > On Thu, Oct 28, 2021 at 12:31:14PM +0200, Christian Brauner wrote: > > From: Laurent Vivier <laurent@xxxxxxxxx> > > > > 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: > > > > [lxc monitor] /var/lib/lxc imp2 > > \_ /sbin/init > > \_ /lib/systemd/systemd-journald > > \_ /lib/systemd/systemd-udevd > > \_ /lib/systemd/systemd-networkd > > \_ /usr/sbin/cron -f -P > > \_ @dbus-daemon --system --address=systemd: --nofork --nopidfile --systemd-activation --syslog-only > > \_ /usr/bin/python3 /usr/bin/networkd-dispatcher --run-startup-triggers > > \_ /usr/sbin/rsyslogd -n -iNONE > > \_ /lib/systemd/systemd-logind > > \_ /lib/systemd/systemd-resolved > > \_ dnsmasq --conf-file=/dev/null -u lxc-dnsmasq --strict-order --bind-interfaces --pid-file=/run/lxc/dnsmasq.pid --liste > > \_ /sbin/agetty -o -p -- \u --noclear --keep-baud console 115200,38400,9600 vt220 > > \_ /sbin/agetty -o -p -- \u --noclear --keep-baud pts/0 115200,38400,9600 vt220 > > \_ /sbin/agetty -o -p -- \u --noclear --keep-baud pts/1 115200,38400,9600 vt220 > > \_ /sbin/agetty -o -p -- \u --noclear --keep-baud pts/2 115200,38400,9600 vt220 > > \_ /sbin/agetty -o -p -- \u --noclear --keep-baud pts/3 115200,38400,9600 vt220 > > \_ [lxc monitor] /var/lib/lxc alp1 > > \_ /usr/libexec/qemu-binfmt/ppc64le-binfmt-P /sbin/init /sbin/init > > \_ /usr/libexec/qemu-binfmt/ppc64le-binfmt-P /lib/systemd/systemd-journald /lib/systemd/systemd-journald > > \_ /usr/libexec/qemu-binfmt/ppc64le-binfmt-P /lib/systemd/systemd-udevd /lib/systemd/systemd-udevd > > \_ /usr/libexec/qemu-binfmt/ppc64le-binfmt-P /usr/sbin/cron /usr/sbin/cron -f -P > > \_ /usr/libexec/qemu-binfmt/ppc64le-binfmt-P /lib/systemd/systemd-resolved /lib/systemd/systemd-resolved > > \_ /usr/libexec/qemu-binfmt/ppc64le-binfmt-P /lib/systemd/systemd-logind /lib/systemd/systemd-logind > > > > Link: https://lore.kernel.org/r/20191216091220.465626-2-laurent@xxxxxxxxx > > [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 > > Cc: Sargun Dhillon <sargun@xxxxxxxxx> > > Cc: Serge Hallyn <serge@xxxxxxxxxx> > > I *think* this is ok. I'm still trying to convince myself that there is > no way for evict_inode() to run after the kfree(ns->binfmt_misc), but > it doesn't look like there is. > > Does this memory (as the number of register entries grows) need to be > accounted for and/or limited ? Good point. We should pass GFP_KERNEL_ACCOUNT andor use a cache with SLAB_ACCOUNT. I'll fix that up. Christian
