2017年1月23日月曜日

Docker vulnerability (CVE-2016-9962) PoC with SELinux (Again)

This blog is for following up to reproduce CVE-2016-9962(vulnerability for Docker) and how can we mitigate it by using SELinux.
(Written by Kazuki Omo:ka-omo@sios.com).

Reference

Docker vulnerability (CVE-2016-9962) PoC with SELinux

https://bugzilla.redhat.com/show_bug.cgi?id=1409531

https://bugzilla.suse.com/show_bug.cgi?id=1012568#c2

Mistake in Previous PoC

I sent previous PoC result to SELinux , I got result I did mistake in Previous PoC. Actually, I didn't use SELinux Access Control in Previous PoC
In Previous PoC, "[PoC] run container(sh) in shell1;"
[root@localhost ~]# runc run ctr

/ #
But Finally I found the "runc" program is working in "unconfined_t" domain. From another terminal, I checked runc domain;
[root@fedora25 ~]# ps axZ|grep runc
unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 1578 pts/0 Sl+
0:00 runc run ctr
So this means runc is working in unconfined_t domain, then that runc is having lots of permissions(actually un-confined) from SELinux.
This is a reason why SELinux couldn't mitigate my previous PoC.

Assign "container_t" domain on runc.

When I checked Fedora25 SELinux Policy, I found that container_t domain is switched from container_runtime_t(which is domain for docker process, etc.). And container_runtime_t is transited from initrc domain by exec container_runtime_exec_t file(/usr/bin/runc), such as;
[root@fedora25 ~]# ls -lZ /usr/bin/runc
-rwxr-xr-x. 1 root root system_u:object_r:container_runtime_exec_t:s0 5016704 Jan 20 19:26 /usr/bin/runc



./container.cil:(typetransition initrc_domain container_runtime_exec_t process container_runtime_t)
Then, for doing PoC more close to existance situation, we need to run "runc" as "container_t" domain.
For running "runc" as "container_t" domain, we need to add several policy (typetransition rule and more allow rule) to transit from unconfined_t to container_t domain. Also I changed PoC directory from /root to /tmp.

Changed PoC directory.

For more easy to write Policy for this PoC, I changed PoC directory to /tmp /PoC-CVE-2016-9962;
[root@fedora25 PoC-CVE-2016-9962]# pwd
/tmp/PoC-CVE-2016-9962
[root@fedora25 PoC-CVE-2016-9962]# ls -l
total 4
-rw-r--r--.  1 root root 2364 Jan 20 09:04 config.json
drwxr-xr-x. 18 root root  380 Jan 23 12:33 rootfs

Additional Policy

Just for PoC, I made below policy rule file as "/root/custom_policy/runc.cil";
(typetransition unconfined_usertype container_runtime_exec_t process container_t
)
(roletransition unconfined_r container_runtime_exec_t process system_r)

(allow container_t user_tmp_t (file (open read execute execute_no_trans)))
(allow container_t var_run_t (dir (write add_name create setattr remove_name rmd
ir)))
(allow container_t var_run_t (fifo_file (create setattr unlink read open)))
(allow container_t ptmx_t (chr_file (read write open ioctl)))
(allow container_t devpts_t (chr_file (setattr read write open ioctl getattr)))
(allow container_t root_t (dir (mounton)))
(allow container_t user_tmp_t (dir (mounton write add_name create remove_name rm
dir)))
(allow container_t user_tmp_t (lnk_file (read)))
(allow container_t proc_t (filesystem (mount remount)))
(allow container_t tmpfs_t (filesystem (mount remount)))
(allow container_t tmpfs_t (dir (setattr write add_name create mounton)))
(allow container_t devpts_t (filesystem (mount)))
(allow container_t sysfs_t (filesystem (mount)))
(allow container_t cgroup_t (filesystem (remount)))
(allow container_t tmpfs_t (lnk_file (create)))
(allow container_t tmpfs_t (chr_file (create setattr read write open getattr ioc
tl append)))
(allow container_t tmpfs_t (file (open create mounton)))
(allow container_t proc_t (dir (mounton)))
(allow container_t proc_t (file (mounton)))
(allow container_t sysctl_irq_t (dir (mounton)))
(allow container_t sysctl_t (dir (mounton)))
(allow container_t sysctl_t (file (mounton)))
(allow container_t proc_kcore_t (file (mounton)))
(allow container_t nsfs_t (file (getattr read open)))
(allow container_t var_run_t (file (create read write open unlink)))
(allow container_t sysfs_t (dir (mounton)))
(allow container_t kernel_t (unix_stream_socket (read write)))
(allow init_t kernel_t (unix_stream_socket (read write)))
(allow container_t init_t (unix_stream_socket (read write)))
[root@fedora25 custom_policy]# cat runc.cil
(typetransition unconfined_usertype container_runtime_exec_t process container_t)
(roletransition unconfined_r container_runtime_exec_t process system_r)

(allow container_t user_tmp_t (file (open read execute execute_no_trans)))
(allow container_t var_run_t (dir (write add_name create setattr remove_name rmdir)))
(allow container_t var_run_t (fifo_file (create setattr unlink read open)))
(allow container_t ptmx_t (chr_file (read write open ioctl)))
(allow container_t devpts_t (chr_file (setattr read write open ioctl getattr)))
(allow container_t root_t (dir (mounton)))
(allow container_t user_tmp_t (dir (mounton write add_name create remove_name rmdir)))
(allow container_t user_tmp_t (lnk_file (read)))
(allow container_t proc_t (filesystem (mount remount)))
(allow container_t tmpfs_t (filesystem (mount remount)))
(allow container_t tmpfs_t (dir (setattr write add_name create mounton)))
(allow container_t devpts_t (filesystem (mount)))
(allow container_t sysfs_t (filesystem (mount)))
(allow container_t cgroup_t (filesystem (remount)))
(allow container_t tmpfs_t (lnk_file (create)))
(allow container_t tmpfs_t (chr_file (create setattr read write open getattr ioctl append)))
(allow container_t tmpfs_t (file (open create mounton)))
(allow container_t proc_t (dir (mounton)))
(allow container_t proc_t (file (mounton)))
(allow container_t sysctl_irq_t (dir (mounton)))
(allow container_t sysctl_t (dir (mounton)))
(allow container_t sysctl_t (file (mounton)))
(allow container_t proc_kcore_t (file (mounton)))
(allow container_t nsfs_t (file (getattr read open)))
(allow container_t var_run_t (file (create read write open unlink)))
(allow container_t sysfs_t (dir (mounton)))
(allow container_t kernel_t (unix_stream_socket (read write)))
(allow init_t kernel_t (unix_stream_socket (read write)))
(allow container_t init_t (unix_stream_socket (read write)))

Load Additional Policy to PoC system

On the PoC system
[root@fedora25 ~]# semodule -i /root/custom_policy/runc.cil

Check custom policy is working or not.

After load runc.cil, run "runc run ctr" in a terminal;
[root@fedora25 PoC-CVE-2016-9962]# runc run ctr
/ #
Then open another terminal and check this "runc" program is working as "container_t" domain;
[root@fedora25 ~]# ps axZ|grep runc
unconfined_u:system_r:container_t:s0-s0:c0.c1023 6799 pts/1 Sl+   0:00 runc run ctr
If the "runc" is not working on container_t domain, chack /var/log/audit/audit.log and maybe add several another rules to runc.cil.

PoC(Again!)

  • open 2 terminals(shell1, shell2).
  • Check SELinux is enabled;
    [root@localhost ~]# getenforce
    Enforcing
    
  • run container(sh) in shell1;
    [root@localhost ~]# runc run ctr
    / #
    
  • run new container in shell2 with "runc exec" command. It is pausing 500sec;
    [root@localhost ~]# runc exec ctr sh
    
  • run "ps ax" in shell1. You can see shell2 process;
    [root@localhost ~]# runc run ctr
    / # ps ax
    PID   USER     TIME   COMMAND
        1 root       0:00 sh
        6 root       0:00 /proc/self/exe init
       11 root       0:00 ps ax
    / #
    
  • In above case, check /proc/6/fd by ls command. You see path as fd/4;
    / # ls -la /proc/6/fd/
    total 0
    dr-x------    2 root     root             0 Jan 16 06:43 .
    dr-xr-xr-x    9 root     root             0 Jan 16 06:43 ..
    lrwx------    1 root     root            64 Jan 16 06:43 0 -> /dev/pts/4
    lrwx------    1 root     root            64 Jan 16 06:43 1 -> /dev/pts/4
    lrwx------    1 root     root            64 Jan 16 06:43 2 -> /dev/pts/4
    lrwx------    1 root     root            64 Jan 16 06:43 3 -> socket:[40487]
    lr-x------    1 root     root            64 Jan 16 06:43 4 -> /run/runc/ctr
    lrwx------    1 root     root            64 Jan 16 06:43 5 -> /dev/pts/4
    lr-x------    1 root     root            64 Jan 16 06:43 6 -> pipe:[40496]
    l-wx------    1 root     root            64 Jan 16 06:43 7 -> /dev/null
    / #
    
  • Do ls /etc/shadow file by using "/proc/6/fd/4/../../../etc/shadow" path;
    / # ls -l /proc/6/fd/4/../../../etc/shadow
    ls: /proc/6/fd/4/../../../etc/shadow: Permission denied
    
Check /var/log/audit/audit.log;
    type=AVC msg=audit(1485143271.659:3107): avc:  denied  { getattr } for  pid=8847 comm="ls" path="/etc/shadow" dev="dm-0" ino=785423 scontext=unconfined_u:system_r:container_t:s0-s0:c0.c1023 tcontext=system_u:object_r:shadow_t:s0 tclass=file permissive=0
Fine, Now SELinux is protecting to search /etc/shadow file.
  • Also you can't read /etc/shadow file because the file permission is "000".
    / # cat /proc/6/fd/4/../../../etc/shadow
    cat: can't open '/proc/6/fd/4/../../../etc/shadow': Permission denied
    
If I change /etc/shadow file as "755" permission, the DAC control will permit to read the file.
    [root@fedora25 ~]# chmod 755 /etc/shadow
    [root@fedora25 ~]# ls -lh /etc/shadow
    -rwxr-xr-x. 1 root root 1.3K Jan 20 08:30 /etc/shadow
But still I can't read /etc/shadow file ;
    / # cat /proc/6/fd/4/../../../etc/shadow
    cat: can't open '/proc/6/fd/4/../../../etc/shadow': Permission denied
I could see in the /var/log/audit/audit.log file that the"read" action is denied by SELinux;
    type=AVC msg=audit(1485143502.183:3311): avc:  denied  { read } for  pid=9418 comm="cat" name="shadow" dev="dm-0" ino=785423 scontext=unconfined_u:system_r:container_t:s0-s0:c0.c1023 tcontext=system_u:object_r:shadow_t:s0 tclass=file permissive=0

Conclusion

Now we know from PoC that we could mitigate CVE-2016-9962 by enabling SELinux.
So we should enable SELinux in container environment also and making more safety(and keeping to have mitigate way even if in 0-day situation) environment.

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