Serving up Sftp and AppArmor September 9, 2009

Recently I decided to replace NFS on a small network with something that was more secure and more resistant to network failures (as in, Nautilus wouldn’t hang because of a symlink to a directory in an autofs mounted NFS share). Most importantly, I wanted something that was secure, simple and robust. I naturally thought of SFTP, but there were at least two problems with a naive SFTP implementation, both of which I decided I must solve to meet the ‘secure’ criteria:

• shell access to the file server
• SFTP can’t restrict users to a particular directory

Of course, there are other alternatives that I could have pursued: sshfs, shfs, SFTP with scponly, patching OpenSSH to support chrooting (see ‘Update’ below), NFSv4, IPsec, etc. Adding all the infrastructure to properly support NFSv4 and IPsec did not meet the simplicity requirement, and neither did running a patched OpenSSH server. sshfs, shfs, and SFTP with scponly did not really fit the bill either.

What did I come up with? A combination of SFTP, a hardened sshd configuration and AppArmor on Ubuntu.

SFTP setup
This was easy because sftp is enabled by default on Ubuntu and Debian systems. This should be enough:

$ sudo apt-get install openssh-server
Just make sure you have the following set in /etc/ssh/sshd_config (see man sshd_config for details).

Subsystem sftp /usr/lib/openssh/sftp-server

With that in place, all I needed to do was add users with strong passwords:

$ sudo adduser sftupser1
$ sudo adduser sftpuser2

Then you can test if it worked with:

$ sftp sftpuser1@server
sftp> ls /
/bin ...


Hardened sshd
Now that SFTP is working, we need to limit access. One way to do this is via a Match rule that uses a ForceCommand. Combined with AllowUsers, adding something like this to /etc/ssh/sshd_config is pretty powerful:

AllowUsers adminuser sftpuser1@192.168.0.10 sftpuser2
Match User sftpuser1,sftpuser2
    AllowTcpForwarding no
    X11Forwarding no
    ForceCommand /usr/lib/openssh/sftp-server -l INFO

Remember to restart ssh with ‘/etc/init.d/ssh restart’. The above allows normal shell access for the adminuser, SFTP-only access to sftpuser1 from 192.168.0.10 and to sftupser2 from anywhere. One can imagine combining this with ‘PasswordAuthentication no’ or GSSAPI to enforce more stringent authentication so access is even more tightly controlled.

AppArmor
The above does a lot to increase security over the standard NFS shares that I had before. Good encryption, strong authentication and reliable UID mappings for DAC (POSIX discretionary access controls) are all in place. However, it doesn’t have the ability to confine access to a certain directory like NFS does. A simple AppArmor profile can achieve this and give even more granularity than just DAC. Imagine the following directory structure:

• /var/exports (top-level ‘exported’ filesystem (where all the files you want to share are))
• /var/exports/users (user-specific files, only to be accessed by the users themselves)
• /var/exports/shared (a free-for-all shared directory where any user can put stuff. The ‘shared’ directory has ‘2775’ permissions with group ‘shared’)

Now add to /etc/apparmor.d/usr.lib.openssh.sftp-server (and enable with ‘sudo apparmor_parser -r /etc/apparmor.d/usr.lib.openssh.sftp-server’):

#include <tunables/global>
/usr/lib/openssh/sftp-server {
  #include <abstractions/base>
  #include <abstractions/nameservice>
 
  # Served files
  # Need read access for every parent directory
  / r,
  /var/ r,
  /var/exports/ r,
 
  /var/exports/**/ r,
  owner /var/exports/** rwkl,
 
  # don't require ownership to read shared files
  /var/exports/shared/** rwkl,
}

This is a very simple profile for sftp-server itself, with access to files in /var/exports. Notice that by default the owner must match for any files in /var/exports, but in /var/exports/shared it does not. AppArmor works in conjunction with DAC, so that if DAC denies access, AppArmor is not consulted. If DAC permits access, then AppArmor is consulted and may deny access.

Summary
This is but one man’s implementation for a simple, secure and robust file service. There are limitations with the method as described, notably managing the sshd_config file and not supporting some traditional setups such as $HOME on NFS. That said, with a little creativity, a lot of possibilities exist for file serving with this technique. For my needs, the combination of standard OpenSSH and AppArmor on Ubuntu was very compelling. Enjoy!

Update
OpenSSH 4.8 and higher (available in Ubuntu 8.10 and later) contains the ChrootDirectory option, which may be enough for certain environments. It is simpler to setup (ie AppArmor is not required), but doesn’t have the same granularity and sftp-server protection that the AppArmor method provides. See comment 32 and comment 34 for details. Combining ChrootDirectory and AppArmor would provide even more defense in depth. It’s great to have so many options for secure file sharing! :)