Disclamer: This is not an official Google product.

Overview

pam-cryptsetup provides a PAM module that allows LUKS-based disk encryption passwords to be kept in sync with account passwords automatically based on factors like if the user has decrypted the disk successfully previously.

The project as a whole consists of two parts: a PAM module pam_cryptsetup.so for triggering on user authentication, and a helper program pam_cryptsetup_helper to perform the actual encryption checks and modifications required.

Building and Installing

Built with GNU Autotools and includes an autogen.sh script:

1. ./autogen.sh
2. ./configure
3. make [prefix={path}]
4. make check # (optional)
5. sudo make install [prefix={path}]

Depends on:

• GLib
• Linux-PAM
• libcryptsetup
• libdevicemapper
• Autoconf, Automake, Libtool

The Autotools Prefix

Due to this project's use of GNU Autotools, all the components are built and installed relative to a user-definable prefix via make prefix={path}. Any time this documentation references {prefix}, substitute the value given to make, or /usr/local if none was given.

Additionally, when using make install, it is required to manually link pam_cryptsetup.so to the proper PAM modules directory (usually /lib/security) from the build default {prefix}/lib/security. This can be avoided by defining the prefix to be empty via make install prefix=.

Components

pam_cryptsetup.so

The pam_cryptsetup.so module attaches to a PAM authentication or credential stack, and passes information provided to the pam_cryptsetup_helper executable to enact any changes that might be necessary.

Module Configuration

When adding this module to a PAM stack, it is recommended to use either the [default=ignore] or optional flags and to place the entry for the module at the very end of the PAM config file sequence.

Module Arguments:

Declaration	Default	Description
crypt_name=<container>	(none)	Name of encrypted /dev/mapper based container to probe. Required for operation.
debug=<state>	false	If set to 'true', provide additional module execution details via pam_syslog.
background=<state>	true	If set to 'false', authentication module will wait for helper process to complete

Module Examples:

Authentication mode

Authentication mode is most useful when a user's login password can be changed by external factors, such as in an LDAP authentication environment.

The following example changes would be placed in /etc/pam/common.auth

For an encrypted device at /dev/mapper/rootdev_crypt:

<snip>
auth [default=ignore] pam_cryptsetup.so crypt_name=rootdev_crypt

Square brackets ([ and ]) can be used for device names with spaces:

<snip>
auth [default=ignore] pam_cryptsetup.so [crypt_name=my special device]

And if your device has a really weird name, closing square brackets must be escaped by using \]:

<snip>
auth [default=ignore] pam_cryptsetup.so [crypt_name=my [extra\] special device]

Credential mode

As an alternative authentication mode, the module can instead be used in credential mode, and will run only when a user account password is set via the passwd command. This is recommended for setups where the password is never expected to change outside of the passwd command being run.

For credential mode, changes should be made in /etc/pam/passwd

For an encrypted device at /dev/mapper/rootdev_crypt:

<snip>
password [default=ignore] pam_cryptsetup.so crypt_name=rootdev_crypt

pam_cryptsetup_helper

The pam_cryptsetup_helper executable receives data via unnamed piping and command line arguments when invoked by the pam_cryptsetup.so module, and uses the data passed to decide if updates to the encrypted disk are necessary.

A Note on Security

Because this helper module handles sensitive information (notably password and volume keys) outside the secure PAM context, the following steps are taken to prevent leaking data:

• memlockall() is called at the start of the process to prevent it or any libraries' memory from being paged to disk during runtime.
• Variables that held sensitive information are zeroed before being freed via either a native implementation of explicit_bzero or an included equivalent.
• While non-sensitive data like username or crypt name are passed via argument, password tokens are read via inter-process anonymous pipes.

Additionally, while this executable can be called manually if one knows the proper invocation, it is not setuid, and requires the caller have root permissions in order for changes to be made to the encrypted disk.

Helper Crypt Slot Cache

All user information, such as which (if any) slot was most recently unlocked with their login password, is recoded in {prefix}/var/pam-cryptsetup/slots as a GLib string array. The default format, before any unlocking has happened, is as follows:

['', '', '', '', '', '', '', '']

where each string associates with one of the 8 LUKS slots (numbered 0-7) available. Once a user's password has successfully unlocked a slot on the disk, their username is added to the string associated with the number slot unlocked.

For example, if we had a user 'kathy' unlock slot 3, the cache would be updated as follows:

['', '', '', 'kathy', '', '', '', '']

Next time kathy authenticates, this info will be used to determine the appropriate action for the helper to take.

Helper Operation

During a run, the module takes the following information into account:

• Username
• Password
• Old Password (cred mode only)
• Cache entries
• Crypt slots unlock-able using
  • password (auth mode)
  • old password (cred mode)

Once all available information is gathered, the action taken is decided by the following logic:

Authentication mode:

• No action if:
  • User is not recorded in-cache, and password does not unlock any slot
  • User is recorded in-cache, and password unlocks the associated slot
• Update disk and cache slot if:
  • User is recorded in-cache, and password does not unlock recorded slot
• Update cache if:
  • User is recorded in-cache, and password unlocks different slot than the one recorded
  • User is not recorded in-cache, and password unlocks a slot

Password mode:

• No action if:
  • Old password does not unlock a slot
• Update disk if:
  • Old password unlocks a slot
• ADDITIONALLY update cache if:
  • User entry exists in a different cache slot compared to new crypt slot

Note: While credential mode will interact with the cache in some cases, it is not meant to be used in addition to authentication mode; using it as such is untested and considered unsupported at this time!

Potential Future Improvements

• Save cache to drive encryption header using LUKS2 unbound slots

  • Would allow easier drive transfer to new systems while keeping existing pam-cryptsetup info intact.
  • Security-conscious might appreciate a less-easily-accessible record of users who can unlock the drive with their passwords.

• Reintroduce multi-threading in helper application (unlikely)

  • Code was messy
  • Performance benefits don't matter when helper runs in the background.

• Utilize kernel keychain for retrieving active volume key

  • Would allow dropping the direct libdevmapper usage
  • Would require that user's kernel is set to store the volume key in an accessible keychain.