pam_e4crypt

This is a PAM module for unlocking transparently encrypted directories on ext4.

Since version 4.1, the Linux kernel supports transparent encryption in ext4. The mechanism relies on the keyrings facility of the kernel.

This module will create keys for (de)ciphering files and file-names in encrypted directories during the authentication phase. During the session setup phase, those keys are added to the session keyring, giving the user (instant) access to directories for which she previously set a "policy" matching her passphrase (e.g. using e4crypt from the e2fsprogs).

Note that the only encryption mode currently supported is aes256-xts.

Google's fscrypt has the same purpose but is not restricted to ext4's transparent encryption feature and far better maintained. Thus, fscrypt is probably better suited for production machines.

Pitfalls

Users should be aware of the following when using (or thinking about using) this module:

• Files and directories are usually cached by the kernel for performance reasons. When using ext4 transparent encryption, decrypted content ends up in those caches and may thus (still) be visible to users with sufficient access rights (e.g. root) during or even after a session (until the caches are cleared).
• Obviously, this module will only unlock files and directories if it will be invoked, e.g. if PAM is used for login. Thus, background services can not access encrypted files directories if the files are not unlocked (that's the whole point of this module -- duh). TL;DR: don't complain about cronjobs failing while trying to access encrypted files.
• Changing the password used for login does (currently) not change the encryption key of affected directories. Hence, those directories will not be unlocked after a password-change.

Using this module

This module should be invoked late during the authentication phase as well as early during the session phase, after an invocation of the pam_keyinit module. The module should be marked as required for the authentication phase and either required or optional for the session phase, e.g.:

auth ...
auth        required        pam_e4crypt.so

account ...
password ...

session     required        pam_keyinit.so
session     required        pam_e4crypt.so
session ...

During the authentication phase, keys are generated from the user password. Hence, the module should be invoked late, at a point where the password is available.

During the session phase, the generated keys are added to the session keyring. Obviously, the keyring has to be initialized. Hence, pam_keyinit or an equivalent module has to invoked prior to pam_e4crypt. Since other session modules may access encrypted directories, it is highly recommended to have pam_e4crypt invoked early.

As this module is considered experimental, users also may want to specify onerr=succeed. Furthermore, it has been reported that having the module required for the session phase may break some setups. Hence, new users are encouraged making the module optional for that phase, at least initially.

Salt

Users must provide a salt (up to 16 bytes) for generation of a key. If no salt is supplied, no key will be generated. By default, the (raw) salt is read from $HOME/.ext4_encryption_salt. You can initialize this file with the following commands:

head -c 16 /dev/urandom > ~/.ext4_encryption_salt

Alternatively, the module can be configured to read the salt from files in a dedicated directory using the saltpath parameter:

auth        required        pam_e4crypt.so  saltpath=/home/.e4crypt

The module will then read the salt from /home/.e4crypt/$USER

Note that the e4crypt will read salt from various places by default. When using the e4crypt add_key command, specify the intended user's salt via the -S option for compatibility with this module.

Keyring

By default, keys are added to the session keyring. Using the keyring argument in the PAM config, it is possible to specify an alternative keyring to which the keys should be added. Use like:

session     required        pam_e4crypt.so  keyring=<desc>

As <desc>, one may specify either a keyring's description or one of the "special values" understood by keyctl (1.5), e.g. @u for the user specific keyring.

Dependencies

At runtime, the module requires:

• Linux-Kernel>=4.1
• PAM
• keyutils
• OpenSSL (or some compatible replacement like libressl)
• libuuid (util-linux)

At build time, the developer-packages for all the runtime-dependencies are required in addition to:

• CMake>=3.5
• A C-compiler (e.g. gcc or clang)

Installation

The module is built using CMake. Run

cmake <path-to-source>
make
make install

in the directory in which you indent to build the module. In-tree builds are supported (supply . as the source directory).

Additional cflags and ldflags can be supplied via the CMAKE_C_FLAGS and CMAKE_MODULE_LINKER_FLAGS when generating the build system, e.g.:

cmake -DCMAKE_C_FLAGS="-O2 -fstack-protector-strong" <path-to-source>

The module is automatically installed in a directory security which resides in the same folder as the PAM library. If those directories are symlinks on the target platform, the actual installation path may differ from the installation path originally used by PAM due to the default search order used by CMake.

Albeit the module should still be found by PAM, users may choose to specify the library base path used for installation manually by setting the CMAKE_INSTALL_LIBDIR variable.

For example, run

cmake -DCMAKE_INSTALL_LIBDIR=<lib-base-path> <path-to-source>

to prepare the module for installation to <lib-base-path>/security/.

About passwords, mounts and policies

"policies" are actually keys generated from a passphrase and a salt. This PAM module will generate keys from the user's passphrase and add them as policies, just like e4crypt add_key. If you view you session keyring, e.g. using keyctl show @s, those keys will be visible as "logon" keys with a descriptor prefixed with "ext4:". The hexadecimal string following the prefix is the actual policy descriptor which you can pass to e4crypt set_policy.

The correct process for encrypting a folder once logged in with pam_e4crypt is:

$ keyctl show
Session Keyring
 111111111 --alswrv      0     0  keyring: _ses
 222222222 ----s-rv      0     0   \_ user: invocation_id
 333333333 --als-rv   1000  1000   \_ logon: ext4:abcdef012345678
$ mkdir crypted
$ e4crypt set_policy abcdef012345678 crypted

Licensing

The module is currently licensed under the GPLv2. Have a look at the LICENSE file for more information. Also, you might want to consult a lawyer if you intend to ship this module together which a BSD-licensed version of PAM.

The license was chosen because some code was cargo-culted from the e2fsprogs, which was authored by Michael Halcrow and Ildar Muslukhov, copyrighted by Google and licensed under the GPLv2. Maybe, one day, if all the affected functions are rewritten to a degree where they are legally re-implementations, none of the eventual co-authors object and none of the dependencies switched to GPL-only, I'll re-release the thing under the LGPL or a BSD-license.