WSL Hello sudo

"WSL Hello sudo" is a Linux PAM module and companion Windows CLI apps that realize sudo by biometric login of Windows Hello on Windows Subsystem for Linux (WSL).
This PAM module allows you to authenticate sudo via face recognition, fingerprint authentication, and of couse machine-local PIN. It runs in both WSL and WSL 2.

The Linux PAM module is written in Rust, and Windows CLI apps are written in C#.
Please use it at your own risk. There is no warranty.

"WSL Hello sudo" actually does not modify your sudo command at all. It is a Linux PAM module.
PAM, Plaggable Authentication Module, is a UNIX's module system that provides user authentication mechanisms to applications such as sudo or su. "WSL Hello sudo" is such a PAM module that lets applications use Windows Hello.

Installation and Configuration

Installation

The installation process is very simple.
Please download the latest release package from GitHub Release and unpack it.
Run install.sh inside the directory, and follow the instruction of install.sh

$ wget http://github.com/nullpo-head/WSL-Hello-sudo/releases/latest/download/release.tar.gz
$ tar xvf release.tar.gz
$ cd release
$ ./install.sh

Although you don't have to care about the detailed installation process,
install.sh does following things.

1. Copy small Windows CLI apps that launch Windows Hello to C:\Users\your_account\pam_wsl_hello (default location)
2. Install a PAM module to your WSL system.
3. Create config files in /etc/pam_wsl_hello/
4. Create uninstall.sh

Configuration

"WSL Hello sudo" is not a fork of sudo but a PAM module. So please configure /etc/pam.d/sudo to make it effective.
I strongly recommend to set password of root first so that you can switch to it by su, in case you make some typo in the config of sudo.
Add auth sufficient pam_wsl_hello.so to the top line of your /etc/pam.d/sudo like the following example

#%PAM-1.0

auth       sufficient pam_wsl_hello.so
session    required   pam_env.so readenv=1 user_readenv=0
session    required   pam_env.so readenv=1 envfile=/etc/default/locale user_readenv=0
@include common-auth
@include common-account
@include common-session-noninteractive

Even if you fail to authenticate via Windows Hello, sudo moves on to the regular password authentication by this setting with sufficient.

Other applications that authenticate users such as su can also utilize Windows Hello by this module.
Even so, I strongly recommend you to make either sudo or su free from this module to prevent from being locked out

Troubleshooting

Windows Hello window appears in background.

The Windows Hello dialog sometimes appears in background. In some cases, it even fails to recognize your face with some weird error message. It seems a bug of Windows API. In that case, restarting Windows a couple of times might solve the problem.

"Windows Hello is not invoked! sudo just prompts password!"

Maybe some error is happening. Unfortunately, sudo suppresses error messages from PAM modules.
To debug "WSL Hello sudo", make it effective for su instead of sudo. su shows error messages from PAM modules, so you can see what is going on.

For your information, the setting for su will be like the example below. I will show only relevant two lines.

auth       sufficient pam_rootok.so
auth       sufficient pam_wsl_hello.so  

Build

The Linux PAM module of "WSL Hello sudo" is written in Rust, and the Windows CLI apps are written in C#.
So, cargo and Visual Studio are required to build it.

Before building "WSL Hello sudo", add the path to MSBuild.exe to PATH environment variable of bash on WSL, not Windows.
If you build Windows CLI apps with your Visual Studio GUI, you can ignore that. In my environment, MSBuild lives in /mnt/c/Program Files (x86)/Microsoft Visual Studio/2019/Community/MSBuild/Current/Bin/

To build "WSL Hello sudo", just run make.

$ git clone https://github.com/nullpo-head/WSL-Hello-sudo.git
$ cd WSL-Hell-sudo
$ make

It invokes cargo and MSBuild.exe properly.

Whether you're using bash or Windows, the repository must be in the Windows filesystem, otherwise MSBuild.exe won't work.

Internals

Windows Hello maintains RSA key-pairs for each Windows user in its TPM hardware, and tells success of authentication by signing given contents by the private key. To utilize its API, "WSL Hello sudo" contains small Windows CLI apps that return public key and singned signature of given content. On the other hand, the PAM module of "WSL Hello sudo" remembers the public keys of each Windows user who corresponds to each Linux user. So, the PAM module authenticates the given Linux user by the following process.

0. The PAM module is launched by sudo and receives a Linux user to be authenticated
1. The PAM module launches the companion Windows app and sends a random value via WSL's interop bridge
2. The companion Windows app invokes Windows Hello
3. Windows Hello makes a signature of the given input by the private key of the current Windows user
4. The companion Windows app returns the signature
5. The PAM module verifies the signature by the public key of the Windows user who corresponds to the given Linux user.