sudo_pair

sudo_pair is a plugin for sudo that requires another human to approve and monitor privileged sudo sessions.

About

sudo is used by engineers daily to run commands as privileged users. But on some sensitive systems, you really want to ensure that no individual can act entirely autonomously. At Square, this includes applications that manage our internal access-control systems, store accounting ledgers, or even move around real money. This plugin allows us to ensure that no user can act entirely on their own authority within these systems.

This plugin and its components are still in prerelease, as we want to get feedback from the open-source community before officially releasing 1.0.

Installation

WARNING: Misconfiguring sudo can lock you out of your machine. Test this in a throwaway environment.

For now, sudo_pair must be compiled from source. It is a standard Rust project, and the following should suffice to build it on any recent version of Rust:

git clone https://github.com/square/sudo_pair.git
cd sudo_pair
cargo build --release

Once built, the plugin itself will need to be installed in a place where sudo can find it. Generally this is under /usr/libexec/sudo (on macOS hosts it's /usr/local/libexec/sudo). An appropriate approval script must be installed into the PATH. A directory must be created for sudo_pair to manage the sockets it uses for communication between plugin and client. And finally, sudo must be configured to load and use the plugin.

# WARNING: these files may not be set up in a way that is suitable
# for your system. Proceed only on a throwaway host.

# install the plugin shared library
install -o root -g root -m 0644 ./target/release/libsudopair.dylib /usr/libexec/sudo

# create a socket directory
install -o root -g root -m 0644 -d /var/run/sudo_pair

# install the approval script; as currently configured, it denies access
# to users approving their own sudo session and may lock you out
install -o root -g root -m 0755 ./sample/bin/sudo_approve /usr/bin/sudo_approve

# your `/etc/sudo.conf` may already have entries necessary for sudo to
# function correctly; if this is the case, the two files will need to be
# merged
install -o root -g root -m 0644 ./sample/etc/sudo.conf /etc/sudo.conf

# if these prompts don't work for you, they're configurable via a simple
# templating language explained later in the README
install -o root -g root -m 0644 ./sample/etc/sudo.prompt.user /etc/sudo.prompt.user
install -o root -g root -m 0644 ./sample/etc/sudo.prompt.pair /etc/sudo.prompt.pair

This only places the plugin files into their expected locations. The plugin will not be enabled yet until you follow the configuration steps below.

Configuration

/etc/sudoers

By default, /etc/sudoers will not tell logging plugins to log output for any commands. You will need to enable this by either telling sudo to enable logging for all commands (and opt out any commands you wish to bypass pairing for) or by opting individual commands into logging.

Example (default to log, opt out of individual commands):

Defaults log_output

%wheel ALL = (ALL) NOLOG_OUTPUT: /bin/cat, /bin/ls

Example (opt into individual commands)

%wheel ALL = (ALL) LOG_OUTPUT: /usr/bin/visudo

/etc/sudo.conf

The plugin can be provided several options to modify its behavior. These options are provided to the plugin by adding them to the end of the Plugin line in /etc/sudo.conf.

Example:

Plugin sudo_pair sudo_pair.so socket_dir=/var/tmp/sudo_pair gids_exempted=42,109

The full list of options are as follows:

• binary_path (default: /usr/bin/sudo_approve)

  This is the location of the approval binary. The approval command itself needs to run under the privileges of the destination user or group, and this is done so using sudo, so it must be exempted from requiring its own pair approval.

• user_prompt_path (default: /etc/sudo_pair.prompt.user)

  This is the location of the prompt template to display to the user invoking sudo; if no template is found at this location, an extremely minimal default will be printed. See the Prompts section for more details.

• pair_prompt_path (default: /etc/sudo_pair.prompt.pair)

  This is the location of the prompt template to display to the user being asked to approve the sudo session; if no template is found at this location, an extremely minimal default will be printed. See the Prompts section for more details.

• socket_dir (default: /var/run/sudo_pair)

  This is the path where this plugin will store sockets for sessions that are pending approval. This directory must be owned by root and only writable by root, or the plugin will abort.

• gids_enforced (default: 0)

  This is a comma-separated list of gids that sudo_pair will gate access to. If a user is sudoing to a user that is a member of one of these groups, they will be required to have a pair approve their session.

• gids_exempted (default: none)

  This is a comma-separated list of gids whose users will be exempted from the requirements of sudo_pair. Note that this is not the opposite of the gids_enforced flag. Whereas gids_enforced gates access to groups, gids_exempted exempts users sudoing from groups. For instance, this setting can be used to ensure that oncall sysadmins can respond to outages without needing to find a pair.

  Note that root is always exempt.

Prompts

This plugin allows you to configure the prompts that are displayed to both users being asked to find a pair and users being asked to approve another user's sudo session. If prompts aren't configured (or can't be found on the filesystem), extremely minimal ones are provided as a default.

The contents of the prompt files are raw bytes that should be printed to the user's terminal. This allows fun things like terminal processing of ANSI escape codes for coloration, resizing terminals, and setting window titles, all of which are (ab)used in the sample prompts provided.

These prompts also implement a simple %-escaped templating language. Any known directive preceded by a % character is replaced by an expansion, and anything else is treated as a literal (e.g., %% is a literal %, and %a is a literal a).

Available expansions:

• %b: the name of the appoval _b_inary
• %B: the full path to the approval _B_inary
• %C: the full _C_ommand sudo was invoked as (recreated as best-effort)
• %d: the cw_d_ of the command being run under sudo
• %h: the _h_ostname of the machine sudo is being executed on
• %H: the _H_eight of the invoking user's terminal, in rows
• %g: the real _g_id of the user invoking sudo
• %p: the _p_id of this sudo process
• %u: the real _u_id of the user invoking sudo
• %U: the _U_sername of the user running sudo
• %W: the _W_idth of the invoking user's terminal, in columns

Approval Scripts

The provided approval script is just a small (but complete) example. As much functionality as possible has been moved into the plugin, with one (important, temporary) exception: currently, the script must verify that the user approving a sudo session is not the user who is requesting the session.

Other than that, the only thing required of the "protocol" is to:

• connect to a socket (as either the user or group being sudoed to)
• wire up the socket's input and output to the user's STDIN and STDOUT
• send a y to approve, or anything else to decline
• close the socket to terminate the session

As it turns out, you can pretty much just do this with socat:

socat STDIO /path/to/socket

The script included with this project isn't much more than this. It performs a few extra niceties (implicitly sudos if necessary, turns off terminal echo, disables Ctrl-C, and kills the session on Ctrl-D), but not much more. Ctrl-C was disabled so a user who's forgotten that this terminal is being used to monitor another user's session doesn't instinctively kill it with Ctrl-C.

Limitations

Sessions under sudo_pair can't be piped to.

Allowing piped data to standard input, as far as I can tell, likely results in a complete bypass of the security model here. Commands can often accept input on stdin, and there's no reasonable way to show this information to the pair.

Security Model

This plugin allows users to sudo -u ${user} to become a user or sudo -g ${group} to gain an additional group.

When a user does this, a socket is created that is owned and only writable by ${user} (or ${group}). In order to connect to that socket, the approver must be able to write to files as that ${user} (or ${group}). In other words, they need to be on the other side of the airtight hatchway. In practical terms, this means the approver needs to also be able to sudo to that user or group.

To facilitate this, the plugin exempts the approval script from the requirement to have a pair. And the sample approval script automatically detects the user or group you need to become and runs sudo -u ${user} (or sudo -g ${group}) implicitly.

As a concrete example, these are the sockets opened for sudo -u root, sudo -u nobody, and sudo -g sys:

drwxr-xr-x   3 root    wheel     96 May  8 09:17 .
s-w-------   1 root    wheel      0 May  8 09:16 1882.29664.sock    # sudo -u root
s-w-------   1 nobody  wheel      0 May  8 09:17 1882.29921.sock    # sudo -u nobody
s----w----   1 root    sys        0 May  8 09:18 1882.29994.sock    # sudo -g sys

The only people who can approve a sudo session to a user or group must also be able to sudo as that user or group.

Due to limitations of the POSIX filesystem permission model, a user may sudo to a new user (and gain its groups) or sudo to a new group (preserving their current user), but not both simultaneously.

Project Layout

This project is composed of three Rust crates:

• sudo_plugin-sys: raw Rust FFI bindings to the sudo_plugin(8) interface
• sudo_plugin: a set of Rust structs and macros to simplify writing plugins
• sudo_pair: the implementation of this plugin

Dependencies

Given the security-sensitive nature of this project, it is an explicit goal to have a minimal set of dependencies. Currently, those are:

• rust-lang/libc
• rust-lang-nursery/rust-bindgen
• rust-lang-nursery/failure
• rust-lang-nursery/error-chain (to be removed)

Contributions

Contributions are welcome! This project should hopefully be small (~500loc for the plugin itself, ~1kloc for the wrappers around writing plugins) and well-documented enough for others to participate without difficulty.

Pick a TODO and get started!

Bugs

Please report non-security issues on the GitHub tracker. Security issues are covered by Square's bug bounty program.

License

sudo_pair is distributed under the terms of the Apache License (Version 2.0).

See LICENSE-APACHE for details.