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 --releaseOnce 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.pairThis 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_enforcedflag. Whereasgids_enforcedgates access to groups,gids_exemptedexempts 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_ommandsudowas invoked as (recreated as best-effort)%d: the cw_d_ of the command being run undersudo%h: the _h_ostname of the machinesudois being executed on%H: the _H_eight of the invoking user's terminal, in rows%g: the real _g_id of the user invokingsudo%p: the _p_id of thissudoprocess%u: the real _u_id of the user invokingsudo%U: the _U_sername of the user runningsudo%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
yto 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/socketThe 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 thesudo_plugin(8)interfacesudo_plugin: a set of Rust structs and macros to simplify writing pluginssudo_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:
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.