hg-admin-tools

A set of tools for managing authorization and access control for

ssh-based Mercurial repositories

Paul Crowley, paul@lshift.net, 2008

This software may be used and distributed according to the terms

of the GNU General Public License, incorporated herein by reference.

WHAT IT GIVES YOU

These tools make it easier to provide a centralized repository host

with read/write access to many repositories for many developers.

Access control is managed with a special repository on the server

called "hgadmin"; pushes to this repository immediately change the

rules that are in effect.

Inside "hgadmin" is a "keys" directory containing the SSH keys of all

developers who have access, and a file "hg-ssh-access.conf" which

gives a set of rules defining who can do what to what.

HOW IT WORKS

All of the repositories controlled by these tools are owned by a

single user (the "hg" user in what follows), but many remote users can

act on them.  We don't use file permissions to achieve that - instead,

developers log in as the "hg" user when they connect to the repository

host using ssh, using ssh URLs of the form

"ssh://hg@repository-host/repository-name".  A restricted shell

prevents them from using this access for unauthorized purposes.

Developers are authenticated only using SSH keys; no other form of

authentication is supported.  When a developer attempts to connect to

a repository via ssh, the SSH daemon searches for a match for that

user's key in ~hg/.ssh/authorized_keys.  If the developer is

authorised to connect to the repository they will have an entry in

this file.  The entry includes a "command" prefix which specifies that

the restricted shell should be used; this shell is passed an argument

identifying the developer.  The shell parses the command the developer

is trying to execute, and consults a rules file to see if that

developer is allowed to perform that action on that repository.  The

bulk of the work of the restricted shell is done by the Python program

"hg-ssh", but the shell script "hg-ssh-wrapper" sets up some

configuration so that you can change it to suit your local

installation.

The file ~hg/.ssh/authorized_keys is generated by "refresh-auth",

which recurses through a directory of files containing SSH keys and

generates an entry in authorized_keys for each one, using the name of

the key file as the identifier for the developer.  These keys will

live in the "keys" subdirectory of a repository called "hgadmin".  A

hook in this repository re-runs "refresh-auth" on the most recent

version after every push.

Finally, a hook in an extension is run for each changeset that is

remotely committed, which uses the rules file to determine whether to

allow the changeset.

GETTING STARTED

This is only one setup - it can be tweaked in many ways, and is as

specific as it is only in the interests of brevity.

You, and all users of this repository host, will need SSH public key

authentication set up, preferably working with ssh-agent so you don't

have to type in your passphrase all the time.  I assume you've done

that in what follows, so if you've done something different you'll

need to change it appropriately.

Issue these commands to get the repository host started.  These are

written out here rather than encapsulated in a script because many of

them may need to be different for your local setup.  You will need

root access on the repository host, because you need to create a new

user.

   ssh -A repository-host

   ssh-add -L >> /tmp/my-ssh-public-key

   sudo adduser --system --shell /bin/sh --group --disabled-password \

     --gecos "Mercurial repositories" hg

   sudo -u hg -H -s

   cd

   mkdir -p admin repos/hgadmin/keys/admin .ssh

   cd admin

   hg clone http://hg.opensource.lshift.net/hg-admin-tools

   cp hg-admin-tools/hg-ssh-wrapper hg-admin-tools/remote-hgrc ~

   cd ../repos/hgadmin

   hg init .

   echo "init user=admin/*" > hg-ssh-access.conf

   cp /tmp/my-ssh-public-key keys/admin/myname

   hg add

   hg commit -m "initial commit"

   cp ~/admin/hg-admin-tools/hgadmin-hgrc .hg/hgrc

   ../../admin/hg-admin-tools/refresh-auth ./hg-ssh-wrapper

   exit

   exit

You are now the sole user able to change and create repositories on

this repository host.  To administer these controls (and test your

access), check out hgadmin:

   mkdir ~/hg

   cd ~/hg

   hg clone ssh://hg@repository-host/hgadmin

   cd hgadmin

You can now add other users by putting their keys in an appropriate

subdirectory of the "keys" directory, and control their access by

editing hg-ssh-access.conf.  Changes will take effect as soon as you

push them to "ssh://hg@repository-host/hgadmin".

Users authorized to do so can now also create new repositories on this

host with "clone":

  hg clone . ssh://hg@repository-host/my-project-name

HG-SSH-ACCESS.CONF

Each line of hg-ssh-access.conf has the following syntax:

<rule> <condition> <condition> ...

Rule is one of

init - allow any operation, including the creation of new repositories

write - allow reads and writes to this file in this repository

read - allow the repo to be read but reject matching writes

deny - deny all requests

A condition is a globpattern matched against a relative path, one of:

user=<globpattern> - user's key

repo=<globpattern> - repo (as the user supplies it)

file=<globpattern> - file in the repo

branch=<globpattern> - name of the branch

The first rule in the file which has all its conditions satisfied is

used to determine whether an action is allowed.

Paths cannot contain any special characters except "/"; glob patterns

cannot contain any special characters except "/" and "*".  "*" matches

zero or more characters not including "/" while "**" matches zero or

more characters including "/".

Blank lines and lines that start with "#" are ignored.

FILE CONDITIONS

The rules file is used to make four decisions:

- Whether to allow a repository to be created

- Whether to allow access to a repository

- Whether to allow a changeset on a particular branch at all

- Whether to allow a changeset to change a particular file

When the first two of these decisions are being made, nothing is known

about what files might be changed, and so all file conditions

automatically succeed for the purpose of such decisions.  This means

that doing tricky things with file conditions can have

counterintuitive consequences:

- You cannot limit read access to a subset of a repository with a

"read" rule and a file condition: any user who has access to a

repository can read all of it and its full history.  Such a rule can

only have the effect of masking a later "write" rule, as in this

example:

   read repo=specialrepo file=dontwritethis

   write repo=specialrepo

allows all users to read specialrepo, and to write to all files

*except* that any changeset which writes to "dontwritethis" will be

rejected.

- For similar reasons, don't give "init" rules file conditions.

- Don't try to deny write access to a particular file on a particular

branch - a developer can write to the file on another branch and then

merge it in.  Either deny all writes to the branch from that user, or

allow them to write to all the files they can write to on any branch.

In other words, something like this will have the intended effect

  write user=docs/* branch=docs file=docs/*

But something like this will not have the intended effect; it will

effectively allow these users to write to any file on any branch, by

writing it to "docs" first:

  write user=docs/* branch=docs

  write user=docs/* file=docs/*

  read user=docs/*

LOCKING YOURSELF OUT

If you find yourself "locked out" - that is, that you no longer have

the permissions needed in hgadmin - you can break back in again if

you're able to become the "hg" user on the repository host.  Once you

are that user, delete ~hg/.ssh/authorized_keys (to stop any user who

might have access but shouldn't from using the repository while you

fix things).  Then go into ~hg/repos/hgadmin, do an "hg update", edit

things to your satisfaction, and commit the change.  Finally, run

~/admin/hg-admin-tools/refresh-auth to regenerate

~hg/.ssh/authorized_keys. 

THANKS

Thanks for reading this far.  If you use hg-admin-tools, please tell

me about it.

Paul Crowley, 2008