do=goto leaks page existence

[ikiwiki.git] / doc / todo / Zoned_ikiwiki.mdwn

[[!toc levels=3]]

# Zoned ikiwiki

## The idea

The idea behind this would be to have one ikiwiki behave as a dynamic private wiki in a specified area
and a more static publiczone wiki.

## Use cases

This can be more or less difficult depending on the use case.

### Purely static public zone with a single, controlled-access inward zone

For this case, only a known set of people are authorized to see the inward zone
or edit anything. Everybody else sees only the public zone. This use case is mostly
easy to handle now, as long as access to things like the `recentchanges` page and
repository browser are not granted for the public zone. In particular, the features
that allow information exposure via edit access are not of concern in this case.

### Static public zone, more than one controlled inward zone

In this case, the known, controlled set of people with special access are divided
into groups with access to different (or overlapping) zones. The public still sees only a static zone.

Here, some of the harder issues, like information disclosure via edit access, do apply,
but only to members of the known, controlled groups. How much of a problem that is
depends on _how sensitive_ the information is that each group might reveal from another
zone. The rcs logs will show when a page has been edited to contain an [[ikiwiki/directive/inline]]
directive or other trick to reveal information, so if it is enough to treat the trusted users' conduct
as a management issue ("don't do that, please") then the risks can be acceptable in this case.

### Public zone allows contribution/editing by external users

This case is the most difficult to cover at present, and probably shouldn't be attempted
without solutions to most or all of the **obstacles** identified here. 

## Implementation techniques

### Edit control by user and pagespec: lockedit

This works today, using the [[plugins/lockedit]] plugin. Because the `user` predicate
can be part of a [[ikiwiki/PageSpec]], this is all we need to flexibly control edit access
using any authentication method `ikiwiki` supports.

### View control in the `http` server

We already can more or less do this for example with [[httpauth|/plugins/httpauth/]], `.htaccess` files and a proper `httpauth_pagespec`.

_Drawbacks:_ might be fiddly to configure and require maintaining two different user/pass logbases (native ikiwiki
signin), or impractical if ikiwiki is using an authentication method not natively supported
in the `http` server (e.g., OpenID).

### View control in ikiwiki CGI

By requiring access to private zones to go through an ikiwiki CGI wrapper,
any ikiwiki-supported authentication method can be used, and the accessible
pages can be specified using the `user` predicate with [[ikiwiki/PageSpec]]s,
just as with the [[plugins/lockedit]] plugin.

The [[plugins/contrib/signinview]] plugin implements this idea, using very
simple configuration that is possible even in shared-hosting environments
without complete access to the `http` server configuration, as long as
`.htaccess` files or their equivalent can be created. The top directory of
a private zone needs only a `.htaccess` file with `Deny from All` or
`Require all denied` (or other equivalent directive for the `http` server
in use), and a `403` error handler of `{$cgiurl}?do=view`.

The plugin emits response headers intended to discourage non-private caches
from retaining the retrieved content. (They are already supposed to avoid
caching any response to a request with an `Authorization` header, but this
plugin can be used with any ikiwiki-supported auth method, not all of which
require that header.)

A plugin like [[plugins/contrib/pagespec_alias]] can be very useful for
defining a group of authorized users:

    us: user(alice) or user(bob) or user(clotaldo)

so that zone access can be a simple [[ikiwiki/PageSpec]]:

    us() and ours/*

*Drawbacks:* The private zones no longer reap all the benefits of a static
wiki generator, as a (fairly heavy) ikiwiki CGI wrapper must be started for
each access. (On the other hand, all it needs to do after confirming authorization
is basically `cat` the statically-generated page with appropriate response headers,
keeping the code simple and easy to audit.)

This can be adequate for a case where the static, public zone could receive a lot
of traffic, with the private zone(s) accessed only by a known small group of people.

### View control with a FastCGI Authorizer

A plugin implementing a [FastCGI](http://www.fastcgi.com/)
[Authorizer](http://www.fastcgi.com/drupal/node/6?q=node/22#S6.3) could provide
the same benefits as [[plugins/contrib/signinview]] (any ikiwiki-supported auth
method, simple zone definition with [[ikiwiki/PageSpec]]s) with less overhead
per access. It would also be simpler than [[plugins/contrib/signinview]] by
leaving it as the `http` server's responsibility to generate the proper headers
and serve the content.

Caching proxies are already supposed to avoid caching any response to a request
that included an `Authorization` header. For some ikiwiki-supported auth methods,
that header might not be needed in the request, and care may be needed to configure
the server to emit other necessary response headers to discourage caching of
content from a private zone.

*Drawbacks:* Not yet implemented, someone would have to do it.
It's not clear [[what code changes fastcgi|todo/fastcgi or modperl installation instructions]]
would require in ikiwiki. An Authorizer seems like a good place to start because of its
limited, simple functionality--but as it could make use of any ikiwiki-supported auth method,
evaluate `PageSpec`s, and the like, it could still run a non-trivial amount of the code.

## Obstacles

A number of ikiwiki features aren't (yet) designed with zoning in mind,
and it will take some effort both to identify them all, and to think out how they
could be addressed. This section invites brainstorming of both kinds.
This might eventually merit a separate page [[Zoned ikiwiki obstacles]]
but I'll begin it here.

Note that not all of these issues will be problems for all **zoned ikiwiki use cases**.

### Leakage of page existence by `do=goto`

An unauthorized client can use a `do=goto` request to find out whether a
page exists (will be forbidden to view it) or not (will be forbidden to create it).

My first idea was to fix this all within [[plugins/contrib/signinview]] by hooking
`cgi` first and checking for `goto` and an unauthorized page. But checking authorization
requires session info, not loaded at `cgi` hook time. Next idea was to somehow skip the rest of
the chain of `cgi` hooks, preventing `goto` from handling the request, and handling
it again in `sessioncgi`. But 'skip the rest of this chain' doesn't seem to be something
a hook can return.

Hmm, maybe change the `do` parameter to something other than `goto` before the `goto` hook
can see it, _then_ handle it later in `sessioncgi`?

### Backlinks

What is problematic is when you link a public page in a private page : 
a backlink will be generated from the public page to the private page.

As noted in [[per_page_ACLs]] in the end users through backlink 
navigation will frequently hit HTTP/401 deterring browsing as well as for the admin at false-positive logwatching.

One can radically [[disable backlinks feature|todo/allow_disabling_backlinks]] but then no more neat backlink navigation that
is really good to have in both area.

Another way of just preventing this backlink leak in that case would be sufficient via i.e a *privatebacklinks* config and
a patch like this one [[!toggle id="backlinkpatch" text="(show)"]].

[[!toggleable id="backlinkpatch" text="""
Comments are welcome.

[[mathdesc]]


<pre>
diff --git a/IkiWiki.pm b/IkiWiki.pm
--- a/IkiWiki.pm
+++ b/IkiWiki.pm
@@ -294,6 +294,14 @@ sub getsetup () {
                safe => 1,
                rebuild => 1,
        },
+       privatebacklinks => {
+               type => "pagespec",
+               example => "",
+               description => "PageSpec controlling which backlinks are private (ie users/*)",
+               link => "ikiwiki/PageSpec",
+               safe => 1,
+               rebuild => 1,
+       },
        hardlink => {
                type => "boolean",
                default => 0,
diff --git a/IkiWiki/Render.pm b/IkiWiki/Render.pm
--- a/IkiWiki/Render.pm
+++ b/IkiWiki/Render.pm
@@ -52,7 +52,8 @@ sub backlinks ($) {
                        $p_trimmed=~s/^\Q$dir\E// &&
                        $page_trimmed=~s/^\Q$dir\E//;
                               
-               push @links, { url => $href, page => pagetitle($p_trimmed) };
+               push @links, { url => $href, page => pagetitle($p_trimmed) }
+               unless defined $config{privatebacklinks} && length $config{privatebacklinks} && pagespec_match($p, $config{privatebacklinks}) && !pagespec_match($page, $config{privatebacklinks}) ;
        }
        return @links;
 }

</pre>
"""]]

In use cases where the main concern about backlinks is only the bad user experience when links are
shown that lead to access denial when clicked, a workable
solution could be to make the backlinks `div` invisible in `local.css`.

### recentchanges page

An accessible `recentchanges` page can include links to changes to pages
that should not be accessible. Even if the links cannot be followed, the
existence of the pages and their edit history are leaked. If rcs integration
is configured, those links on the `recentchanges` page can leak complete contents
through the **rcs browser**.

It can be helpful to generate separate `recentchanges` pages for different zones.
The [[plugins/recentchanges]] plugin already allows this--a `recentchanges` page
can be created anywhere, just by using the `recentchanges` directive
with the right [[ikiwiki/PageSpec]] for the zone it should cover--except that it cannot yet
be configured to generate a different `recentchanges` link destination into pages
in different zones. So, it would be helpful if its configuration could allow multiple
`recentchangespage` values, paired with `PageSpec`s for the pages on which they
should be used.

### rcs browser

If the repository browser is accessible, potentially all content can be exposed.
Even if links to the repository browser are not generated into public wiki pages,
if a user can obtain or guess the repository browser URL and construct arbitrary
requests, information can be revealed.

Solutions could involve authnz features of the revision control systems themselves
and their associated repository browsers; for example, `svn` supposedly has such
features, and recent versions of `viewvc` supposedly honor them. But such features
may not be available for every rcs, and where they are available, they'll have to
be configured separately and differently from ikiwiki itself. They might not support
the same auth methods (e.g. OpenID) being used by the wiki itself.

Another approach would be for ikiwiki's own rcs plugin to generate a CGI wrapper
that invokes the repository browser CGI (which itself would _not_ be made
executable via `http` request). The `historyurl` and `diffurl` would then refer
to this wrapper. (In fact, they would not have to be specified in the config file,
as the plugin would know where it generated them. Instead, what would need to be
specified would be the filesystem path for the rcs browser being wrapped). The
wrapper could dissect the request parameters, identify the pages being accessed,
and subject them to the same accessibility tests used for the wiki. The rcs browser
itself needs to be configured to use the wrapper URL in all its generated links,

This might not be very hard to do with `gitweb` as it is already implemented in Perl.
The wrapper could probably import it and use its already-supplied routines to parse
the request into the affected file names, and probably complete the whole request
without a second `exec`. Other rcs backends might or might not be as easy.

### Search

If [[plugins/search]] is enabled, private content is indexed and
searchable to the public.

### Information leaks allowed by edit access

> Have you considered all the ways that anyone with edit access to the
> public wiki could expose information from the public wiki? For example,
> you could inline all the private pages into a public page. --[[Joey]]

Many ikiwiki features could give information exposure opportunities to someone
with edit access. The list here is surely incomplete, and would take a purposeful
review of the code and plugins (including third-party plugins) to complete.

* Directives that can inline information from other pages
    * [[ikiwiki/directive/inline]] *the most obvious one*
    * [[ikiwiki/directive/map]]
    * [[ikiwiki/directive/brokenlinks]] ?
    * [[ikiwiki/directive/orphans]] ?
    * [[ikiwiki/directive/linkmap]] ?
    * _others_?
* Not to forget `contrib` plugins
    * [[plugins/contrib/report]] ?
    * _others_?

Note that, _with_ the right controls on who can edit the pages and insert
the directives, the fact that a public page can inline stuff from private
pages can be very useful. Public pages can be created that are populated
by selected content that's maintained on the private side. The [[ikiwiki/directive/if]]
directive can be used in the private content to control what parts can be
inlined into public pages. All of this is in ikiwiki today.