Re: [PoC] Federated Authn/z with OAUTHBEARER

On Mon, Jan 13, 2025 at 5:00 PM Jacob Champion
<jacob(dot)champion(at)enterprisedb(dot)com> wrote:
> So I think what I'm going to need to do is modify v41-0003 to allow
> the mechanism to politely hang up the connection while the flow is in
> progress.

This is done in v42. The rough conversation now looks like this:

Issuer User libpq Backend
| | |
| x -----> x -----> o [1] Startup Packet
| | | |
| | x <----- x [2] OAUTHBEARER Request
| | | |
| | x -----> x [3] Parameter Discovery
| | | |
| | x <----- x [4] Parameters Stored
| | | |
| | x -----> x [5] Finish OAUTHBEARER
x <----> x <----> x | [6] OAuth Handshake
x <----> x <----> x <----- o [7] Server Hangs Up
x <----> x <----> x
| | |
| | x -----> o [8] New Startup Packet
| | | |
| | x <----- x [9] OAUTHBEARER Request
| | | |
o | x -----> x [10] Send Token
| | |
| <----- x <----- x [11] Connection Established
| | |
x <----> x <----> x
x <----> x <----> x [11] Use the DB
. . .
. . .
. . .

The key change is that the client sends the final OAUTHBEARER response
_before_ beginning the OAuth flow, allowing the server to concurrently
close its side of the discovery connection (steps 5-7). This requires
only a single change to the proposed SASL_ASYNC feature in v41-0003:
when a mechanism returns SASL_ASYNC during exchange(), it may also
specify an output response packet to send before switching over to
async auth. (This is similar to how mechanisms may send a response
packet when returning SASL_FAILED.)

That change simplifies the description of the flow a bit, too, and
I've updated the documentation. If an OAuth flow needs to run, two
connections to the server will be made. The only way to skip the
discovery connection now is if a (custom) hook has a token cached.

> This further decouples the lifetimes of the mechanism and
> the async auth -- the async state now has to live outside of the SASL
> exchange --

The only part of the state that I had to move was the token itself,
which now lives in conn->oauth_token. This is cleaned up with a new
pqClear- function, so that it can live across connection attempts and
be proactively cleared from memory after a successful connection.

> but I think it's probably more architecturally sound.

As evidence, this change flushed out a few bugs and provided the basis
to fix every TODO in fe-auth-oauth.c, so I'm pretty happy with it:
- an AuthenticationSASLFinal message is not allowed, as OAUTHBEARER
does not specify any additional server data (this bug goes all the way
back to v1)
- a server is not allowed to switch discovery URLs on a client between
connection attempts
- a server is not allowed to override a previously determined oauth_scope
- the connection is retried only if a conn->oauth_token was not
initially set (this simplifies conn->oauth_want_retry considerably)
- require_auth=oauth will complain if a discovery connection lets the client in

The existing Perl tests were not affected by this refactoring, other
than a latent test bug that got caught with the fallout. The advisory
Python tests (which pin behavior on the wire) needed more changes.
I've also added some tests to 002_client.pl which have the custom hook
misbehave in various ways and pin the expected error messages.

v41-0005 has probably outlived its usefulness by now, and I've folded
those changes into v42-0004.

Thanks!
--Jacob