Re: postgres_fdw, dblink, and CREATE SUBSCRIPTION security

On 2/6/23 08:22, Robert Haas wrote:
> I don't think that's quite the right concept. It seems to me that the
> client is responsible for informing the server of what the situation
> is, and the server is responsible for deciding whether to allow the
> connection. In your scenario, the client is not only communicating
> information ("here's the password I have got") but also making demands
> on the server ("DO NOT authenticate using anything else"). I like the
> first part fine, but not the second part.

For what it's worth, making a negative demand during authentication is
pretty standard: if you visit example.com and it tells you "I need your
OS login password and Social Security Number to authenticate you," you
have the option of saying "no thanks" and closing the tab.

It's not really about protecting the server at that point; the server
can protect itself. It's about protecting *you*. Allowing the proxy to
pin a specific set of authentication details to the connection is just a
way for it to close the tab on a server that would otherwise pull some
other piece of ambient authority out of it.

In a hypothetical world where the server presented the client with a
list of authentication options before allowing any access, this would
maybe be a little less convoluted to solve. For example, a proxy seeing
a SASL list of

- ANONYMOUS
- EXTERNAL

could understand that both methods allow the client to assume the
authority of the proxy itself. So if its client isn't allowed to do
that, the proxy realizes something is wrong (either it, or its target
server, has been misconfigured or is under attack), and it can close the
connection *before* the server runs login triggers.

> In this kind of scenario, the client has no business
> demanding that the server authenticate using the password rather than
> anything else. The server, not the client, is in charge of deciding
> which connections to accept; the client's job is only to decide which
> connections to proxy.

This sounds like a reasonable separation of responsibilities on the
surface, but I think it's subtly off. The entire confused-deputy problem
space revolves around the proxy being unable to correctly decide which
connections to allow unless it also knows why the connections are being
authorized.

You've constructed an example where that's not a concern: everything's
symmetrical, all proxies operate with the same authority, and internal
users are identical to external users. But the CVE that led to the
password requirement, as far as I can tell, dealt with asymmetry. The
proxy had the authority to connect locally to a user, and the clients
had the authority to connect to other machines' users, but those users
weren't the same and were not mutually trusting.

> And the human being is responsible for making
> sure that the combination of those two things implements the intended
> security policy.

Sure, but upthread it was illustrated how difficult it is for even the
people implementing the protocol to reason through what's safe and
what's not.

The primitives we're providing in the protocol are, IMO, difficult to
wield safely for more complex use cases. We can provide mitigations, and
demand that the DBA reason through every combination, and tell them
"don't do that" when they screw up or come across a situation that our
mitigations can't paper over. But I think we can solve the root problem
instead.

> We
> could do something that looks more like a series of if-then rules,
> e.g.
>
> target-host 127.0.0.0/8 => reject
> authentication-method scram => accept
> reject

Yeah, I think something based on allow/deny is going to be the most
intuitive.

> But it's only a hop, skip and a jump from there to something that
> looks an awful lot like a full-blown programing language, and maybe
> that's even the right idea, but, oh, the bike-shedding!

Eh. Someone will demand Turing-completeness eventually, but you don't
have to listen. :D

--Jacob