Preventing non-superusers from altering session authorization

Hi all,

I briefly mentioned this issue in another mailing thread [0].

Currently, a user is allowed to execute SET SESSION AUTHORIZATION [1]
if the role they connected to PostgreSQL with was a superuser at the
time of connection. Even if the role is later altered to no longer be a
superuser, the session can still execute SET SESSION AUTHORIZATION, as
long as the session isn't disconnected. As a consequence, if that role
is altered to no longer be a superuser, then the user can use SET
SESSION AUTHORIZATION to switch to another role that is a superuser and
regain superuser privileges. They can even re-grant themselves the
superuser attribute.

It is possible that the user had already run SET SESSION AUTHORIZATION
to set their session to a superuser before their connecting role lost
the superuser attribute. In this case there's not much we can do.

Also, from looking at the code and documentation, it looks like SET
SESSION AUTHORIZATION works this way intentionally. However, I'm unable
to figure out why we'd want it to work this way.

I've attached a patch that would fix this issue by checking the catalog
to see if the connecting role is currently a superuser every time SET
SESSION AUTHORIZATION is run. However, according to the comment I
deleted there's something invalid about reading the catalog from that
function, though I wasn't able to understand it fully.

One downside is that if a user switches their session authorization to
some role, then loses the superuser attribute on their connecting role,
they may be stuck in a that role with no way to reset their session
authorization without disconnecting and reconnecting.

Thanks,
Joe Koshakow

[0]
https://www.postgresql.org/message-id/CAAvxfHco7iGw4NarymhfLWN6PjzYRrbYFt2BnSFeSD5sFzqEJQ%40mail.gmail.com
[1] https://www.postgresql.org/docs/15/sql-set-session-authorization.html