Re: Non-superuser subscription owners

On Fri, Feb 3, 2023 at 3:47 AM Andres Freund <andres(at)anarazel(dot)de> wrote:
> On 2023-02-02 09:28:03 -0500, Robert Haas wrote:
> > I don't know what you mean by this. DML doesn't confer privileges. If
> > code gets executed and runs with the replication user's credentials,
> > that could lead to privilege escalation, but just moving rows around
> > doesn't, at least not in the database sense.
>
> Executing DML ends up executing code. Think predicated/expression
> indexes, triggers, default expressions etc. If a badly written trigger
> etc can be tricked to do arbitrary code exec, an attack will be able to
> run with the privs of the run-as user. How bad that is is influenced to
> some degree by the amount of privileges that user has.

I spent some time studying this today. I think you're right. What I'm
confused about is: why do we consider this situation even vaguely
acceptable? Isn't this basically an admission that our logical
replication security model is completely and totally broken and we
need to fix it somehow and file for a CVE number? Like, in released
branches, you can't even have a subscription owned by a non-superuser.
But any non-superuser can set a default expression or create an enable
always trigger and sure enough, if that table is replicated, the
system will run that trigger as the subscription owner, who is a
superuser. Which AFAICS means that if a non-superuser owns a table
that is part of a subscription, they can instantly hack superuser.
Which seems, uh, extremely bad. Am I missing something?

Based on other remarks you made upthread, it seems like we ought to be
doing the actual replication as the table owner, since the table owner
has to be prepared for executable code attached to the table to be
re-run on rows in the table at any table when somebody does a REINDEX.
And then, in master, where there's some provision for non-superuser
subscription owners, we maybe need to re-think the privileges required
to replicate into a table in the first place. I don't think that
having I/U/D permissions on a table is really sufficient to justify
performing those operations *as the table owner*; perhaps the check
ought to be whether you have the privileges of the table owner.

--
Robert Haas
EDB: http://www.enterprisedb.com