Re: Non-superuser subscription owners

On Wed, Feb 22, 2023 at 12:18 PM Jeff Davis <pgsql(at)j-davis(dot)com> wrote:
> There are two questions:
>
> 1. Is the security situation with logical replication bad? Yes. You
> nicely summarized just how bad.
>
> 2. Is it the same situation as accessing a table owned by a user you
> don't absolutely trust?
>
> Regardless of how the second question is answered, it won't diminish
> your point that logical replication is in a bad state. If another
> situation is also bad, we should fix that too.

Well said.

> So I don't think "shouldn't" is quite good enough. In the first place,
> the user needs to know that the risk exists. Second, what if they
> actually do want to access a table owned by someone else for whatever
> reason -- how do they do that safely?

Good question. I don't think we currently have a good answer.

> I can't resist mentioning that these are all SECURITY INVOKER problems.
> SECURITY INVOKER is insecure unless the invoker absolutely trusts the
> definer, and that only really makes sense if the definer is a superuser
> (or something very close). That's why we keep adding exceptions with
> SECURITY_RESTRICTED_OPERATION, which is really just a way to silently
> ignore the SECURITY INVOKER label and use SECURITY DEFINER instead.

That's an interesting way to look at it. I think there are perhaps two
different possible perspectives here. One possibility is to take the
view that you've adopted here, and blame it on SECURITY INVOKER. The
other possibility, at least as I see it, is to blame it on the fact
that we have so many places to attach executable code to tables and
very few ways for people using those tables to limit their exposure to
such code. Suppose Alice owns a table and attaches a trigger to it. If
Bob inserts into that table, I think we have to run the trigger,
because Alice is entitled to assume that, for example, any BEFORE
triggers she might have defined that block certain kinds of inserts
are actually going to block those inserts; any constraints that she
has applied to the table are going to be enforced against all new
rows; and any default expressions she supplies are actually going to
work. I think Bob has to be OK with those things too; otherwise, he
just shouldn't insert anything into the table.

But Bob doesn't have to be OK with Alice's code changing the session
state, or executing DML or DDL with his permissions. I wonder if
that's where we should be trying to insert restrictions here. Right
now, we think of SECURITY_RESTRICTED_OPERATION as a way to prevent a
function or procedure that runs under a different user ID than the
session user from poisoning the session state. But I'm thinking that
maybe the problem isn't really with code running under a different
user ID. It's with running code *provided by* a different user ID.
Maybe we should stop thinking about the security context as something
that you set when you switch to running as a different user ID, and
start thinking about it as something that needs to be set based on the
relationship between the user that provided the code and the session
user. If they're not the same, some restrictions are probably
appropriate, except I think in the case where the user who provided
the code can become the session user anyway.

> Another option is having some kind SECURITY NONE that would run the
> code as a very limited-privilege user that can basically only access
> the catalog. That would be useful for running default expressions and
> the like without the definer or invoker needing to be careful.

This might be possible, but I have some doubts about how difficult it
would be to get all the details right. We'd need to make sure that
this limited-privilege user couldn't ever create a table, or own one,
or be granted any privileges to do anything other than the minimal set
of things it's supposed to be able to do, or poison the session state,
etc. And it would have weird results like current_user returning the
name of the limited-privilege user rather than any of the users
involved in the operation. Maybe that's all OK, but I find it more
appealing to try to think about what kinds of operations can be
performed in what contexts than to invent entirely new users.

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