Re: Allow matching whole DN from a client certificate

From: Andrew Dunstan <andrew(at)dunslane(dot)net>
To: Jacob Champion <pchampion(at)vmware(dot)com>, "daniel(at)yesql(dot)se" <daniel(at)yesql(dot)se>
Cc: "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Allow matching whole DN from a client certificate
Date: 2021-01-28 22:10:35
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On 1/28/21 11:39 AM, Jacob Champion wrote:
> On Wed, 2021-01-27 at 15:42 -0500, Andrew Dunstan wrote:
>> I'm not sure where we want to go with the present patch. Do we want to
>> go with what we have and document the limitations more, or try to do
>> something more elaborate? If so, exactly what?
> After sleeping on it:
> I think that if you expect the 99% use case to be in the vein of what
> you originally proposed:
> dn /^C=US,ST=North.Carolina,O=test,OU=eng,CN=andrew$ andrew
> where the *entire* DN is pinned, there are no characters outside the
> ASCII subset, and no subgroup matching is required to find the mapped
> username (i.e. there's one line for every user of the system), then
> this approach would work.

I think this is the 99% use case, TBH. It's certainly what I was
originally asked for.

> (I'd still recommend switching to use the RFC
> flag to OpenSSL, to ease future improvements.) There should be a bunch
> of warning documentation saying not to do anything more complex unless
> you're an expert, and that the exact regular expression needed may
> change depending on the TLS backend.

I'll play around with the RFC flag.

> If you want to add UTF-8 support to the above, so that things outside
> ASCII can be matched nicely, then the ASN1_STRFLGS_ESC_MSB flag should
> be removed from the _print_ex() call per OpenSSL documentation, and we
> should investigate how it plays with other non-UTF-8 database
> encodings. That seems like work but not a huge amount of it.

How about if we remove ASN1_STRFLGS_ESC_MSB when the server encoding is
UTF8? That should be an extremely simple change.

> But if you think users are going to try to match more complex regular
> expressions, for example to be able to peel out a portion of the DN to
> use as a mapped username, or match just a few specific RDNs for
> authorization, then I think a more elaborate approach is needed to
> avoid handing people a dangerous tool. Most of the DN-matching regex
> examples I've seen on self-help sites have been wrong, in that they
> match too much.
> Unfortunately I don't really know what that solution should look like.
> A DSL for filtering on RDNs would be a lot of work, but it could
> potentially allow LDAP to be mapped through pg_ident as well?

In the end it will be up to users to come up with expressions that meet
their usage. Yes they could get it wrong, but then they can get so many
things wrong ;-)



Andrew Dunstan

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