Horton

Re: Postgres 11 release notes

Bruce Momjian <bruce@momjian.us>

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From: Bruce Momjian <bruce@momjian.us>
To: Michael Paquier <michael@paquier.xyz>
Cc: Heikki Linnakangas <hlinnaka@iki.fi>, PostgreSQL-development <pgsql-hackers@postgresql.org>, Magnus Hagander <magnus@hagander.net>, Stephen Frost <sfrost@snowman.net>
Date: 2018-05-17T13:55:34Z
Lists: pgsql-hackers 
On Wed, May 16, 2018 at 09:09:22PM -0400, Bruce Momjian wrote:
> > > FYI, I think the server could also require channel binding for SCRAM. We
> > > already have scram-sha-256 in pg_hba.conf, and I think
> > > scram-sha-256-plus would be reasonable.
> > 
> > Noted as well.  There is of course the question of v10 libpq versions
> > which don't support channel binding, but if an admin is willing to set
> > up scram-sha-256-plus in pg_hba.conf then he can request his users to
> > update his drivers/libs as well.
> 
> Yes, I don't see a way around it.  Once you accept that someone in the
> middle can change what you request undetected, then you can't do 
> fallback.  Imagine a man-in-the-middle with TLS where the
> man-in-the-middle allows the two end-points to negotiate the shared
> secret, but the man-in-the-middle forces a weak cipher.  This is what is
> happening with Postgres when the man-in-the-middle forces a weaker
> authentication method.

Technically, you can do automatic fallback if the fallback has
man-in-the-middle and downgrade protection.  Technically, because TLS is
already active when we start authentication negotiation, we don't need
downgrade protection as long as we have man-in-the-middle protection.

Unfortunately, only SCRAM with channel binding and sslmode=verify-full
have man-in-the-middle protection.  Therefore, downgrading from SCRAM
with channel binding to SCRAM without channel binding, MD5, or
'password' is only safe if sslmode=verify-full is enabled.  Technically
MD5, or 'password' has weak or non-existent replay protection, but if we
are requiring TLS, then that doesn't really matter, assuming we have
man-in-the-middle protection.

I don't know if falling back from SCRAM with channel binding to a lesser
authentication methods only if sslmode=verify-full is enabled is really
helpful to anyone since it requires certificate installation.

TLS has similar downgrade issues:

	http://www.educatedguesswork.org/2012/07/problems_with_secure_upgrade_t.html

but many of its downgrade options have downgrade protection, and you
don't lose man-in-the-middle protection by downgrading. 
Man-in-the-middle protection via certificate checking happens
independent of the TLS version being used, which is not the case for
Postgres authentication downgrade options.

-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

+ As you are, so once was I.  As I am, so you will be. +
+                      Ancient Roman grave inscription +

Commits

  1. doc: update PG 11 release notes

  2. Fix misspelled pg_trgm contrib name in PostgreSQL 11 release notes

  3. Doc: clarify release note text about v11's new window function features.

  4. Improve wording of release notes item

  5. Fix typos in release notes

  6. Doc: preliminary list of PG11 major features.

  7. Make numeric power() handle NaNs according to the modern POSIX spec.

  8. Various improvements of skipping index scan during vacuum technics

  9. Revert back-branch changes in power()'s behavior for NaN inputs.

  10. Avoid wrong results for power() with NaN input on more platforms.

  11. Avoid wrong results for power() with NaN input on some platforms.

  12. Skip full index scan during cleanup of B-tree indexes when possible

  13. Rewrite the code that applies scan/join targets to paths.

  14. Postpone generate_gather_paths for topmost scan/join rel.

  15. Add casts from jsonb

  16. Make plpgsql use its DTYPE_REC code paths for composite-type variables.

  17. Don't allow VACUUM VERBOSE ANALYZE VERBOSE.

  18. Pass InitPlan values to workers via Gather (Merge).

  19. Account for the effect of lossy pages when costing bitmap scans.

  20. Allow no-op GiST support functions to be omitted.

  21. Rearm statement_timeout after each executed query.

  22. Push limit through subqueries to underlying sort, where possible.