Date: Sun, 19 May 2019 19:38:45 +0200
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Dean Rasheed <dean.a.rasheed@gmail.com>,
	Stephen Frost <sfrost@snowman.net>,
	Andres Freund <andres@anarazel.de>,
	PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Subject: Re: Multivariate MCV stats can leak data to unprivileged users
Message-ID: <20190519173845.tgf7ku3vigwgndn5@development>
References: 
 <CAEZATCW3iAJnU8opUCXb2i_-8syU7on4cdXLWHBeWr04v67HoQ@mail.gmail.com>
 <4E0E0C14-C476-446A-835F-CC3558709DA9@anarazel.de>
 <20190518190042.2y33h4toxk4fxv5t@development>
 <568.1558208711@sss.pgh.pa.us>
 <20190518232812.qpnjmydlx3uzzsj7@development>
 <20866.1558223060@sss.pgh.pa.us>
 <20190518234841.GE6197@tamriel.snowman.net>
 <CAEZATCUcoAKrn-1NFkDN7md-wt6CQev0JXa+Tv31T9Jjkxgx5Q@mail.gmail.com>
 <23229.1558275148@sss.pgh.pa.us>
 <23772.1558276123@sss.pgh.pa.us>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii; format=flowed
Content-Disposition: inline
In-Reply-To: <23772.1558276123@sss.pgh.pa.us>
User-Agent: NeoMutt/20180716-1444-295967-dirty
Precedence: bulk

On Sun, May 19, 2019 at 10:28:43AM -0400, Tom Lane wrote:
>I wrote:
>> I wonder ... another way we could potentially do this is
>
>> create table pg_statistic_ext_data(
>>     stxoid oid,  -- OID of owning pg_statistic_ext entry
>>     stxkind char, -- what kind of data
>>     stxdata bytea -- the data, in some format or other
>> );
>
>> The advantage of this way is that we'd not have to rejigger the
>> catalog's rowtype every time we think of a new kind of extended
>> stats.  The disadvantage is that manual inspection of the contents
>> of an entry would become much harder, for lack of any convenient
>> output function.
>
>No, wait, scratch that.  We could fold the three existing types
>pg_ndistinct, pg_dependencies, pg_mcv_list into one new type, say
>"pg_stats_ext_data", where the actual storage would need to have an
>ID field (so we'd waste a byte or two duplicating the externally
>visible stxkind field inside stxdata).  The output function for this
>type is just a switch over the existing code.  The big advantage of
>this way compared to the current approach is that adding a new
>ext-stats type requires *zero* work with adding new catalog entries.
>Just add another switch case in pg_stats_ext_data_out() and you're
>done.
>

The annoying thing is that this undoes the protections provided by special
data types generated only in internally. It's not possible to generate
e.g. pg_mcv_list values in user code (except for C code, at which points
all bets are off anyway). By abandoning this and reverting to bytea anyone
could craft a bytea and claim it's a statistic value.

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services