Re: libpq compression (part 2)

Greetings,

* Justin Pryzby (pryzby(at)telsasoft(dot)com) wrote:
> On Fri, Jan 07, 2022 at 01:46:00PM -0500, Bruce Momjian wrote:
> > On Sat, Jan 1, 2022 at 11:25:05AM -0600, Justin Pryzby wrote:
> > > Thanks for working on this. The patch looks to be in good shape - I hope more
> > > people will help to review and test it. I took the liberty of creating a new
> > > CF entry. http://cfbot.cputube.org/daniil-zakhlystov.html
> > >
> > > +zpq_should_compress(ZpqStream * zpq, char msg_type, uint32 msg_len)
> > > +{
> > > + return zpq_choose_compressor(zpq, msg_type, msg_len) == -1;
> > >
> > > I think this is backwards , and should say != -1 ?
> > >
> > > As written, the server GUC libpq_compression defaults to "on", and the client
> > > doesn't request compression. I think the server GUC should default to off.
> > > I failed to convince Kontantin about this last year. The reason is that 1)
> > > it's a new feature; 2) with security implications. An admin should need to
> > > "opt in" to this. I still wonder if this should be controlled by a new "TYPE"
> > > in pg_hba (rather than a GUC); that would make it exclusive of SSL.
> >
> > I assume this compression happens before it is encrypted for TLS
> > transport. Second, compression was removed from TLS because there were
> > too many ways for HTTP to weaken encryption. I assume the Postgres wire
> > protocol doesn't have similar exploit possibilities.
>
> It's discussed in last year's thread. The thinking is that there tends to be
> *fewer* exploitable opportunities between application->DB than between
> browser->app.

Yes, this was discussed previously and addressed.

> But it's still a known concern, and should default to off - as I said.

I'm not entirely convinced of this but also am happy enough as long as
the capability exists, no matter if it's off or on by default.

> That's also why I wondered if compression should be controlled by pg_hba,
> rather than a GUC. To require/allow an DBA to opt-in to it for specific hosts.
> Or to make it exclusive of ssl. We could choose to not suppose that case at
> all, or (depending on the implement) refuse that combination of layers.

I'm definitely against us deciding that we know better than admins if
this is an acceptable trade-off in their environment, or not. Letting
users/admins control it is fine, but I don't feel we should forbid it.

As for the details of how we allow control over it, I suppose there's a
number of options. Having it in the HBA doesn't seem terrible, though I
suspect most will just want to enable it across the board and having to
have "compression=allowed" or whatever added to every hba line seems
likely to be annoying. Maybe a global GUC and then allow the hba to
override?

Thanks,

Stephen