On Mon, Oct 11, 2021 at 01:30:38PM -0400, Stephen Frost wrote:
> Greetings,
>
> > Keep in mind that in our existing code (not my patch), the LSN is zero
> > for unlogged relations, a fixed value for some GiST index pages, and
> > unchanged for some hint bit changes. Therefore, while we can include
> > the LSN in the IV because it _might_ help, we can't rely on it.
>
> Regarding unlogged LSNs at least, I would think that we'd want to
> actually use GetFakeLSNForUnloggedRel() instead of just having it zero'd
> out. The fixed value for GiST index pages is just during the index
Good idea. For my patch I had to use a WAL-logged dummy LSN, but for
our use, re-using a fake LSN after a crash seems fine, so we can just
use the existing GetFakeLSNForUnloggedRel(). However, we might need to
use the part of my patch that removes the assumption that unlogged
relations always have zero LSNs, because right now they are only used
for GiST indexes --- I would have to research that more.
> Yes, that's the direction that I was thinking also and specifically with
> XTS as the encryption algorithm to allow us to exclude the LSN but keep
> everything else, and to address the concern around the nonce/tweak/etc
> being the same sometimes across multiple writes. Another thing to
> consider is if we want to encrypt zero'd page. There was a point
> brought up that if we do then we are encrypting a fair bit of very
> predictable bytes and that's not great (though there's a fair bit about
> our pages that someone could quite possibly predict anyway based on
> table structures and such...). I would think that if it's easy enough
> to not encrypt zero'd pages that we should avoid doing so. Don't recall
> offhand which way zero'd pages were being handled already but thought it
> made sense to mention that as part of this discussion.
Yeah, I wanted to mention that. I don't see any security difference
between fully-zero pages, pages with headers and no tuples, and pages
with headers and only a few tuples. If any of those are insecure, they
all are. Therefore, I don't see any reason to treat them differently.
--
Bruce Momjian <bruce@momjian.us> https://momjian.us
EDB https://enterprisedb.com
If only the physical world exists, free will is an illusion.
