Re: storing an explicit nonce

Hi,

On 2021-05-25 15:34:04 -0400, Bruce Momjian wrote:
> My point is that we have to full-page-write cases where we change the
> nonce --- we get a new LSN/nonce for free if we are using the LSN as the
> nonce. What has made this approach much easier is that you basically
> tie a change of the nonce to require a change of LSN, since you are WAL
> logging it and every nonce change has to be full-page-write WAL logged.
> This makes the LSN-as-nonce less fragile to breakage than a custom
> nonce, in my opinion, which may explain why my patch is so small.

This disregards that we need to be able to increment nonces on standbys
/ during crash recovery.

It may look like that's not needed, with an (wrong!) argument like: The
only writes come from crash recovery, which always are associated with a
WAL record, guaranteeing nonce increases. Hint bits are not an issue
because they don't mark the buffer dirty.

But unfortunately that analysis is wrong. Consider the following
sequence:

1) replay record LSN X affecting page Y (FPI replay)
2) write out Y, encrypt Y using X as nonce
3) crash
4) replay record LSN X affecting page Y (FPI replay)
5) hint bit update to Y, resulting in Y'
6) write out Y', encrypt Y' using X as nonce

While 5) did not mark the page as dirty, it still modified the page
contents. Which means that we'd encrypt different content with the same
nonce - which is not allowed.

I'm pretty sure that there's several other ways to end up with page
contents that differ, despite the LSN not changing.

Greetings,

Andres Freund