Re: "WIP: Data at rest encryption" patch and, PostgreSQL 11-beta3

Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> > If the WAL *is* encrypted, the state at this point is that the block
> > is unreadable, because the first 4kB of the block is the first half of
> > the bits that resulted from encrypting 8kB of data that includes the
> > new record, and the second 4kB of the block is the second half of the
> > bits that resulted from encrypting 8kB of data that did not include
> > the new record, and since the new record perturbed every bit on the
> > page with probability ~50%, what that means is you now have garbage.
> > That means that not only did we lose the new record, but we also lost
> > the 3.5kB of good data which the page previously contained. That's
> > NOT ok. Some of the changes associated with those WAL records may
> > have been flushed to disk already, or there may be commits in there
> > that were acknowledged to the client, and we can't just lose them.
>
> ISTM that this is only a problem if you choose the wrong encryption
> method. One not-wrong encryption method is to use a stream cipher
> --- maybe that's not the exact right technical term, but anyway
> I'm talking about a method which notionally XOR's the cleartext
> data with a random bit stream generated from the encryption key
> (probably along with other knowable inputs such as the block number).
> In such a method, corruption of individual on-disk bytes doesn't
> prevent you from getting the correct decryption of on-disk bytes
> that aren't corrupted.

We actually use a block cipher (with block size 16 bytes), as opposed to
stream cipher. It's true that partial write is a problem because if a single
bit of the cipher text changed, decryption will produce 16 bytes of
garbage. However I'm not sure if partial write can affect as small unit as 16
bytes.

Nevertheless, with the current version of our patch, PG should be resistant
against such a partial write anyway because we chose to align XLOG records to
16 bytes (as long as the encryption is enabled) for the following reasons:

If one XLOG record ends and the following one starts in the same encryption
block, both records can get corrupted during streaming replication. The
scenario looks like: 1) the first record is written on master (the unused part
of the block contains zeroes), 2) the block is encrypted and its initial part
(i.e. the number of bytes occupied by the first record in the plain text) is
streamed to slave, 3) the second record is written on master, 4) the
containing encryption block is encrypted again and the trailing part (i.e. the
number of bytes occupied by the second record) is streamed, 5) decryption of
the block on slave will produce garbage and thus corrupt both records. This is
because the trailing part of the block was filled with zeroes during
encryption, but it contains different data at decryption time.

Alternative approach to this replication problem is that walsender decrypts
the stream and walreceiver encrypts it again. While this can provide us with
the advantage to have master and slave encrypted with different keys, this
approach brings some additional complexity. For example, pg_basebackup would
need to deal with encryption.

This design decision can be changed, but there's one more thing to consider:
if the XLOG stream is decrypted, the decryption cannot be disabled unless the
XLOG records are aligned to 16 bytes (and in turn, the XLOG alignment cannot
be enabled w/o initdb).

--
Antonin Houska
https://www.cybertec-postgresql.com