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Bug#795735: partman-crypto: always encrypt swap

On Thu, Oct 19, 2017 at 04:37:54PM -0400, Daniel Kahn Gillmor wrote:
> It's a shame that encrypted swap by default hasn't happened yet for
> debian.

> actual hardware performance
> ---------------------------
> I suspect the cost is negligible on most hardware today, particularly
> when compared to the disk I/O.  If you're swapping, you're likely to be
> waiting for the disk, not waiting for the CPU.  That said, i agree that
> users with specialized situations ought to be able to disable this.  But
> the default should still be on.

For clarity, when I stated that, I wasn't using it as an argument
against enabling encryption of the swap partition by default; it's just
that saying "there is no performance impact when doing something extra"
is objectively wrong. The performance impact may be negligable; but when
you need to do more work, you need to actually do it and there will be
an impact. It will not matter in the common case, but there will be
exceptions where it will matter, and therefore we need to continue
supporting that.

Pet peeves, I suppose.

> suspend-to-disk
> ---------------
> If the user suspends to disk, then the memory will be written to disk.
> this is definitely a leak.  However, we currently write the memory to
> disk *without* suspending to disk, so even if we don't handle
> suspend-to-disk "safely" it's still a win to encrypt swap, because we
> protect the people who do *not* suspend to disk.  So that's the simplest
> solution to the suspend-to-disk problem: just punt on it for now, and
> leave that case unprotected.
> If suspend-to-disk (or rather, resume-from-disk) is the only problem,
> then we should look for ways to opportunistically take advantage of
> other non-disk hardware on which we could store any ephemeral keys
> needed for restoration.
> For example, on systems with rewritable nvram, it's conceivable that we
> could suspend to the encrypted volume, and then stash the ephemeral
> encryption key in nvram.  Upon resume, read the key from nvram into main
> memory, clear the nvram, and restore from the encrypted volume.  This
> isn't perfectly secure (an attacker with both the disk and the nvram can
> recover your memory from the suspend file) but it is a significant win
> against an attacker who physically removes the hard disk.
> So i think we ought to outline the steps that need to be taken to make
> this happen by default.  Which pieces need to be updated, and how?

The initramfs code that does the actual resume should be updated to read
the key from wherever it was written.

On systems that don't have writable nvram, we could still support both
encrypted swap and suspend-to-disk provided the system has an
alternative location to write that session's encryption key to (which
I'll inaccurately call "the session key" for now -- it's not really a
session key, but it's close enough).

If the kernel needs to read the session key, then it can't be on a
partition that needs to be live. The initramfs would need to mount that
partition to be able to see whether there is a session key that needs to
be used to do the resume-from-disk, which would cause a journal replay;
that would invalidate the suspended image. What we could do, however, is
if the user has a separate /boot partition, to make the "suspend" script
write the session key to a well-known file in that partition, and then,
just before actually doing the suspend, *unmounting* the /boot
partition. The resume code in the initramfs could then mount the /boot
partition and figure out if there is a session key; if there is, it
would read the key, unmount the partition again, and then proceed to
decrypt the swap partition and resume the system.

If a user installs a system with encrypted swap but no separate /boot
partition, the suspend-to-disk code could/should then produce an error
message to the effect that the user needs to either disable encrypted
swap, or create a separate /boot partition. If partman is then also
updated to default to having a separate /boot partition, we would be
able to support encrypted swap as well as suspend-to-disk virtually

On a side node, if the system has a TPM that is not disabled, we should
also attempt to encrypt the session key (regardless of where it is
stored) using the TPM. That way, an attacker who gains access of the
hard disk and the (encrypted) session key but not the system itself
cannot decrypt the swap partition, since a TPM is supposed to not allow
keys to be removed from it.

Could you people please use IRC like normal people?!?

  -- Amaya Rodrigo Sastre, trying to quiet down the buzz in the DebConf 2008

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