Re: [PATCH 1/3] keys: add KEY_SPEC_DM_VERITY_KEYRING

[David Howells <dhowells@redhat.com>]

Jarkko Sakkinen <jarkko@kernel.org> wrote:

> This is great for discussion but what we want for the commit message
> is just motivation and resolution.

Actually, I think it's useful that Andrew wrote up the issues in the commit
message - and I think it shows part of the motivation.  The 'writing a fake
/proc/keys line in the description' is something I hadn't considered.

> I don't think we need all this just to say that /proc/keys in a racy
> query mechanism for production, which is an issue for dm-verity, given
> that nothing else is available.

I think at some point, we will need a system call to search all for all
accessible keys matching certain criteria by actually walking the key
database.  The problem there is that there may be multiple hits, so we may
need something like:

	int count = find_key(key_serial_t start_id,
			     const char *type, const char *desc_prefix,
			     key_serial_t *results, size_t results_size,
			     unsigned int flags);

Allowing you to do:

	key_serial_t dm_key;
	int n = find_key(0, "keyring", ".dm_verity", &dm_key, 1,
			 FIND_KEY_EXACT_DESC);

This wouldn't be as fast as a direct lookup since it would have to walk the
key tree, doing name comparisons and perm checks on each key of the type.

> And secondly special keys are meant for implicit keyrings so isn't
> that all there's to it?

I have no particular objection to setting aside a block of negative key IDs
for special keyrings that need to be accessed a lot - though I would make
common reg/unreg functions that take the ID to be registered and, say, set the
block at -257..-512.  Moving the BFP keyring to -257 and DM to -258.

David