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

[Andrew Halaney <andrew@amutable.com>]

On Fri, Sep 18, 2026 at 09:05:26AM +0100, David Howells wrote:
> 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'll defer to what you all want in the message here, I found it valuable
but I trend on the side of overly verbose admittedly!

> 
> > 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.

To be clear are you suggesting I do that for v2 here? Happy to make the
change and add some reuse to the registration functions, etc. I'm
guessing its fine to change the bpf id since its still only in -next?

The only awkward bit with making that more generic is that dm-verity
isn't __ro_after_init since its coming from a module possibly, and
because of the module usage I also protected it with a spinlock in case
someone's accessing it while you unload the module. Could just use one
spinlock for the whole generic array, and drop the __ro_after_init I
suppose.

Let me know if I'm not following properly!

Thanks,
Andrew