[RFC] Keyrings: How to make them more useful

[David Howells]

James Bottomley <James.Bottomley at HansenPartnership.com> wrote:

> One of the problems I keep tripping over is different special casing
> for user keyrings (which are real struct key structures) and system
> keyrings which are special values of the pointer in struct key *.

It's meant to be like that.  The trusted system keyrings are static within
system_keyring.c and not so easily accessible by kernel modules for
direct modification, bypassing the security checks.

Obviously this is merely a bit of obscurity and enforcement isn't possible
against kernel code that is determined to modify those keyrings or otherwise
interfere in the verification process.

> For examples of what this special handling does, just look at things
> like bpf_trace.c:bpf_lookup_{user|system}_key
> 
> Since the serial allocation code has a hard coded not less than 3
> (which looks for all the world like it was designed to mean the two
> system keyring id's were never used as user serial numbers)

That's just a coincidence.  The <3 thing predates the advent of those system
keyring magic pointers.

> I think we could simply allow the two system keyring ids to be passed into
> lookup_user_key() (which now might be a bit misnamed) and special case not
> freeing it in put_key().

If you want to make lookup_user_key() provide access to specific keyrings like
this, just use the next negative numbers - it's not like we're likely to run
out soon.

But I'd rather not let lookup_user_key() return pointers to these keyrings...

David