linux-kernel - Re: RFC: 'ioctl' for keyrings

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Date:	Thu, 13 Mar 2014 12:57:14 -0400
From:	Mimi Zohar <zohar@...ux.vnet.ibm.com>
To:	David Howells <dhowells@...hat.com>
Cc:	torvalds@...ux-foundation.org, dmitry.kasatkin@...il.com,
	keyrings@...ux-nfs.org, linux-security-module@...r.kernel.org,
	linux-kernel@...r.kernel.org, David Safford <safford@...ibm.com>
Subject: Re: RFC: 'ioctl' for keyrings

On Thu, 2014-03-13 at 15:47 +0000, David Howells wrote: 
> Hi Linus,
> 
> I've encountered a situation where I could do with providign certain key types
> with their own operations and I'm wondering as to the best way.
> 
> The problem I've been trying to deal with is to improve quota management on
> keys and introduce LRU key discard when a quota is overrun.  This requires me
> to overhaul all the ->instantiate() and ->update() key type operations as they
> can ask for more quota.
> 
> What I want to do is to use the preparsing mechanism I introduced for
> asymmetric key handling for both operations.  This has the advantage that all
> parse errors and invalid arguments can be found upfront before we start
> allocating things that are difficult to back out of - and means that we're less
> likely to encounter ENOMEM in the type op functions.
> 
> Now, ->instantiate() is meant to set the payload on a key and ->update() is
> meant to replace the payload on a key.  There is, currently, no way to modify a
> key to some lesser extent.
> 
> However, in the process of doing this, I've come to realise that the trusted
> key type and the encrypted key type use their ->update() operations to make a
> partial alteration to a key.
> 
> Unfortunately, this does not lend itself to preparsing because the result
> depends on the key we're going to update - and we don't necessarily know the
> key yet.
> 
> Further, this behaviour is also broken because ->update() may be called by
> add_key() if it finds a matching key; but you cannot rely on add_key() adding
> vs updating because it may race with someone else doing an add_key().  To make
> matters yet more interesting, at least with encrypted_update(), the parameters
> expected in the payload depend on whether you want to create a key or alter a
> key.
> 
> I can fix this in one of a number of ways:
> 
>  (1) Provide a generic control operation (analogous with ioctl()) that allows
>      the user to make some general operation on a key (querying it, altering
>      it, interacting with hardware).
> 
>  (2) Provide an alter operation that only allows the key to be altered.
>      Looking at trusted_update(), though, I have a suspicion that this may not
>      be sufficient as that also seems to invoke an interaction with the TPM.
> 
>  (3) Provide separate, specific keyctl functions for the special operations
>      required by encrypted and trusted keys (and other key types potentially)
>      that are then validated in the core and routed to the key type.
> 
> I would prefer (2) or perhaps (3), I think.
> 
> As I understand the code, I think operations being performed from ->update()
> are:
> 
>  (a) Resealing a key with a new pcrs (trusted).
> 
>  (b) Changing the master key (encrypted).
> 
> Mimi, Dmitry: is this list right?

In addition to resealing trusted keys to a new TPM PCR value, there are
a few other options that can be modified (eg. keyauth, blobauth,
pcrlock).  Encrypted keys can be encrypted/decrypted with a new master
key (trusted or user key type).

Mimi

> Note that if I do provide some more appropriate vector, I will probably have to
> have special code for handling encrypted and trusted keyrings in the core,
> rejecting attempts to update in key_create_or_update() and redirecting them in
> key_update() in security/keys/key.c
> 
> David
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