Re: GSSAPI Proxy initiative

[Nico Williams <nico@cryptonector.com>]

On Thu, Nov 3, 2011 at 5:16 PM, Myklebust, Trond
<Trond.Myklebust@netapp.com> wrote:
>> It is ok to use keyring if that's deemed the right place for session keys, but I
>> think you already have structures where you currently store them so I don't
>> thik you necessarily need to change that part of the kernel implementation.
>
> No, but we still need to be able to do recovery of rpcsec_gss contexts once they are broken, and right now we have a major flaw due to the fact that recovery depends on a lot of small processes and data that is allowed to be swapped out at the moment when we need them the most (i.e. in a memory reclaim situation).
>
> If the server reboots while our client is in the middle of writing back a file (or several files), then the client needs to recover those rpcsec_gss contexts that authenticate the processes which own any dirty pages that remain to be written out.
> Key security is an irrelevant concern once your kernel deadlocks in an OOM state.

Ah, this problem.  Hopefully the client has enough resources to thrash
a lot in the process but still manage to recover.  A better solution
(see below) is possible, but will require more protocol/mechanism
work.

>> Currently credential caches are stored in files, is there a problem with that
>> model ? Do you need access to credential caches from the kernel when
>> under memory pressure ?
>
> Yes, there is a major problem with that model, and yes we do potentially need access to credential caches when in a recovery situation (which is a situation when we are usually under memory pressure).

Ideally we could store in each RPCSEC_GSS context (not GSS context)
enough state on the client side to recover quickly when the server
reboots.  How would we do this?  Suppose the server gives the client a
"ticket", and a key much like the Kerberos ticket session key is
agreed upon or sent by the server -- that could be stored in the
RPCSEC_GSS context and could be used to recover it quickly for
recovery from server reboot.  I'm eliding a lot of details here, but I
believe this is fundamentally workable.

A similar solution would be to store some GSS "sub-credential" in the
RPCSEC_GSS context, but this would work for Kerberos and maybe not so
well for other mechanisms -- and even with Kerberos, the service
ticket might be expired when it comes time to recover.  So I prefer
the RPCSEC_GSS-level solution I mentioned above.

If you agree with me on this then this sub-thread will be best moved
to the NFSv4 WG, particularly if we agree on a protocol-level
solution.

Nico
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