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Subject: Re: [PATCH Version 2 0/3] GSSD: Use gss-ctx keys and gsskeyd to
 sync Kerberos credentials and kernel gss_contexts.
From: Simo Sorce <simo@redhat.com>
To: "Adamson, Andy" <William.Adamson@netapp.com>
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        "<linux-nfs@vger.kernel.org>" <linux-nfs@vger.kernel.org>
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Date: Tue, 22 Oct 2013 12:09:22 -0400
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On Tue, 2013-10-22 at 15:32 +0000, Adamson, Andy wrote:
> On Oct 22, 2013, at 11:02 AM, Simo Sorce <simo@redhat.com> wrote:
> 
> > On Tue, 2013-10-22 at 10:22 -0400, andros@netapp.com wrote:
> >> From: Andy Adamson <andros@netapp.com>
> >> 
> >> This is an RFC patchset, which will be used for testing.
> >> 
> >> This patchset requires the "SUNRPC: destroy gss_cred and context on Kerberos credential destruction" kernel patchset.
> >> 
> >> We need to do a lot of testing to ensure that once kdestroy and gss-ctx
> >> gss_user_destroy is called, all existing buffered
> >> writes using the 'destroyed gss credential + context' are serviced.
> >> 
> >> Differences from version 1:
> >> 
> >> - moved from nfstgt_login and nfstgt_logout to gsskeyd.
> >> - gsskeyd automatically creates gss-ctx key on kinit and destroys the gss-ctx
> >> key on kdestroy.
> >> 
> >> gsskeyd will need to act differently for different krb5 credential caches.
> > 
> > Why ?
> 
> Just need different parsing to know which directory to poll. 
> > 
> >> For example, some versions of gssd store FILE credentials in FILE:/tmp/krb5cc_<UID>
> >> while this code, written for fedora 19 uses FILE:/run/user/<UID>/krb5cc/tgt.
> > 
> > This is incorrect, Fedora 19 use DIR type ccaches, so you should
> > reference a cache withing the dir type, which will notmally be something
> > like: DIR:/run/user/<uidnumber>/krb5cc for the whole collection or
> > DIR::/run/user/<uidnumber>/krb5cc/txtXXXXXX for the sepcific cache.
> > 
> > However note that due to issues with DIR type caches we are moving to a
> > new KEYRING based type of cache in F20, so assuming any specific type of
> > cache is a dead start.
> 
> But I'm not assuming any kind of kerberos credential cache! I just coded to one type for this RFC.
> Didn't you see my KEYRING based ccache comment below?

Ah sorry, I had not noticed.
 
> >> As Trond suggested, if we keep gsskeyd separate from gssd, we could set up a
> >> configuration file along the lines of the keytools' request-key.conf file to
> >> allow both NFS and CIFS (and other filesystems) to install plugin handlers
> >> for kinit/kdestroy events.
> > 
> > Given in many cases (the desirable ones at least) kerberos credentials
> > are not inited via kinit, but rather by things like pam_krb5, sssd, or
> > directly imported via sshd, I am trying to understand how you are going
> > to address the majority of these cases.
> 
> All of these cases use a kerberos credential cache which if is of type
> FILE the gsskeyd can poll inotify and if it is of type KEY: then
> gsskeyd adds the gss-ctx key to the Kerberos keyring (that is the
> theory :)) so the same code services kinit/pam_krb5

You wouldn't be able to see it if I set a custom ccache file though
export KRB5CCNAME=FILE:/random/sir/foofile
 
> > Should users put gsskeyd in their .profile/.bashrc files or something ?
> > 
> >> Else, we could have gssd be the process to poll inotify (given
> >> that it already polls rpc_pipefs) and then just have it fork off the
> >> subprocess if and when it sees an interesting event.
> > 
> > What is an 'interesting' event ?
> 
> TGT (used for NFS TGS) creation/destruction

Shouldn't you care for the specific NFS ticket only ?
Although kdestroy is quite blunt, in theory users can simply destroy the
specific NFS ticket and keep their TGT.

Also I see a problem with this approach. In order to make user life
easier we reinit ccaches in sssd when a user for example unlocks the
screen. If you track a specific TGT or NFS ticket, this would cause the
immediate destruction of credentials in the nfs client which will also
kill access to the NFS server. I think this would cause quite some grief
to users.

> >> We need to investigate how this works when the kernel keyring is used for
> >> Kerberos credentials. I believe that in this case gsskeyd can add the gss-ctx
> >> key to the kerberos keyring, and it will get destroyed along with all other
> >> keys at kdestroy.
> > 
> > Is there any reason why you are doing this work with an utility that is
> > separate from gssd or libkrb5 ?
> 
> Yes  - to allow both NFS and CIFS (and other filesystems) to install
> plugin handlers for kerberos credential cache creation/destruction
> events.
> 
> But, having gssd do it is fine, or having a client plugin in libkrb5
> is also fine.

> > 
> > I think the only sensible way to handle something like this is by adding
> > support directly in libkrb5, I do not see something external ever
> > working reliably.
> 
> I agree that I should persue a libkrb5 approach.

Keep in mind the point above though.
The issues here is that you do not know what the intention was if you
try a completely automated approach. you should probably interact with
the login service and act conditionally in any way.

I think it may be safer for you to provide a hook that the system can
call when a user fully logs out. And use that hook to kill any
credential associated to a particular UID.

Done this way you do not need to poll anything nor change libkrb5. And
you also allow the creation of a tool that the user can run to force the
destruction of his credentials if he is not using a compliant login
manager. Or if he wants to explicitly kill NFS creds in the kernel for
whatever reason.

> > I am not against it for testing purposes, but then does it need to be
> > committed to nfs-utils ?
> 
> Did you not see the "This is an RFC patchset"? I'm not asking for a commit to nfs-utils, but "request for comments" which you just did!  Thanks :)

Yes I was providing feedback :-)

Simo.

-- 
Simo Sorce * Red Hat, Inc * New York