On Mar 18, 2006, at 4:34 AM, [EMAIL PROTECTED] wrote:
On Mar 17, 2006, at 7:33 PM, Jeffrey Altman wrote:
I am cc'ing the [email protected] mailing list because
this is
really an OpenAFS discussion. [EMAIL PROTECTED] is meant to be a
mailing
list focused on development of the MIT Kerberos reference
implementation.
The fundamental issue being discussed here is whether the
Kerberos.App
display of the Kerberos Credential Cache contents can be used as an
indication by end users that the AFS kernel module contains tokens
for
that user. Hank is claiming that presence of an "afs/[EMAIL PROTECTED]"
service ticket in the credential cache is an indicator that there are
AFS tokens installed in the AFS kernel module.
I believe that end users should be discouraged from checking the
Kerberos credential cache to see if they have AFS tokens because
doing
so is fundamentally flawed. There are many reasons why tokens
might be
removed from the AFS kernel after their initial installation let
alone
reasons why tokens might not be able to be stored in the first place.
Therefore, using the Kerberos credential cache as a replacement for
the
"tokens" command or a GUI token display will only make the lives of
end
users and those that support them more difficult.
If there is a concern that the presence of the AFS service ticket
will
be misinterpreted as meaning that tokens are present then perhaps the
thing to do is modify aklog and anything that derives from its code
base
to not use the default credential cache and instead use a local
memory
cache. We could make this the default behavior and allow the default
credential cache be used when "-d" is specified on the command
line to
allow the presence of the service tickets to be used for debugging
purposes.
One other place that the credential cache and the AFS token get out
of sync is when trying to renew the TGT. Renewal of the TGT and AFS
token is a whole subject on its own, but suffice it to say that
running "kinit -R" will renew the TGT, but remove the AFS service
ticket, while the AFS token remains unchanged.
This is not an issue for the KLL loginLogout aklog plugin. The KLL
loginLogout plugins run whenever new tickets are acquired, including
when renewal occurs. All of this support works great except when
Kerberos tickets are acquired during console login.
During console login the KLL is called by the Kerberos authplugin
which is loaded by authorizationhost. In Tiger authorizationhost
runs as root or the securityagent user depending on whether
",privileged" was specified in /etc/authorization. The
authorizationhost is spawned by the SecurityAgent and is not part of
the chain of processes which will eventually be the user's login
session. Unfortunately AFS only associates tokens with a user by two
mechanisms: magic entries in the user's group list or the user's
uid. Since the KLL loginLogout plugins have neither the user's uid
or access to the user's group list, they cannot store AFS tokens
properly during login.
Now I believe I asked Conrad Sauerwald to insert some code into the
Kerberos authplugin which tries to work around this issue. If
",privileged" is specified and the authorizationhost is running as
root, the plugin will try to call KLStoreNewInitialTicketCredentials
() with its effective uid temporarily set to the user's effective
uid. Which means it might be possible to get the uid-based storage
working so long as ",privileged" is specified. Folks should
definitely test this though because I haven't actually tried it since
before Tiger shipped.
Note that specifying ",privileged" isn't a big deal because the KLL
needs to run as root to read your /etc/krb5.keytab anyway.
The best way to solve this is to have AFS use the Mac OS X Security
Session model for storing tokens so that it doesn't need to depend on
either the user's uid or their group list. This is what Kerberos
uses for credentials and what most all of the keychain stuff uses.
I'm not sure how much progress has been made, or if the functionality
necessary to make this work for a kernel module is even present. But
really that's the right way to do it.
While not part of the same topic, Derrick Brashear spent time this
week
attempting to prepare a KFM KLL plug-in for aklog that would work on
Tiger and discovered that under Tiger we will not be able to provide
such functionality. We will work with Apple to try to make this
happen
in a future release. For those who are unaware, the KFM KLL plug-ins
have been used in previous releases of MacOS X to allow the Kerberos
initial ticket getting functionality to be extended such that
whenever a
new Kerberos 5 Initial Ticket is obtained a new AFS token would be
acquired at the same time. Without this functionality it is not
possible to provide a single sign-on experience for AFS on Tiger.
(Remember, I'm just a developer who happens to work at Apple; I don't
speak for Apple.)
It turns out I was working this week on something similar, a PAM
module for aklog. This is still highly experimental, but I have
modified aklog to be more reentrant and embeddible, by converting all
the global variables into fields of a structure. I have add an
additional routine that allocates such a structure, which is then
passed to the aklog() routine. So now, the main() routine of the
aklog program calls alloc_aklog_globals() to get a block of memory,
and passes that to the aklog() routine with the arguments.
The PAM modules does the same thing. There are some differences that
are taken care of with a macro definition, like the fact that argv[0]
is the name of the aklog program, but is the first real argument from
the PAM modules. Also, the aklog program exits on error; the PAM
modules does a longjmp back to the aklog() routine to do an error
return (because there is no exit, there might be some memory leaks or
other cleanup that will need to be done).
Creating a loadable PAM module did present some problem. The LWP
code uses assembly language (process.s for ppc and process.i386.s for
x86), but that code references a global variable in way that would
require a writable text segment. So I added the usual assembly goop
to move the relocation info to a data segment. (I had tried to
replace LWP with the pthread code, but was running into a bunch of
undefined symbol problems, so I fell back to LWP. I did notice four
places that allocate pthread mutexes but don't initialize with
PTHREAD_MUTEX_INITIALIZER as per POSIX, and would have made a
difference on Mac OS X, since when uninitialized, it would reside in
common segment, which is not allowed for a bundle. Initializing it
moves it into a data segment.)
So I finally created the PAM module, which only does session open and
close. The open calls aklog to create the token. The close does the
equivalent of unlog, but I then commented that out, since I can
imagine being logging into the GUI, when an ssh session comes in,
renews the AFS token and then removes it on logout, leaving the GUI
without a token. Some mechanism for the last-one-out removing the
token is needed.
So fine, I have a PAM module, but so what? Well, I also wrote a
loginwindow "LoginHook", that is called loginwindow_pam_helper. It
gets run when loginwindow finishes authenticating (presuming /etc/
authorization is set up to do Kerberos authentication). It then does
the usual PAM stuff to open a session and I set up the PAM config
file to call my pam_aklog module. In loginwindow_pam_helper, I use a
kqueue to monitor when loginwindow terminates (usually when the user
logs out, or if loginwindow dies) and then close the session.
Well, this actually all works! I log in, and my AFS tokens are
there. I can even set up a .xlog file and get tokens for multiple
cells.
Now I'm not saying this is the best approach. My thinking was that a
PAM module would be useful for other Unix platforms as well, but
doesn't really fit in so nicely in Mac OS X. The KFM might be a
little cleaner, though Mac OS X specific. But it might be possible
to get loginwindow to do PAM directly, so my loginwindow_pam_helper
wouldn't be needed anymore, and then the PAM module might fit in
better (of course, I'm not on the loginwindow team, so I can't say
they will make this change).
Mac OS X also has it's own way of doing things, like the
Authorization Services framework. Now I'm a Unix guy, so the
Authorization Services framework is new to me. But it might also be
feasible to put in aklog as a plug-in. Then loginwindow could get to
aklog through Authorization Services, and since there is already a
pam_securityserver.so module that calls into the Authorization
Services framework, things like ssh might also be able to get to
aklog. Again this is Mac OS specific, but could be the cleanest way
for both Mac OS X applications and command-line programs to get aklog
access. And since aklog is embeddable, we could build the aklog
program, the aklog PAM module, the aklog Authorization plug-in and
possibly even the KFM KLL plug-in, all using the same base code.
Anyways, Derrick, let me know if I can be of help with the KFM KLL
plug-in (which is something else I don't know much about).
The problem with using a pam plugin is that it won't run every time
new tickets are acquired. The KLL loginLogout plugins even run when
Kerberos.app is automatically renewing your tickets. It is this
seamlessness which probably originally inspired Henry Hotz to want to
use the presence of the afs service ticket to indicate that afs
tokens had been acquired. Now you could write a loginLogout plugin
that calls pam modules, but that will just land you back at the
problem I described above.
You can actually hinge successful Kerberos ticket acquisition on
successful AFS token acquisition. The loginLogout plugin API allows
the plugin to cancel ticket acquisition if the plugin fails. However
this means that if you can't get AFS tokens, you also won't be able
to get tickets -- probably not terribly useful unless you have an AFS
home directory -- then no tokens really does mean you can't do anything.
As a side note the loginLogout plugins are called for all ticket
acquistion. So if the user has two identities such as [EMAIL PROTECTED] and
user/[EMAIL PROTECTED], the plugin needs to be smart about how it gets
tokens when the plugin is called for both of them.
--lxs
Alexandra Ellwood <[EMAIL PROTECTED]>
MIT Kerberos Development Team
<http://mit.edu/lxs/www>
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