reinit_creds (was Re: OpenSSHd barfs upon reauthentication: PAM, Solaris 8)

[Nicolas Williams]

On Wed, Sep 05, 2001 at 02:03:23PM -0700, Darren Moffat wrote:
> >> >Could we please have a clarification on the semantics of
> >> >PAM_CRED_ESTABLISH vs. the semantics of PAM_REINITIALIZE_CREDS?
> >> 
> >> My interpretation is:
> >> 
> >> You call PAM_ESTABLISH_CRED to create them
> >> You call PAM_REINITIALIZE_CRED to update creds that can expire over time,
> >> for example a kerberos ticket.
> 
> Oops.  I meant PAM_REFRESH_CRED
> 
> >PAM_RENEW_CREDS is there for credential renewal (i.e., ticket renewal,
> >in the Kerberos case). That's clear from its name.
> 
> It is called PAM_REFRESH_CRED.

Yes, sorry.

> >PAM_REINITIALIZE_CREDS is, well, completely undocumented (I'll recheck
> >the XSSO docs). The OpenSSH interpretation is clear.
> 
> Agreed and it is very ambiquous.  I think it is quite easy to make
> the assumption that ESTABLISH and REINITIALIZE care synonymous but both
> are different from REFRESH.

Again, Sun treats REINITIALIZE as a synonym of REFRESH.

> >And, IMO, as I think about it, the OpenSSH interpretation makes plenty
> >of sense. Consider an app that will not fork() a child that runs as the
> >PAM_USER (e.g., a web server) but which nonetheless needs the user's
> >Kerberos creds -- why bother creating a user-owned ccache then?
> 
> I can see that from a PAM view point but it won't really work from
> a Kerberos view point (it isn't how kerberos was designed to work).

Sure it can work from the Kerberos pov.

Like this: take an app that doesn't fork, but which authenticates
multiple users, and does not "become" them, and which impersonates each
of those users, at times, using the per-user ccache -- as long as the
app can read each per-user ccache and as long as it keeps track of whose
is whose it can do this.



> >> "The pam_setcred() function is used to establish, modify, or delete the 
> >> credentials of the current user associated with the authentication handle,
> >> pamh. "
> >
> >Why does that description not jive with my interpretation of the OpenSSH
> >interpretation of the pam_setcred() flags' semantics? I mean, I don't
> >see why.
> 
> I guess it does at the PAM level but at the level of Kerberos the creds
> are always placed in a cred file owned by that user since their is no
> concept of acting onbehalf of another.

Wrong. See above. Applications can impersonate Kerberos principals,
provided they have the creds for those principals.

> >I've written parts of a PAM_KRB5 (based on Frank Cusack's original)
> >which behaves slightly differently: its auth method saves the user's
> >Kerberos creds in a memory ccache and stuffs that into the pam handle as
> >pam data, and its setcred method actually creates the file ccache (and
> >destroys the memory ccache).
> 
> The Solaris one does that as well, the ccache file is created as
> 	/tmp/krb5cc_<uid> with permissions 600 and owner <uid>

I'll double check.

> >> It will always create a cred cache file owned by PAM_USER, the only
> >> way you could get the effect you describe above is if you called
> >> pam_setcred with PAM_USER as root, changed PAM_USER using pam_set_item
> >> to be <user> - but this isn't what OpenSSH does (and it is wrong anyway).
> >
> >This is silly -- why create a user-owned ccache *before* the account
> >management part of PAM has been called and suceeded??
> 
> Because you are supposed to call pam functions in this order:
> 
> pam_start(pamh,...);
> pam_authenticate(pamh, ...);
> pam_acct_mgmt(pamh, 
> pam_setcred(pamh, PAM_ESTABLISH_CRED)
> ...
> pam_setcred(pamh, PAM_DELETE_CRED);
> pam_end(pamh);

Clearly -- no disagreement there. I'm asking why create the user-owned
ccache in pam_authenticate() (which is what you said Sun's pam_krb5
does).

> >In fact, it seems clear to me that no user-owned ccache should be
> >created outside pam_krb5:pam_sm_setcred().
> 
> In Solaris it isn't.

You seem to be mis-interpreting the man page bit you quoted --
pam_krb5:pam_sm_authenticate() should not create the file ccache (an
internal memory ccache is needed though to communicate the creds to the
pam_krb5:pam_sm_setcred() function (via pam data)).

> >> I'm not sure the call to pam_setcred with PAM_REINITIALIZE_CREDS is
> >> actually required in session.c.  Why was it put there ?  Is there a
> >> particular pam module that causes a problem ?
> >
> >See above about apps that don't fork() a process that runs as the
> >PAM_USER.
> 
> I don't believe that is well defined for Kerberos and it isn't how
> pam_krb5 in Solaris is designed to work.

Sure it is. An app can use multiple ccaches and creds no problem. As
long as it keeps them straight.

> --
> Darren J Moffat


Nico
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