Re: Login PAM interaction suspect

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From: Nicolas François <nekral lists gmail com> To: Pluggable Authentication Modules <pam-list redhat com> Subject: Re: Login PAM interaction suspect Date: Tue, 15 Nov 2011 22:17:52 +0100 Hello,

On Tue, Nov 15, 2011 at 11:57:05AM -0500, David Mitton wrote:

 I am designing a PAM module to serve as a backup authentication
mechanism for a device when it has lost network connectivity to it's
LDAP server.  There is no local password containing file on the
system. The credential used has OTP-ish properties and encodes a
privilege level as well.
 Upon successful authentication, I planned on the module fabricating
a one line passwd file that would be timestamped and deleted past
 A helper nsslib function will deal with fronting the "user"
information to login and the system.

 I was wondering which api service to put the logic the builds the
authenticated user information.
 I was thinking that pam_setcred() would be appropriate.

I have no strong opinion, but this could be pam_authenticate() or

2) The bigger picture:  notice that getpwnam() and initgroups() has
been called before any of this.
   So if pam_setcred() or pam_open_session() have any effect on
PAM_USER, the user database, or the information returned by nss
functions they will not be noticed by login.

Note also in the pam_setcred manpage:

											   On a Linux system the user's
	   UID and GID's are credentials too. However, it has been decided that
	   these properties (along with the default supplementary groups of which
	   the user is a member) are credentials that should be set directly by
	   the application and not by PAM. Such credentials should be established,
	   by the application, prior to a call to this function. For example,
	   initgroups(2) (or equivalent) should have been performed.

(This is usually not completely implemented because pam_open_session may
need privileges)

I have no intent of having my pam modules change the attributes of the running process. They are communicating with the database module that will provide login (or any other PAM authenticating process) with the information about the user that just authenticated. It's up to the authentication application (login) to change what it needs based on that info. I don't wish to commit that information until the point that I know that the session will go forward.

PAM_USER is also documented as
		   The username of the entity under whose identity service will be
		   given. That is, following authentication, PAM_USER identifies the
		   local entity that gets to use the service. Note, this value can be
		   mapped from something (eg., "anonymous") to something else (eg.
		   "guest119") by any module in the PAM stack. As such an application
		   should consult the value of PAM_USER after each call to a PAM

From an application point of view it would be much easier if the list of
PAM stack functions which can change PAM_USER were much more limited
(pam_authenticate, pam_acct_mgmt, (pam_chauthtok?))

I would exclude pam_setcred, pam_open_session and pam_close_session from
the list because I would not know in the application how to react to such
change of PAM_USER.

I would exclude other PAM functions because I do not see any use case to
change PAM_USER in these other functions.

This was discussed in some other forum (which I lost my breadcrumbs to).
It's moot to me, as I currently don't plan on changing that value.
But login should not assume that getpwnam(PAM_USER) will work until committed with a setcred.

In my case, if "alice" logs in, there is no "alice" record for getpwnam() to find until my PAM module creates one and passes it to my nsslib module.

It seems that the work around is to do it in pam_acct_mgmt, but I think it stinks.


Best Regards,

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