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Re: Login PAM interaction suspect



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
> usage.
>  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
pam_acct_mgmt().

> 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)

PAM_USER is also documented as
       PAM_USER
           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
           function.

>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.

Best Regards,
-- 
Nekral


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