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Description of problem:
In case of Smart Card authentication, the krb5_child of sssd runs as root in order to be able to access the pcscd socket and relies on setting the KRB5CCNAME environment variable to access the ccache on behalf of the user.

However, with KCM, root cannot access another user's ccache, see e.g. this explanation by MIT krb5 upstream:
https://github.com/krb5/krb5/pull/557#issuecomment-254834623

Therefore we need to obtain the credentials as a user who can talk to pcscd (typically root) but then drop the privileges to the user who is logging and and save the credentials to the ccache as that user.

Version-Release number of selected component (if applicable):
up to sssd-2.0-24

How reproducible:
always

Steps to Reproduce:
1. login to an IPA client with a smart card
2. klist
3.

Actual results:
credential cache KCM:$uid not found

Expected results:
a valid credential cache

Additional info:
see e.g. https://bugzilla.redhat.com/show_bug.cgi?id=1441764#c8 or a thread on freeipa-users titled "smartcard auth + kerberos ticket?" from Nov-15 2018.

Upstream ticket:
https://pagure.io/SSSD/sssd/issue/3903

* master: e49e9f727e4960c8a0a2ed50488dac6e51ddf284

Hi Scott, can you please qa_ack this bug? You found the original issue in https://bugzilla.redhat.com/show_bug.cgi?id=1441764#c8 so I guess you know how to reproduce.

Verified.

Version ::

sssd-2.0.0-43.el8.x86_64
sssd-kcm-2.0.0-43.el8.x86_64

Results ::


# p11tool --provider /usr/lib64/opensc-pkcs11.so --export 'pkcs11:model=PKCS%2315;manufacturer=Aventra%20Ltd.;serial=7055056447986431;token=singleuser1-01%20%28MyEID%29;id=%01;object=cert.01;type=cert' --outfile /tmp/card.crt

# ipa certmap-match /tmp/card.crt
---------------
2 users matched
---------------
  Domain: EXAMPLE.COM
  User logins: singleuser1

  Domain: ad.test
  User logins: adcertsingleuser1
----------------------------
Number of entries returned 2
----------------------------

# su - singleuser1 -c "kdestroy -A"

# su - singleuser1 -c "su - singleuser1 -c klist"
PIN for singleuser1-01 (MyEID)
Ticket cache: KCM:614800473:39355
Default principal: singleuser1

Valid starting       Expires              Service principal
02/11/2019 14:50:28  02/12/2019 14:50:21  krbtgt/EXAMPLE.COM