[krbdev.mit.edu #8885] PKINIT calls responder twice

[Greg Hudson via RT]

Mon Mar 09 15:33:03 2020: Request 8885 was acted upon.
 Transaction: Ticket created by ghudson at mit.edu
       Queue: krb5
     Subject: PKINIT calls responder twice
       Owner: Nobody
  Requestors: ghudson at mit.edu
      Status: new
 Ticket <URL: https://krbdev.mit.edu/rt/Ticket/Display.html?id=8885 >


During a PKINIT AS-REQ, there are two rounds of client padata processing, one
to generate the request and one to validate the KDC response and compute the
reply key.

During the first round, pkinit_client_prep_questions() calls
pkinit_identity_initialize(), which calls crypto_load_certs() with
defer_id_prompts=TRUE. This accumulates a list of identities in
id_cryptoctx->deferred_ids. pkinit_client_prep_questions () then generates a
PKINIT question containing the list of identities to be prompted for.

During the second round, pkinit_client_prep_questions() is again called. The
pkinit_identity_initialize() call is skipped (because
reqctx->identity_initialized is true), but the list of deferred identities is
still present, so a question is again generated. PKINIT does not need access to
client certificates while processing the KDC response, so the question is
pointless and creates a bad user experience.

The simplest way to to suppress the second question is probably to check that
the padata type is KRB5_PADATA_PK_AS_REQ.

The double-responder behavior is already visible in t_pkinit.py tests, such as
the "FILE identity, password on key (responder)". We simply need to count the
number of "OK:" messages in the output to see how many responder calls are
made.

Reported by Russ Allbery.