Fix handling of FILE ccache removed creds #1246
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commit a9bd3c6 ("Fix racy file ccache corruption in cred_delete()") implemented krb5_cc_remove_cred() for "FILE" ccaches by overwriting the removed credential endtime value with zero (Unix Epoch). However, it did not modify fcc_get_next() to filter out these deleted entries. As a result, invalid credentials can be returned from the FILE ccache where endtime < starttime. RFC4120 requires endtime >= starttime for all tickets. MIT Kerberos since d3b39a8bac6206b5ea78b0bf6a2958c1df0b0dd5 ("Implement krb5_cc_remove_cred for remaining types") modifies a removed cred by setting endtime = 0 authtime = -1 and then filters out removed creds from the fcc_next_cred() results. In 2013 Heimdal broke interop with MIT processes that share the FILE ccache by implementing remove by setting "endtime = 0" and now MIT has broken interop with the Heimdal implementation of fcc_remove_cred() by checking for both "endtime = 0" and "authtime = -1". This change filters results from fcc_get_next() when the "endtime == 0" which is acceptable because a KDC is not permitted to return a ticket with an endtime == 0.
MIT commit d3b39a8bac6206b5ea78b0bf6a2958c1df0b0dd5 implemented krb5_cc_remove_cred() for FILE ccaches by setting endtime to zero and authtime to minus one and then filtering out those credentials from get_next(). This change sets "authtime = -1" for the removed cred to permit MIT krb5 to ignore removed credentials from a shared FILE ccache.
gssproxy stores its credentials to the ccache with endtime == 0. Therefore, it is not safe to skip ccache entries unless authtime is -1 as well.
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For compatibility with MIT both endtime and authtime must be modified in order for the cred to be ignored when iterating the ccache contents.
Heimdal fcc_next_cred() must ignore removed credentials when iterating and not pass them back to the application. Returning a cred with endtime < authtime violates the spirit of RFC4120 which requires that a KDC not return a cred with an endtime < the starttime or authtime.