gensec returns the wrong error to kerberos errors like Ticket Expired and clock skew issues

[Richard Sharpe]

On Sat, Nov 7, 2015 at 7:10 PM, Richard Sharpe
<realrichardsharpe at gmail.com> wrote:
> On Sat, Nov 7, 2015 at 12:32 AM, Stefan Metzmacher <metze at samba.org> wrote:
>> Am 06.11.2015 um 03:22 schrieb Richard Sharpe:
>>> On Wed, Nov 4, 2015 at 11:23 AM, Jeremy Allison <jra at samba.org> wrote:
>>>> On Wed, Nov 04, 2015 at 11:14:50AM -0800, Richard Sharpe wrote:
>>>>> On Wed, Nov 4, 2015 at 10:22 AM, Jeremy Allison <jra at samba.org> wrote:
>>>>>> On Wed, Nov 04, 2015 at 10:00:48AM -0800, Richard Sharpe wrote:
>>>>>>> Hi folks,
>>>>>>>
>>>>>>> A capture I have indicates that when a Windows server gets a
>>>>>>> KRB5KRB_AP_ERR_TKT_EXPIRED error it returns
>>>>>>> STATUS_MORE_PROCESSING_REQUIRED along with an SPNEGO negTokenTarg with
>>>>>>> the Kerberos error blob in it.
>>>>>>>
>>>>>>> Samba, and it looks like gensec, folds that down to LOGON_FAILED,
>>>>>>> which makes it very hard for admins to figure out what the real error
>>>>>>> is.
>>>>>>>
>>>>>>> Is there a bugzilla on this?
>>>>>>>
>>>>>>> If I get a chance I will try to provide a fix.
>>>>>>
>>>>>> I think that is intentional in order to prevent
>>>>>> username guessing attacks.
>>>>>
>>>>> That doesn't even pass the smell test. The KDC is responsible for
>>>>> preventing password guessing games.
>>>>
>>>> Sure, but smbd is also. Remember we have our own passdb
>>>> code which has to implement the same protections.
>>>>
>>>> However, I haven't looked at that bit of gensec recently
>>>> so you may be right here :-).
>>>
>>> It looks like perhaps in
>>> source3/librpc/crypto/gse.c:gse_get_server_auth_token in the default
>>> arm of the switch, if gse_min is KRB5KRB_AP_ERR_TKT_EXPIRED or
>>> KRB5KRB_AP_ERR_TKT_NYV we should return
>>> NT_STATIS_MORE_PROCESSING_REQUIRED.
>>>
>>> I will see if this produces a response more to the liking of Windows.
>>
>> Can you file a bug report and attach network captures against windows
>> and against
>> Samba?
>
> OK, I will likely file a bug, but it looks like this is a problem to
> do with the version of MIT Kerberos 5 you are using.
>
> Having done some debugging and a lot of code inspection, it looks like
> this is not something that Samba can get right or wrong. The problem
> is in gss_accept_sec_context in versions up to at least 1.10.3 and
> perhaps later, but I have not checked all versions. Also, I have not
> checked versions earlier than 1.10.3 in CentOS 6.6 so it might be a
> regression in that version.
>
> In version 1.10.3+ (from CentOS/RHEL 6.6 krb5-1.10.3-37) there is a
> path where it seems to forget to create an output token on an error
> where the AP request was decoded, however, 10.13.2 does not seem to
> have this bug.

OK, after much screwing around with SPEC files, I managed to build the
RPMs I needed with krb5-1.13.2 in them, installed them on my test
machine, set the time forward and backward, and Samba generates the
correct responses (that is, the same as Windows does.)

So this is not a Samba bug, but I will still file a bug in case
someone else searches for this issue as well.

-- 
Regards,
Richard Sharpe
(何以解憂？唯有杜康。--曹操)