[SSSD] sssdpac_verify() treats krb5_pac_verify() error as fatal - mspac_verify() does not
Thomas Sondergaard
ts at medical-insight.com
Mon Aug 4 14:10:04 UTC 2014
On 04-08-2014 15:38, Jakub Hrozek wrote:
> On Mon, Aug 04, 2014 at 03:10:17PM +0200, Thomas Sondergaard wrote:
>> On 04-08-2014 13:40, Jakub Hrozek wrote:
>>> On Mon, Aug 04, 2014 at 02:24:47PM +0300, Alexander Bokovoy wrote:
>>>> On Sun, 03 Aug 2014, Thomas Sondergaard wrote:
>>>>> Hello,
>>>>>
>>>>> I've run into a GSSAPI authentication problem that has caused me a little
>>>>> time to diagnose, but turns out to only occur with sssd_pac_plugin.so
>>>>> present. I am not an AD, Kerberos or sssd expert, so let me just present
>>>>> the information I have collected.
>>>>>
>>>>> Our code, which uses GSSAPI, fails with the following error on a Fedora 20
>>>>> box with sssd-client (sssd_pac_plugin.so) installed with the following
>>>>> error codes:
>>>>>
>>>>> major: 851968='Unspecified GSS failure. Minor code may provide more
>>>>> information', minor: 22='Invalid argument'
>>>>>
>>>>> KRB5_TRACE does not give any useful indication for what goes wrong.
>>>>>
>>>>> With sssd-client (sssd_pac_plugin.so) removed the GSSAPI authentication
>>>>> works. With KRB5_TRACE I get the following warning:
>>>>>
>>>>> [23801] 1407018816.872001: PAC checksum verification failed:
>>>>> -1765328196/Bad encryption type
>>>>>
>>>>> Looking at the code, I find that without sssd_pac_plugin.so installed the
>>>>> following verification function is used:
>>>>>
>>>>> static krb5_error_code
>>>>> mspac_verify(krb5_context kcontext,
>>>>> krb5_authdata_context context,
>>>>> void *plugin_context,
>>>>> void *request_context,
>>>>> const krb5_auth_context *auth_context,
>>>>> const krb5_keyblock *key,
>>>>> const krb5_ap_req *req)
>>>>> {
>>>>> krb5_error_code code;
>>>>> struct mspac_context *pacctx = (struct mspac_context *)request_context;
>>>>>
>>>>> if (pacctx->pac == NULL)
>>>>> return EINVAL;
>>>>>
>>>>> code = krb5_pac_verify(kcontext, pacctx->pac,
>>>>> req->ticket->enc_part2->times.authtime,
>>>>> req->ticket->enc_part2->client, key, NULL);
>>>>> if (code != 0)
>>>>> TRACE_MSPAC_VERIFY_FAIL(kcontext, code);
>>>>>
>>>>> /*
>>>>> * If the above verification failed, don't fail the whole
>>>>> authentication,
>>>>> * just don't mark the PAC as verified. A checksum mismatch can occur
>>>>> if
>>>>> * the PAC was copied from a cross-realm TGT by an ignorant KDC, and
>>>>> Apple
>>>>> * Mac OS X Server Open Directory (as of 10.6) generates PACs with no
>>>>> * server checksum at all.
>>>>> */
>>>>> return 0;
>>>>> }
>>>>>
>>>>>
>>>>> This incarnation of the mspac_verify() function is from
>>>>>
>>>>> commit 76ebe5d07c1002b674eb1c4e3ab35f6001eec91c
>>>>> Author: Greg Hudson <ghudson at mit.edu>
>>>>> Date: Wed Feb 16 23:34:37 2011 +0000
>>>>>
>>>>> Don't reject AP-REQs based on PACs
>>>>>
>>>>> Experience has shown that it was a mistake to fail AP-REQ verification
>>>>> based on failure to verify the signature of PAC authdata contained in
>>>>> the ticket. We've had two rounds of interoperability issues with the
>>>>> hmac-md5 checksum code, an interoperability issue OSX generating
>>>>> unsigned PACs, and another problem where PACs are copied by older KDCs
>>>>> from a cross-realm TGT into the service ticket. If a PAC signature
>>>>> cannot be verified, just don't mark it as verified and continue on
>>>>> with the AP exchange.
>>>>>
>>>>> ticket: 6870
>>>>> target_version: 1.9.1
>>>>> tags: pullup
>>>>>
>>>>> git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@24640
>>>>> dc483132-0cff-0310-8789-dd5450dbe970
>>>>>
>>>>>
>>>>>
>>>>> With sssd-client (sssd_pac_plugin.so) installed the function
>>>>> sssdpac_verify is used instead, which looks like this
>>>>> (sssd-client-1.11.6-1):
>>>>>
>>>>> static krb5_error_code sssdpac_verify(krb5_context kcontext,
>>>>> krb5_authdata_context context,
>>>>> void *plugin_context,
>>>>> void *request_context,
>>>>> const krb5_auth_context
>>>>> *auth_context,
>>>>> const krb5_keyblock *key,
>>>>> const krb5_ap_req *req)
>>>>> {
>>>>> krb5_error_code kerr;
>>>>> int ret;
>>>>> krb5_pac pac;
>>>>> struct sssd_context *sssdctx = (struct sssd_context *)request_context;
>>>>> struct sss_cli_req_data sss_data;
>>>>> int errnop;
>>>>>
>>>>> if (sssdctx == NULL || sssdctx->data.data == NULL) {
>>>>> return EINVAL;
>>>>> }
>>>>>
>>>>> kerr = krb5_pac_parse(kcontext, sssdctx->data.data,
>>>>> sssdctx->data.length, &pac);
>>>>> if (kerr != 0) {
>>>>> return EINVAL;
>>>>> }
>>>>>
>>>>> kerr = krb5_pac_verify(kcontext, pac,
>>>>> req->ticket->enc_part2->times.authtime,
>>>>> req->ticket->enc_part2->client, key, NULL);
>>>>> if (kerr != 0) {
>>>>> return EINVAL;
>>>>> }
>>>>>
>>>>> sss_data.len = sssdctx->data.length;
>>>>> sss_data.data = sssdctx->data.data;
>>>>>
>>>>> ret = sss_pac_make_request(SSS_PAC_ADD_PAC_USER, &sss_data,
>>>>> NULL, NULL, &errnop);
>>>>> if (ret != 0) {
>>>>> /* Ignore the error */
>>>>> }
>>>>>
>>>>> return 0;
>>>>> }
>>>>>
>>>>> The krb5_pac_verify header documentation from krb5.h has this to say:
>>>>>
>>>>> * @note A checksum mismatch can occur if the PAC was copied from a
>>>>> cross-realm
>>>>> * TGT by an ignorant KDC; also Apple Mac OS X Server Open Directory (as of
>>>>> * 10.6) generates PACs with no server checksum at all. One should
>>>>> consider
>>>>> * not failing the whole authentication because of this reason, but,
>>>>> instead,
>>>>> * treating the ticket as if it did not contain a PAC or marking the PAC
>>>>> * information as non-verified.
>>>>>
>>>>> So, sssdpac_verify() treats krb5_pack_verify() errors as fatal, whereas
>>>>> krb5's mspac_verify() does not. This is all the information that I have
>>>>> gathered. It looks to me like sssd-client (sssd_pac_plugin.so) is not
>>>>> doing this right, but I'm looking forward to your comments.
>>>> I think you are on the right way here. We need to shortcut to return 0
>>>> there instead of EINVAL because unverified PAC wouldn't need to be added
>>>> to the cache.
>>>>
>>>> Jakub, can you change the if (kerr != 0) {
>>>> return EINVAL;
>>>> }
>>>>
>>>> to if (kerr != 0) {
>>>> return 0;
>>>> }
>>>>
>>>> here?
>>>>
>>>> Unfortunately, since tracing code is not available outside internals of
>>>> libkrb5, we cannot inject TRACE_MSPAC_VERIFY_FAIL(kcontext, kerr); here.
>>>
>>> Thanks for looking into the issue. Attached is a patch that changes the
>>> verification failure. Thomas, can you test the patch? If not, what
>>> version on what OS are you running? Perhaps I can prepare you a test
>>> build..
>>
>> Don't worry about a test build. I will test your patch.
>>
>> Regards,
>> Thomas
>
> Sorry about the delay, I was in a meeting until now. There is a test
> build available here:
> https://jhrozek.fedorapeople.org/sssd-test-builds/sssd-pac-verify/
Your patch solves the problem. I have not tested whether it introduces
other problems, but my code works now with the new sssd-client installed.
Thanks!
Thomas
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