Re: Fedora 32 rpc.gssd misbehavior

[Chuck Lever <chuck.lever@oracle.com>]

> On Jul 30, 2020, at 3:10 PM, Simo Sorce <simo@redhat.com> wrote:
> 
> On Thu, 2020-07-30 at 13:59 -0400, Chuck Lever wrote:
>>> On Jul 30, 2020, at 1:08 PM, Robbie Harwood <rharwood@redhat.com> wrote:
>>> 
>>> Simo Sorce <simo@redhat.com> writes:
>>> 
>>>> On Wed, 2020-07-29 at 14:27 -0400, Chuck Lever wrote:
>>>>>> On Jul 29, 2020, at 1:19 PM, Chuck Lever <chuck.lever@oracle.com> wrote:
>>>>>> 
>>>>>> Hi!
>>>>>> 
>>>>>> I recently updated my test systems from EL7 to Fedora 32, and
>>>>>> NFSv4.0 with Kerberos has stopped working.
>>>>>> 
>>>>>> I mount with "klimt.ib" as before. The client workload stops
>>>>>> dead when the server tries to perform its first CB_RECALL.
>>>>>> 
>>>>>> I added some client instrumentation:
>>>>>> 
>>>>>> kernel: NFSv4: Callback principal (nfs@klimt.ib.1015granger.net) does not match acceptor (nfs@klimt.ib).
>>>>>> kernel: NFS: NFSv4 callback contains invalid cred
>>>>>> 
>>>>>> I boosted gssd verbosity, and it says:
>>>>>> 
>>>>>> rpc.gssd[986]: doing downcall: lifetime_rec=72226 acceptor=nfs@klimt.ib
>>>>>> 
>>>>>> But it knows the full hostname for the server:
>>>>>> 
>>>>>> rpc.gssd[986]: Full hostname for 'klimt.ib' is 'klimt.ib.1015granger.net'
>>>>>> 
>>>>>> 
>>>>>> The acceptor appears to come from the Kerberos library. Shouldn't
>>>>>> it be canonicalized? If so, should the Kerberos library do it, or
>>>>>> should gssd? Since this behavior appeared after an upgrade, I
>>>>>> suspect a Kerberos library regression. But it could be config-
>>>>>> related, since both systems were re-imaged from the ground up.
>>>>>> 
>>>>>> Also noticing some other problems on the server (missing hostname
>>>>>> strings in debug messages, sssd_kcm infinite loops, and gssd
>>>>>> sending garbage to the client after the NULL request that
>>>>>> establishes the callback context).
>>>>>> 
>>>>>> But let's look at the client acceptor problem first.
>>>>> 
>>>>> I believe I found the problem.
>>>>> 
>>>>> 8bffe8c5ec1a ("gssd: add /etc/nfs.conf support") added a number of gssd config
>>>>> options to /etc/nfs.conf, including "avoid-dns". The default setting of avoid-
>>>>> dns is 1. When I set this option on my client system explicitly to 0, NFSv4.0
>>>>> with Kerberos works again.
>>>>> 
>>>>> Is there a reason the default setting is 1?
>>>>> 
>>>> 
>>>> Now that you mention DNS, this may be an interaction between a new
>>>> default in Fedora 32 and how your environment is setup re DNS.
>>>> 
>>>> In F32 we changed the option dns_canonicalize_hostname from 'true' to
>>>> 'fallback'.
>>>> This is a transitional state to eventually move it to 'false' at some
>>>> point in the future.
>>>> 
>>>> What it changes in practice is that it will first try the name passed
>>>> in *as is* and only as a fallback try a CNAME if the name passed is not
>>>> resolved as an A name. If you have principals in the KDC for both
>>>> names, but you do not have keys in the keytab for both, you can have
>>>> transitional issues.
>>>> 
>>>> Additionally we discovered a bug that causes non qualified names to
>>>> fail resolution with the 'fallback' option.
>>>> If your name in the principal is really not qualified it will try to
>>>> qualify it anyway, so if your principal is literally nfs/foo@FOO
>>>> libgssapi may try to use nfs/foo.my.domdain@FOO, where "my.domain" is
>>>> what is defined in resolv.conf search path.
>>>> 
>>>> We are trying to address this regression.
>>>> 
>>>> So try to set dns_canonicalize_hostname to true to see if that may
>>>> influence your issue. If so, please let me know, as we still need to
>>>> address this where possible.
>>> 
>>> Also, please try setting `qualify_shortname = ""`.  (I did update the
>>> config file we ship with Fedora, but upstream's default turns that on.
>>> This is a temporary workaround while we merge something better
>>> upstream.)
>> 
>> For completeness, I tried:
>> 
>> avoid-dns = 1
>> dns_canonicalize_hostname = fallback
>> qualify_shortname = ""
>> 
>> which is the default configuration out of the shrink wrap.
>> 
>> The workload hangs as before, and the acceptor is unqualified:
>> 
>> rpc.gssd[985]: doing downcall: lifetime_rec=84046 acceptor=nfs@klimt.ib
>> 
>> 
>> The test is:
>> 
>> Configured domain name is "1015granger.net"
>> 
>> Fully-qualified client hostname is "manet.ib.granger.net"
>> 
>> Fully-qualified server hostname is "klimt.ib.granger.net"
>> 
>> mount command is "mount -o vers=4.0,sec=sys klimt.ib:/export /mnt"
>> 
>> In this case, both systems have keytabs and service principals, so
>> the client automatically attempts to establish a GSS context for
>> lease management and callback operations. The failure occurs because
>> the server's principal is nfs@klimt.ib.1015granger.net but the
>> acceptor now matches the server hostname from the mount command line,
>> which is not always fully qualified.
> 
> Ok, TBH I personally consider the syntax you  are currently using as
> working by accident and that you should really sue the FQDN on the
> command line (I assume it works that way, right?), however I understand
> this is also technically a regression, that said I do not think we can
> really fix this case because your "shortname" is not short (it has a
> dot in it) so the heuristicts won't trigger to qualify it even when you
> set qualify_shortname="".
> 
> I have the feeling we'll break this case, and our answer will have to
> be "use the fqdn on the command line".

See previous e-mail. Using the shrink wrap default settings, which
includes qualify_shortname="", results in a hang on callback, as
originally observed.

Users will notice this and complain: klimt.ib works for the NFSv3
case and the NFSv4.1 case, and for NFSv4.0 when there is no keytab,
but NFSv4.0,sec=* with a keytab eventually hangs.


--
Chuck Lever