Re: Fedora 32 rpc.gssd misbehavior

Chuck Lever
2020-07-30T18:01:22Z

> 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.

--
Chuck Lever