Re: [Samba] cifs-utils: regression in (mulituser?) mounting 'CIFS VFS: Send error in SessSetup = -126'

[Jeff Layton <jlayton-eUNUBHrolfbYtjvyW6yDsg@public.gmane.org>]

On Fri, 2017-02-10 at 14:14 -0500, Simo Sorce wrote:
> On Fri, 2017-02-10 at 13:30 -0500, Jeff Layton wrote:
> > On Fri, 2017-02-10 at 12:39 -0500, Jeff Layton wrote:
> > > On Fri, 2017-02-10 at 11:15 -0600, Chad William Seys wrote:
> > > > Hi Jeff,
> > > > 
> > > > > So we have a default credcache for the user for whom we are
> > > > > operating
> > > > > as, but we can't get the default principal name from it. My
> > > > > guess is
> > > > > that it's not finding the
> > > > 
> > > > This mount is run by root UID=0 and seems to be find that
> > > > credential 
> > > > cache without problem (earlier in the logs).  The problem seems
> > > > to come 
> > > > in when it tries to find the cache for user with UID=1494 .
> > > > 
> > > > I'm wondering if the scan of /tmp was helpful for finding caches
> > > > of 
> > > > non-same users.
> > > > 
> > > > (I'm a little surprised that there is any attempt to find
> > > > credentials of 
> > > > the non-root user at mount time - what happens if the non-root
> > > > user 
> > > > hasn't kinit-ed yet?  And yet that case worked in the past...)
> > > > 
> > > 
> > > I'm more interested in what the trailing info in your credcache
> > > name is.
> > > In your log output for the working case, there are:
> > > 
> > > /tmp/krb5cc_0
> > > /tmp/krb5cc_1494_sM11PG
> > > 
> > > So first one doesn't have that _XXXXXX trailing bit. Maybe
> > > cifs.upcall
> > > is not guessing that piece of the filename correctly?
> > > 
> > 
> > (cc'ing Nalin, Simo and the linux-cifs ml)
> > 
> > Yeah, it seems pretty likely that that is the problem. My guess is
> > that
> > the extra stuff on the ccname is coming from pam_krb5, which seems to
> > want to create a credcache that is session-specific.
> > 
> > You could play with setting a different ccname_template for pam_krb5
> > that doesn't have the trailing stuff at the end, but it looks like it
> > won't clean them up on logout if you do that. Caveat emptor.
> > 
> > I'm not sure what the right solution is there. For Simo and Nalin:
> > 
> > The upshot here is that we did a big clean up of the cifs-utils code
> > recently, to get it out of the business of scanning /tmp for
> > credcaches.
> > That allows us to have better compatibility with other credcache
> > types
> > (keyring or whatever), and it was always rather nasty anyway.
> > 
> > pam_krb5 wants to make session-specific credcaches however, and
> > cifs.upcall can't easily guess them. cifs.upcall is called from the
> > kernel, so it can't look in the environment or anything to find the
> > credcache.
> > 
> > What's the right approach to fix this? Are we stuck with scanning
> > /tmp
> > forever?
> 
> I think the right approach in the long run will be the KCM we are
> building in SSSD and planning to make the default cache type for F26.
> 
> For file ccaches you are out of luck, even scanning /tmp can fail as
> users can decide to put them elsewhere.
> 
> If a scan need to be made I would rather stat the files and look which
> one belong to the user that start with "/tmp/krb" rather than trying to
> guess the file name. Keep in mind predictable file names in /tmp are
> really not an option so pam_krb5 is doing the right thing in using a
> randomized file name given it runs as root.
> 

Well, that's what we used to do -- do a readdir in /tmp, start looking
for files that might be credcaches. Of course that meant we have to
carry around a bunch of cruft for handling FILE: credcaches that doesn't
really adapt well to DIR: or KEYRING: ones.

I guess I have a philosophical question: how is an application (like
cifs.upcall), which is not descended from the user's session expected to
find a user's credcache? Is that use-case just not really accounted for
buy the krb5 libs?

One thing we could do (though I really don't like it), is to take the
pid value passed in the upcall and scrape that task's
/proc/<pid>/environ file for KRB5CCNAME=. That really is pretty nasty
though.

-- 
Jeff Layton <jlayton-eUNUBHrolfbYtjvyW6yDsg@public.gmane.org>