On Tue, 8 Apr 2014 10:46:52 -0400
Dr Fields James Bruce <bfields@xxxxxxxxxxxx> wrote:
> On Tue, Apr 08, 2014 at 10:23:37AM -0400, Trond Myklebust wrote:
> >
> > On Apr 8, 2014, at 10:03, J. Bruce Fields <bfields@xxxxxxxxxxxx> wrote:
> >
> > > On Tue, Apr 08, 2014 at 09:49:03AM -0400, Jeff Layton wrote:
> > >> On Tue, 8 Apr 2014 08:35:01 -0400
> > >> "J. Bruce Fields" <bfields@xxxxxxxxxxxx> wrote:
> > >>
> > >>> On Tue, Apr 08, 2014 at 08:21:40AM -0400, Jeff Layton wrote:
> > >>>> I've recently been hunting down some problems with delegation handling
> > >>>> and have run across a problem with the client authenticates CB_COMPOUND
> > >>>> requests. I could use some advice on how best to fix it.
> > >>>>
> > >>>> Specifically, check_gss_callback_principal() tries to look up the
> > >>>> callback client and then tries to compare the ticket in it against the
> > >>>> clp->cl_hostname:
> > >>>>
> > >>>> /* Expect a GSS_C_NT_HOSTBASED_NAME like "nfs@serverhostname" */
> > >>>>
> > >>>> if (memcmp(p, "nfs@", 4) != 0)
> > >>>> return 0;
> > >>>> p += 4;
> > >>>> if (strcmp(p, clp->cl_hostname) != 0)
> > >>>> return 0;
> > >>>> return 1;
> > >>>>
> > >>>> The problem is that there is no guarantee that those hostnames will be
> > >>>> the same. If, for instance, I mount "foo:/" and the SPN is
> > >>>> "nfs/foo.bar.baz" that strcmp will return true, and the CB_COMPOUND
> > >>>> request will get tossed out [1]. Ditto if I happen to mount a CNAME of the
> > >>>> server.
> > >>>
> > >>> It sounds like a bug to me that the mount is succeeding without the name
> > >>> matching.
> > >>>
> > >>> The security provided by krb5 is much weaker if we don't check that the
> > >>> name provided on the commandline matches what the server authenticates
> > >>> as.
> > >>>
> > >>
> > >> The logic in gssd for this is pretty awful.
> > >>
> > >> It will basically trust DNS if there is no '.' in the hostname that was
> > >> used at mount time. That'll make it take the address and
> > >> reverse-resolve it.
> > >
> > > Argh, OK, I guess this is the compromise Simo made in "Avoid DNS reverse
> > > resolution for server names (take 3)".
> > >
> > >> We could add yet another band-aid and make it so that DNS is never
> > >> trusted. I'll note that for cifs, we took that route. You have to mount
> > >> the canonical name of the server in order to use krb5.
> > >
> > > I wish we could do that, but I suppose it's too harsh to break
> > > already-working fstabs. Maybe we could phase it in somehow.
> > >
> > >>>> Now that we try to use krb5 on the callback channel even when sec=sys
> > >>>> is specified, this is very problematic.
> > >>>
> > >>> And similarly I think the attempt to opportunistically use krb5 for
> > >>> state management should fail and fall back on auth_sys if the server's
> > >>> name doesn't match.
> > >>>
> >
> > This suggestion makes no sense to me at all. How does it help to fall back to using weak security when the strong security checks fail?
>
> It'd fix this particular problem.
>
> But, I don't know, I'm frankly confused about our security design for
> the NFSv4 state.
>
> When we insist on krb5 (and checked the server name correctly), and
> failed without it, then I feel like I understand what we're doing. Once
> we start trying it and then falling back (as I understand happens for
> the krb5 state in the auth_sys case) I get confused.
>
> > >> Like Trond pointed out, the problem is that gssd doesn't give us that
> > >> info currently. We could change it to do that of course, but that
> > >> basically means revving the downcall.
> > >
> > > It might be easier to rev the upcall so that the kernel could ask gssd
> > > to do strict checking? Since it's just a bunch of name=value pairs it
> > > shouldn't be a huge pain to revise.
> >
> > So what would trigger the kernel to ask for strict checking? Do we add a mount option that says “fail if the server doesn’t authenticate itself”? That would be hard to combine with security negotiation, since it only makes sense for RPCSEC_GSS authentication.
>
> I was thinking about only doing it in the state-establishment case.
> (Since we won't know how to authenticate the callbacks in that case.)
>
> But that would screw up krb5 mounts, I guess, never mind.
>
> Using a fqdn implicitly requests strict checking so a mount option would
> seem redundant.
>
So I guess we have two options to fix this:
1) Change gssd to require the canonical fqdn and not rely on name
resolution. Unfortunately, I think the MIT krb5 libs will still
canonicalize the hostnames by default, so this might not actually fix
anything. See:
http://web.mit.edu/kerberos/krb5-devel/doc/admin/princ_dns.html
...or...
2) Loosen or somehow fix the check in check_gss_callback_principal().
One possibility might be to do a dns_resolver upcall for the host
portion of the SPN, and then compare the address with the server's
address. Ugly, but since we already trust DNS implicitly I guess it's
no less secure...
--
Jeff Layton <jlayton@xxxxxxxxxx>
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