[gssproxy] cifs-utils, Linux cifs kernel client and gssproxy

Thu Dec 17 21:49:33 UTC 2020

On Thu, 2020-12-17 at 15:22 -0600, Steve French wrote:
> A couple of more specific questions:
> 
> 1) Do you have a link to how the various user space tools that would
> often need to use kerberos (e.g. "smbclient" and "smbcacls") integrate
> with this so I could see some examples of how they tie into your
> proxy?

Tools will not need to change one bit.

For client applications the proxy simply allows access to user's
ccaches, or (and this is a feature) allows unattended use of user's
credentials either via user's keytabs or via constrained delegation
using server credentials.

The latter two features are implemented w/o needing any changes on most
client applications (some may need minor changes to the way they invoke
APIs to be less prescriptive, but the changes are generally very
minimal and straightforward).

For the latter two feature the proxy also offers privilege separation,
by handing to user processes an encrypted ccache, and intercepting
GSSAPI calls (via libgssapi mechglue, hence transparent to
applciations).

> 2) Does it use the kernel keyring to store credentials or rely on the
> traditional kerberos key cache?

The latter.

> Presumably there continues to be a drive to keep as many credentials
> as possible in the kernel for maximal security in this very
> challenging recent security landscape.

You can use the KEYRING ccacche type if you want, or you can use the
KCM daemon, or as I said above you could use the gss-proxy privsep
feature. IT really is up to distro defaults/admin preference/choice.

> 3) Besides Kerberos and NTLMSSP what other auth protocols do you
> support in gssproxy (e.g. PKU2U is one I see commonly in the list of
> SPNEGO OIDs during auth).

Currently GSS-Proxy handles only Krb5, but extending it to cover GSS-
NTLMSSP is trivial.
PKU2U would need to be implemented by the krb5 mechanism, but we've
never seen any real demand for that.

> There has been a push recently to move
> away from NTLMv2/NTLMSSP (which is often encapsulated in SPNEGO) to
> stronger 'peer to peer' protocols.   Macs IIRC have peer to peer
> Kerberos and presumably PKU2U (see
> https://tools.ietf.org/id/draft-zhu-pku2u-07.html) is reasonably
> common in Windows.    It would be useful if you already have support
> for PKU2U in your libraries or know how to tie them in so we would not
> have to rely on NTLMSSP/NTLMv2 for peer to peer (systems that are
> joined to a domain like Samba AD or Active Directory or AAD) and could
> improve security in non domain joined cases.

PKU2U is not supported in any library available, so that work would
need to be done there first.

Just to be clear, GSS-Proxy does not implement any mecahnism directly,
It is a mechglue layer used to "proxy" a mechanism so that execution
happens in another context, it still uses libkrb5/libgssapi internally,
so any support needs to be implemented there first.

> 4) Does gssproxy integrate in any way with Samba server? There would
> be strong interest in having easy ways to plug in additional security
> protocols transparently into the client (cifs.ko and user space tools
> like smbclient and smbcacls and Ronnie's libsmb3 etc.) and servers
> (Samba and ksmbd) - so e.g. if O_AUTH became useful for file sharing,

No, what you are looking for is for Samba to use libgssapi instead of
re-implementing most of it on it's own. GSS-Proxy is a GSSAPI mechanism
fundamentally.

> could gssproxy help transparently enable this on the client and server
> (since the SPNEGO flows, at least for SMB3 are fairly opaque and the
> client and server don't really care what auth is negotiated as long as
> the underlying libraries send the right list of auth protocols and
> negotiate the correct 'preferred' one that was requested on mount or
> in the /etc config files).

GSS-Proxy is 2 things:

- A GSSAPI daemon for the kernel so you do not need to implement GSS or
krb5 in there. In this sense it could definitely do that for
cifs.ko/cifsd, as *all* GSSAPI processing is deferred to the proxy, the
kernel only shuffles the blobs of data from kernel to userspace using a
gssapi-like interface implemented via RPC (XDR encoding).
If you look at knfsd you will see zero gssapi work in the kernel.

- A privilege separation daemon so you can allow an app to use GSSAPIwithout having access to "naked" keytabs, via interposing the krb5
mechanism. At the moment, SPNEGO is *entirely* handled in libgssapi,
as it is a meta-mechanism.
So for userspace applications GSS-Proxy (and libgssapi's mechglue
layer)  would need to be changed to intercept SPNEGO, and change the
list of mechanisms available, and then hope the application doesn't
make assumptions about those mechanisms ... in practice it will always
be easier to simply provide a gssapi mechanism (you can provide them as
shared objects and load them into GSSAPI at runtime via
/etc/gss/mech.d/*.conf files, Which is how gssproxy and gss-ntlmssp are
loaded into MIT's libgssapi).

Note that for the kernel case, given the protocol is used directly even
SPNEGO can be easily proxied, so the capabalities for user-space and
kernel are slightly different at this time.

HTH,
Simo.

> On Thu, Dec 17, 2020 at 7:39 AM Simo Sorce <simo at redhat.com> wrote:
> > Hi Steve,
> > 
> > GSSAPI and krb5 as implemented in system krb5 libraries exists from
> > longer than Samba has implemented those capabilities, I do not think it
> > make sense to reason along those lines.
> > 
> > GSS-Proxy has been built with a protocol to talk from the kernel that
> > resolved a number of issues for knfsd (eg big packet sizes when a MS-
> > PAC is present).
> > 
> > And Samba uses internally exactly the same krb5 mechanism as it defers
> > to the kerberos libraries as well.
> > 
> > Additionally GSS-Proxy can be very easily extended to also do NTLMSSP
> > using the same interface as I have built the gssntlmssp long ago from
> > the MS spec, and is probably the most correct NTLMSSP implementation
> > you can find around.
> > 
> > Gssntlmssp also has a Winbind backend so you get automaticaly access to
> > whatever cached credentials Winbindd has for users as a bonus (although
> > the integration can be improved there), yet you *can* use it w/o
> > Winbindd just fine providing a credential file (smbpasswd format
> > compatible).
> > 
> > GSS-Proxy is already integrated in distributions because it is used by
> > knfsd, and can be as easily used by cifsd, and you *should* really use
> > it there, so we can have a single, consistent, maintained, mechanism
> > for server side GSS authentication, and not have to repeat and reinvent
> > kernel to userspace mechanisms.
> > 
> > And if you add it for cifsd you have yet another reason to do it for
> > cifs.ko as well.
> > 
> > Finally the GSS-Proxy mechanism is namespace compatible, so you also
> > get the ability to define different auth daemons per different
> > containers, no need to invent new mechanisms for that or change yet
> > again protocols/userspace to obtain container capabilities.
> > 
> > For the client we'll need to add some XDR parsing functions in kernel,
> > they were omitted from my original patches because there was no client
> > side kernel consumer back then, but it i an easy, mechanical change.
> > 
> > HTH,
> > Simo.
> > 
> > On Wed, 2020-12-16 at 16:43 -0600, Steve French wrote:
> > > generally I would feel more comfortable using something (library or
> > > utility) in Samba (if needed) for additional SPNEGO support if
> > > something is missing (in what the kernel drivers are doing to
> > > encapsulate Active Directory or Samba AD krb5 tickets in SPNEGO) as
> > > Samba is better maintained/tested etc. than most components.  Is there
> > > something in Samba that could be used here instead of having a
> > > dependency on another project - Samba has been doing Kerberos/SPNEGO
> > > longer than most ...?   There are probably others (jra, Metze etc.)
> > > that have would know more about gssproxy vs. Samba equivalents though
> > > and would defer to them ...
> > > 
> > > On Wed, Dec 16, 2020 at 8:33 AM Simo Sorce <simo at redhat.com> wrote:
> > > > Hi Michael,
> > > > as you say the best course of action would be for cifs.ko to move to
> > > > use the RPC interface we defined for knfsd (with any extensions that
> > > > may  be needed), and we had discussions in the past with cifs upstream
> > > > developers about it. But nothing really materialized.
> > > > 
> > > > If something is needed in the short term, I thjink the quickest course
> > > > of action is indeed to change the userspace helper to use gssapi
> > > > function calls, so that they can be intercepted like we do for rpc.gssd
> > > > (nfs client's userspace helper).
> > > > 
> > > > Unfortunately I do not have the cycles to work on that myself at this
> > > > time :-(
> > > > 
> > > > HTH,
> > > > Simo.
> > > > 
> > > > On Wed, 2020-12-16 at 10:01 +0000, Weiser, Michael wrote:
> > > > > Hello,
> > > > > 
> > > > > I have a use-case for authentication of Linux cifs client mounts without the user being present (e.g. from batch jobs) using gssproxy's impersonation feature with Kerberos Constrained Delegation similar to how it can be done for NFS[1].
> > > > > 
> > > > > My understanding is that currently neither the Linux cifs kernel client nor cifs-utils userland tools support acquiring credentials using gssproxy. The former uses a custom upcall interface to talk to cifs.spnego from cifs-utils. The latter then goes looking for Kerberos ticket caches using libkrb5 functions, not GSSAPI, which prevents gssproxy from interacting with it.[2]
> > > > > 
> > > > > From what I understand, the preferred method would be to switch the Linux kernel client upcall to the RPC protocol defined by gssproxy[3] (as has been done for the Linux kernel NFS server already replacing rpc.svcgssd[4]). The kernel could then, at least optionally, talk to gssproxy directly to try and obtain credentials.
> > > > > 
> > > > > Failing that, cifs-utils' cifs.spnego could be switched to GSSAPI so that gssproxy's interposer plugin could intercept GSSAPI calls and provide them with the required credentials (similar to the NFS client rpc.gssd[5]).
> > > > > 
> > > > > Assuming my understanding is correct so far:
> > > > > 
> > > > > Is anyone doing any work on this and could use some help (testing, coding)?
> > > > > What would be expected complexity and possible roadblocks when trying to make a start at implementing this?
> > > > > Or is the idea moot due to some constraint or recent development I'm not aware of?
> > > > > 
> > > > > I have found a recent discussion of the topic on linux-cifs[6] which provided no definite answer though.
> > > > > 
> > > > > As a crude attempt at an explicit userspace workaround I tried but failed to trick smbclient into initialising a ticket cache using gssproxy for cifs.spnego to find later on.
> > > > > Is this something that could be implemented without too much redundant effort (or should already work, perhaps using a different tool)?
> > > > > 
> > > > > [1] https://github.com/gssapi/gssproxy/blob/main/docs/NFS.md#user-impersonation-via-constrained-delegation
> > > > > [2] https://pagure.io/gssproxy/issue/56
> > > > > [3] https://github.com/gssapi/gssproxy/blob/main/docs/ProtocolDocumentation.md
> > > > > [4] https://github.com/gssapi/gssproxy/blob/main/docs/NFS.md#nfs-server
> > > > > [5] https://github.com/gssapi/gssproxy/blob/main/docs/NFS.md#nfs-client
> > > > > [6] https://www.spinics.net/lists/linux-cifs/msg20182.html
> > > > > --
> > > > > Thanks,
> > > > > Michael
> > > > > _______________________________________________
> > > > > gss-proxy mailing list -- gss-proxy at lists.fedorahosted.org
> > > > > To unsubscribe send an email to gss-proxy-leave at lists.fedorahosted.org
> > > > > Fedora Code of Conduct: https://docs.fedoraproject.org/en-US/project/code-of-conduct/
> > > > > List Guidelines: https://fedoraproject.org/wiki/Mailing_list_guidelines
> > > > > List Archives: https://lists.fedorahosted.org/archives/list/gss-proxy@lists.fedorahosted.org
> > > > 
> > > > --
> > > > Simo Sorce
> > > > RHEL Crypto Team
> > > > Red Hat, Inc
> > > > 
> > > > 
> > > > 
> > > > 
> > 
> > --
> > Simo Sorce
> > RHEL Crypto Team
> > Red Hat, Inc
> > 
> > 
> > 
> > 
> 
> 

-- 
Simo Sorce
RHEL Crypto Team
Red Hat, Inc