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

Simo Sorce simo at redhat.com
Wed Dec 16 14:31:40 UTC 2020

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 :-(


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
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Simo Sorce
RHEL Crypto Team
Red Hat, Inc

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