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

Jacob Shivers jshivers at redhat.com
Fri Mar 5 21:29:27 UTC 2021


if nfs-client is to be used as well as cifs-client, then wouldn't a
different socket be required? Testing with cifs-client using the same
socket returns an error from gssproxy:

"cifs sets allow_any_uid with the same socket, selinux_context, and
program as nfs-client!".

Leveraging the built-in functionality of gssproxy is preferable as
that addresses use cases where the process was started manually and
not via systemd. Would it be reasonable to ever add gid filtering to
gssproxy? I ask as that would allow for a group of batch
processes/applications to have access where say applications could be
added to a fixed group name/gid but, different uids may be deployed
over time as different applications are used.

On Tue, Feb 23, 2021 at 2:54 PM Simo Sorce <simo at redhat.com> wrote:
>
> Nice work!
>
> Notice that GSS-Proxy can already perform access control, so you do not
> have to create additional sockets or the systemd script, but you can
> simply create service files in gssproxy that filter based on uid or
> even SELinux context if you like.
>
> There are additional filters based on the calling program too, but
> those are not to be used to enforce security measures as argv[0] is
> under the control of the user.
>
> HTH,
> Simo.
>
> On Tue, 2021-02-23 at 12:42 -0500, Jacob Shivers wrote:
> > I have tested the patches for cifs.upcall and can say that with some
> > additional modifications to gssproxy the end setup brings a degree of
> > feature parity to SMB clients that had been previously exclusive to
> > NFS clients.
> > Deployment does require some additional configuration, including the
> > creation of a drop-in file for gssproxy under /etc/gssproxy and for
> > the gssproxy service managed by systemd
> >
> >
> > ### KDC configuration
> >
> >  *** delegation ***
> >
> > Constrained Delegation (CD) for the SMB host to the Kerberized SMB server
> > Resource Based Constrained Delegation (RBCD) on the Kerberized SMB
> > server to determine which SMB clients can delegate
> >
> >
> > ### SMB client configuration
> >
> >  *** sssd ***
> >
> >  Configuration file modification.
> >
> > Disable using fully qualified domain names as impersonation does not
> > correctly handle fully qualified names at time of ticket acquisition.
> >
> > use_fully_qualified_names = False
> >
> >  *** gssproxy ***
> >
> >  Drop file creation
> >
> > Add a drop file for gssproxy to create the necessary socket and
> > corresponding settings.
> >
> > # cat /etc/gssproxy/99-cifs-client.conf with contents
> > [service/cifs]
> > mechs = krb5
> > socket = /var/lib/gssproxy/cifs.sock
> > cred_store = keytab:/etc/krb5.keytab
> > cred_usage = initiate
> > euid = 0
> > impersonate = yes
> > allow_any_uid = yes
> >
> >  Service drop-in file.
> >
> > Create a drop-in file to limit socket access for an unattended user, if desired.
> >
> > # cat /etc/systemd/system/gssproxy.service.d/90-cifs.conf
> > [Service]
> > # Limit cifs.sock socket file accessiblility to just the unattended user.
> > ExecStartPost=/bin/bash -c 'chmod 660 /var/lib/gssproxy/cifs.sock &&
> > setfacl -m u:chang:rw /var/lib/gssproxy/cifs.sock'
> >
> >
> >  *** cifs.upcall ***
> >
> >  cifs.spnego.conf modification
> >
> > Allow for gssprxy to be used and specify socket file
> >
> > # cat /etc/request-key.d/cifs.spnego.conf
> > create  cifs.spnego    * *  /usr/bin/env GSS_USE_PROXY=yes
> > GSSPROXY_SOCKET=/var/lib/gssproxy/cifs.sock /usr/sbin/cifs.upcall %k
> >
> >    include the '-t' flag to allow for accessing a Kerberized DFS
> > namespace using a domain based mount, i.e //example.net/dfs/share
> >
> > create  cifs.spnego    * *  /usr/bin/env GSS_USE_PROXY=yes
> > GSSPROXY_SOCKET=/var/lib/gssproxy/cifs.sock /usr/sbin/cifs.upcall -t
> > %k
> >
> >
> > The above does allow for unattended users to access a Kerberized SMB
> > share while limiting access to only a specific user:
> >
> > # mount //win2k16-dfs1.example.net/greendale/ /mnt -o
> > sec=krb5,username='TEST-BOX$@EXAMPLE.NET',multiuser
> >
> > # su - jeff
> > Last login: Fri Feb  5 12:14:42 EST 2021 on pts/0
> > [jeff at test-box ~]$ ll /mnt
> > ls: cannot access '/mnt': Permission denied
> > [jeff at test-box ~]$ logout
> >
> > # su - chang
> > Last login: Fri Feb  5 12:14:46 EST 2021 on pts/0
> > [chang at test-box ~]$ klist
> > klist: No credentials cache found (filename: /tmp/krb5cc_602001123)
> > [chang at test-box ~]$ ls -l /mnt
> > total 143
> >
> > drwxr-xr-x. 2 chang domain users      0 Oct  4  2018  DfsrPrivate
> > -rwxr-xr-x. 1 chang domain users      0 Sep 22 10:07  test_file
> > -rwxr-xr-x. 1 chang domain users      0 Dec 14 15:22  whoami
> > drwxr-xr-x. 2 chang domain users      0 Dec  7 12:54  winhome
> >
> > [chang at test-box ~]$ klist
> > klist: No credentials cache found (filename: /tmp/krb5cc_602001123)
> >
> >
> > Feb 23 12:22:44.857956 test-box.example.net su[1672]: (to jeff) root on pts/1
> > Feb 23 12:22:44.866263 test-box.example.net su[1672]:
> > pam_systemd(su-l:session): Cannot create session: Already running in a
> > session or user slice
> > Feb 23 12:22:44.867158 test-box.example.net su[1672]:
> > pam_unix(su-l:session): session opened for user jeff by root(uid=0)
> > Feb 23 12:22:46.253310 test-box.example.net cifs.upcall[1700]: key
> > description: cifs.spnego;0;0;39010000;ver=0x2;host=win2k16-dfs1.example.net;ip4=192.168.124.132;sec=krb5;uid=0x23e1cedc;creduid=0x23e1cedc;pid=0x6a3
> > Feb 23 12:22:46.253335 test-box.example.net cifs.upcall[1700]: ver=2
> > Feb 23 12:22:46.253338 test-box.example.net cifs.upcall[1700]:
> > host=win2k16-dfs1.example.net
> > Feb 23 12:22:46.253342 test-box.example.net cifs.upcall[1700]:
> > ip=192.168.124.132
> > Feb 23 12:22:46.253344 test-box.example.net cifs.upcall[1700]: sec=1
> > Feb 23 12:22:46.253348 test-box.example.net cifs.upcall[1700]: uid=602001116
> > Feb 23 12:22:46.253352 test-box.example.net cifs.upcall[1700]: creduid=602001116
> > Feb 23 12:22:46.253365 test-box.example.net cifs.upcall[1700]: pid=1699
> > Feb 23 12:22:46.253978 test-box.example.net cifs.upcall[1700]:
> > get_cachename_from_process_env: pathname=/proc/1699/environ
> > Feb 23 12:22:46.254995 test-box.example.net cifs.upcall[1700]:
> > get_existing_cc: default ccache is FILE:/tmp/krb5cc_602001116
> > Feb 23 12:22:46.255015 test-box.example.net cifs.upcall[1700]:
> > get_tgt_time: unable to get principal
> > Feb 23 12:22:46.255021 test-box.example.net cifs.upcall[1700]:
> > handle_krb5_mech: getting service ticket for win2k16-dfs1.example.net
> > Feb 23 12:22:46.255024 test-box.example.net cifs.upcall[1700]:
> > handle_krb5_mech: using GSS-API
> > Feb 23 12:22:46.259295 test-box.example.net cifs.upcall[1700]: GSS-API
> > error init_sec_context: Unspecified GSS failure.  Minor code may
> > provide more information
> > Feb 23 12:22:46.259306 test-box.example.net cifs.upcall[1700]: GSS-API
> > error init_sec_context: No Kerberos credentials available (default
> > cache: FILE:/tmp/krb5cc_602001116)
> > Feb 23 12:22:46.259311 test-box.example.net cifs.upcall[1700]:
> > handle_krb5_mech: failed to obtain service ticket via GSS (851968)
> > Feb 23 12:22:46.259314 test-box.example.net cifs.upcall[1700]: Unable
> > to obtain service ticket
> > Feb 23 12:22:46.259323 test-box.example.net cifs.upcall[1700]: Exit
> > status 851968
> > Feb 23 12:22:46.262827 test-box.example.net kernel: CIFS VFS:
> > \\win2k16-dfs1.example.net Send error in SessSetup = -126
> > Feb 23 12:22:47.398266 test-box.example.net su[1672]:
> > pam_unix(su-l:session): session closed for user jeff
> > Feb 23 12:22:49.159640 test-box.example.net su[1702]: (to chang) root on pts/1
> > Feb 23 12:22:49.173264 test-box.example.net su[1702]:
> > pam_systemd(su-l:session): Cannot create session: Already running in a
> > session or user slice
> > Feb 23 12:22:49.173967 test-box.example.net su[1702]:
> > pam_unix(su-l:session): session opened for user chang by root(uid=0)
> > Feb 23 12:22:51.878743 test-box.example.net cifs.upcall[1729]: key
> > description: cifs.spnego;0;0;39010000;ver=0x2;host=win2k16-dfs1.example.net;ip4=192.168.124.132;sec=krb5;uid=0x23e1cee3;creduid=0x23e1cee3;pid=0x6c0
> > Feb 23 12:22:51.878765 test-box.example.net cifs.upcall[1729]: ver=2
> > Feb 23 12:22:51.878769 test-box.example.net cifs.upcall[1729]:
> > host=win2k16-dfs1.example.net
> > Feb 23 12:22:51.878773 test-box.example.net cifs.upcall[1729]:
> > ip=192.168.124.132
> > Feb 23 12:22:51.878776 test-box.example.net cifs.upcall[1729]: sec=1
> > Feb 23 12:22:51.878780 test-box.example.net cifs.upcall[1729]: uid=602001123
> > Feb 23 12:22:51.878783 test-box.example.net cifs.upcall[1729]: creduid=602001123
> > Feb 23 12:22:51.878786 test-box.example.net cifs.upcall[1729]: pid=1728
> > Feb 23 12:22:51.879060 test-box.example.net cifs.upcall[1729]:
> > get_cachename_from_process_env: pathname=/proc/1728/environ
> > Feb 23 12:22:51.879799 test-box.example.net cifs.upcall[1729]:
> > get_existing_cc: default ccache is FILE:/tmp/krb5cc_602001123
> > Feb 23 12:22:51.879819 test-box.example.net cifs.upcall[1729]:
> > get_tgt_time: unable to get principal
> > Feb 23 12:22:51.879824 test-box.example.net cifs.upcall[1729]:
> > handle_krb5_mech: getting service ticket for win2k16-dfs1.example.net
> > Feb 23 12:22:51.879827 test-box.example.net cifs.upcall[1729]:
> > handle_krb5_mech: using GSS-API
> > Feb 23 12:22:51.886573 test-box.example.net gssproxy[1000]:
> > [2021/02/23 17:22:51]: Client [2021/02/23 17:22:51]:
> > (/usr/sbin/cifs.upcall) [2021/02/23 17:22:51]:  connected (fd =
> > 14)[2021/02/23 17:22:51]:  (pid = 1729) (uid = 602001123) (gid =
> > 602000513)[2021/02/23 17:22:51]:  (context =
> > system_u:system_r:kernel_t:s0)[>
> > Feb 23 12:22:51.886573 test-box.example.net gssproxy[1000]: [CID
> > 14][2021/02/23 17:22:51]: Connection matched service cifs
> > Feb 23 12:22:51.886573 test-box.example.net gssproxy[1000]: [CID
> > 14][2021/02/23 17:22:51]: gp_rpc_execute: executing 6
> > (GSSX_ACQUIRE_CRED) for service "cifs", euid: 602001123,socket:
> > /var/lib/gssproxy/cifs.sock
> > Feb 23 12:22:51.886573 test-box.example.net gssproxy[1000]:
> > GSSX_ARG_ACQUIRE_CRED( call_ctx: { "" [  ] } input_cred_handle: <Null>
> > add_cred: 0 desired_name: <Null> time_req: 0 desired_mechs: { }
> > cred_usage: INITIATE initiator_time_req: 0 acceptor_time_req: 0 )
> > Feb 23 12:22:52.346639 test-box.example.net gssproxy[1000]:
> > GSSX_RES_ACQUIRE_CRED( status: { 0 { 1 2 840 113554 1 2 2 } 0 "" "" [
> > ] } output_cred_handle: { "chang at EXAMPLE.NET" [ { "chang at EXAMPLE.NET"
> > { 1 2 840 113554 1 2 2 } INITIATE 36000 0 } ] [ K.....T.....pJv.... ]
> > 0 } )
> > Feb 23 12:22:52.348086 test-box.example.net gssproxy[1000]: [CID
> > 14][2021/02/23 17:22:52]: Connection matched service cifs
> > Feb 23 12:22:52.348086 test-box.example.net gssproxy[1000]: [CID
> > 14][2021/02/23 17:22:52]: gp_rpc_execute: executing 8
> > (GSSX_INIT_SEC_CONTEXT) for service "cifs", euid: 602001123,socket:
> > /var/lib/gssproxy/cifs.sock
> > Feb 23 12:22:52.348086 test-box.example.net gssproxy[1000]:
> > GSSX_ARG_INIT_SEC_CONTEXT( call_ctx: { "" [  ] } context_handle:
> > <Null> cred_handle: { "chang at EXAMPLE.NET" [ { "chang at EXAMPLE.NET" { 1
> > 2 840 113554 1 2 2 } INITIATE 36000 0 } ] [ K.....T.....pJv.... ] 0 }
> > target_name: "cifs at win2k16-dfs1.example.net" mech>
> > Feb 23 12:22:52.348086 test-box.example.net gssproxy[1000]: [CID
> > 14][2021/02/23 17:22:52]: Credentials allowed by configuration
> > Feb 23 12:22:52.357103 test-box.example.net gssproxy[1000]:
> > GSSX_RES_INIT_SEC_CONTEXT( status: { 0 { 1 2 840 113554 1 2 2 } 0 ""
> > "" [  ] } context_handle: { [ ......H............ ] [  ] 0 { 1 2 840
> > 113554 1 2 2 } "chang at EXAMPLE.NET"
> > "cifs/win2k16-dfs1.example.net at EXAMPLE.NET" 36000 432 1 1 }
> > output_token: [ .....>
> > Feb 23 12:22:52.357419 test-box.example.net cifs.upcall[1729]: Exit status 0
> >
> >
> > Ultimately a helper-script packaged with cifs-utils or a separate
> > package entirely could be responsible for adding the file under
> > /etc/gssproxy and the drop-in file for systemd. The helper script
> > could take a series of users/groups as arguments that would limit
> > access to the socket file or there could be a separate config file
> > that is read from to determine if access to the cifs.sock socket
> > should be limited.
> >
> >
> > On Fri, Feb 19, 2021 at 12:38 PM Simo Sorce <simo at redhat.com> wrote:
> > > On Fri, 2021-02-19 at 03:30 -0800, Shyam Prasad N wrote:
> > > > Hi Simo,
> > > >
> > > > > 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.
> > > >
> > > > Could you please point me to the documentation for doing this?
> > >
> > > I do not know if the kernel documents this, but the way gssproxy works
> > > is that when you start the daemon it pokes at the kernel to let it know
> > > the socket is available. So then the kernel opens the socket in the
> > > namespace the proxy is running into (detected from the poking
> > > operation, which is a write in a proc file).
> > >
> > > HTH,
> > > Simo.
> > >
> > > > Regards,
> > > > Shyam
> > > >
> > > > On Thu, Dec 17, 2020 at 5:41 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
> > >
> > >
> > >
> > >
>
> --
> Simo Sorce
> RHEL Crypto Team
> Red Hat, Inc
>
>
>
>




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