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rpcsec_gss support for kernel RPC servers
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This document gives references to the standards and protocols used to
implement RPCGSS authentication in kernel RPC servers such as the NFS
server and the NFS client's NFSv4.0 callback server.  (But note that
NFSv4.1 and higher don't require the client to act as a server for the
purposes of authentication.)

RPCGSS is specified in a few IETF documents:

 - RFC2203 v1: https://tools.ietf.org/rfc/rfc2203.txt
 - RFC5403 v2: https://tools.ietf.org/rfc/rfc5403.txt

There is a third version that we don't currently implement:

 - RFC7861 v3: https://tools.ietf.org/rfc/rfc7861.txt

Background
==========

The RPCGSS Authentication method describes a way to perform GSSAPI
Authentication for NFS.  Although GSSAPI is itself completely mechanism
agnostic, in many cases only the KRB5 mechanism is supported by NFS
implementations.

The Linux kernel, at the moment, supports only the KRB5 mechanism, and
depends on GSSAPI extensions that are KRB5 specific.

GSSAPI is a complex library, and implementing it completely in kernel is
unwarranted. However GSSAPI operations are fundementally separable in 2
parts:

- initial context establishment
- integrity/privacy protection (signing and encrypting of individual
  packets)

The former is more complex and policy-independent, but less
performance-sensitive.  The latter is simpler and needs to be very fast.

Therefore, we perform per-packet integrity and privacy protection in the
kernel, but leave the initial context establishment to userspace.  We
need upcalls to request userspace to perform context establishment.

NFS Server Legacy Upcall Mechanism
==================================

The classic upcall mechanism uses a custom text based upcall mechanism
to talk to a custom daemon called rpc.svcgssd that is provide by the
nfs-utils package.

This upcall mechanism has 2 limitations:

A) It can handle tokens that are no bigger than 2KiB

In some Kerberos deployment GSSAPI tokens can be quite big, up and
beyond 64KiB in size due to various authorization extensions attacked to
the Kerberos tickets, that needs to be sent through the GSS layer in
order to perform context establishment.

B) It does not properly handle creds where the user is member of more
than a few thousand groups (the current hard limit in the kernel is 65K
groups) due to limitation on the size of the buffer that can be send
back to the kernel (4KiB).

NFS Server New RPC Upcall Mechanism
===================================

The newer upcall mechanism uses RPC over a unix socket to a daemon
called gss-proxy, implemented by a userspace program called Gssproxy.

The gss_proxy RPC protocol is currently documented `here
<https://fedorahosted.org/gss-proxy/wiki/ProtocolDocumentation>`_.

This upcall mechanism uses the kernel rpc client and connects to the gssproxy
userspace program over a regular unix socket. The gssproxy protocol does not
suffer from the size limitations of the legacy protocol.

Negotiating Upcall Mechanisms
=============================

To provide backward compatibility, the kernel defaults to using the
legacy mechanism.  To switch to the new mechanism, gss-proxy must bind
to /var/run/gssproxy.sock and then write "1" to
/proc/net/rpc/use-gss-proxy.  If gss-proxy dies, it must repeat both
steps.

Once the upcall mechanism is chosen, it cannot be changed.  To prevent
locking into the legacy mechanisms, the above steps must be performed
before starting nfsd.  Whoever starts nfsd can guarantee this by reading
from /proc/net/rpc/use-gss-proxy and checking that it contains a
"1"--the read will block until gss-proxy has done its write to the file.