92 lines
3.6 KiB
Text
92 lines
3.6 KiB
Text
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rpcsec_gss support for kernel RPC servers
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=========================================
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This document gives references to the standards and protocols used to
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implement RPCGSS authentication in kernel RPC servers such as the NFS
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server and the NFS client's NFSv4.0 callback server. (But note that
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NFSv4.1 and higher don't require the client to act as a server for the
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purposes of authentication.)
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RPCGSS is specified in a few IETF documents:
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- RFC2203 v1: http://tools.ietf.org/rfc/rfc2203.txt
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- RFC5403 v2: http://tools.ietf.org/rfc/rfc5403.txt
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and there is a 3rd version being proposed:
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- http://tools.ietf.org/id/draft-williams-rpcsecgssv3.txt
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(At draft n. 02 at the time of writing)
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Background
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----------
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The RPCGSS Authentication method describes a way to perform GSSAPI
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Authentication for NFS. Although GSSAPI is itself completely mechanism
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agnostic, in many cases only the KRB5 mechanism is supported by NFS
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implementations.
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The Linux kernel, at the moment, supports only the KRB5 mechanism, and
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depends on GSSAPI extensions that are KRB5 specific.
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GSSAPI is a complex library, and implementing it completely in kernel is
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unwarranted. However GSSAPI operations are fundementally separable in 2
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parts:
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- initial context establishment
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- integrity/privacy protection (signing and encrypting of individual
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packets)
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The former is more complex and policy-independent, but less
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performance-sensitive. The latter is simpler and needs to be very fast.
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Therefore, we perform per-packet integrity and privacy protection in the
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kernel, but leave the initial context establishment to userspace. We
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need upcalls to request userspace to perform context establishment.
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NFS Server Legacy Upcall Mechanism
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----------------------------------
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The classic upcall mechanism uses a custom text based upcall mechanism
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to talk to a custom daemon called rpc.svcgssd that is provide by the
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nfs-utils package.
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This upcall mechanism has 2 limitations:
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A) It can handle tokens that are no bigger than 2KiB
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In some Kerberos deployment GSSAPI tokens can be quite big, up and
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beyond 64KiB in size due to various authorization extensions attacked to
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the Kerberos tickets, that needs to be sent through the GSS layer in
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order to perform context establishment.
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B) It does not properly handle creds where the user is member of more
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than a few housand groups (the current hard limit in the kernel is 65K
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groups) due to limitation on the size of the buffer that can be send
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back to the kernel (4KiB).
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NFS Server New RPC Upcall Mechanism
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-----------------------------------
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The newer upcall mechanism uses RPC over a unix socket to a daemon
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called gss-proxy, implemented by a userspace program called Gssproxy.
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The gss_proxy RPC protocol is currently documented here:
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https://fedorahosted.org/gss-proxy/wiki/ProtocolDocumentation
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This upcall mechanism uses the kernel rpc client and connects to the gssproxy
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userspace program over a regular unix socket. The gssproxy protocol does not
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suffer from the size limitations of the legacy protocol.
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Negotiating Upcall Mechanisms
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-----------------------------
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To provide backward compatibility, the kernel defaults to using the
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legacy mechanism. To switch to the new mechanism, gss-proxy must bind
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to /var/run/gssproxy.sock and then write "1" to
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/proc/net/rpc/use-gss-proxy. If gss-proxy dies, it must repeat both
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steps.
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Once the upcall mechanism is chosen, it cannot be changed. To prevent
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locking into the legacy mechanisms, the above steps must be performed
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before starting nfsd. Whoever starts nfsd can guarantee this by reading
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from /proc/net/rpc/use-gss-proxy and checking that it contains a
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"1"--the read will block until gss-proxy has done its write to the file.
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