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diff --git a/Documentation/filesystems/nfs/client-identifier.rst b/Documentation/filesystems/nfs/client-identifier.rst new file mode 100644 index 000000000000..5147e15815a1 --- /dev/null +++ b/Documentation/filesystems/nfs/client-identifier.rst @@ -0,0 +1,216 @@ +.. SPDX-License-Identifier: GPL-2.0 + +======================= +NFSv4 client identifier +======================= + +This document explains how the NFSv4 protocol identifies client +instances in order to maintain file open and lock state during +system restarts. A special identifier and principal are maintained +on each client. These can be set by administrators, scripts +provided by site administrators, or tools provided by Linux +distributors. + +There are risks if a client's NFSv4 identifier and its principal +are not chosen carefully. + + +Introduction +------------ + +The NFSv4 protocol uses "lease-based file locking". Leases help +NFSv4 servers provide file lock guarantees and manage their +resources. + +Simply put, an NFSv4 server creates a lease for each NFSv4 client. +The server collects each client's file open and lock state under +the lease for that client. + +The client is responsible for periodically renewing its leases. +While a lease remains valid, the server holding that lease +guarantees the file locks the client has created remain in place. + +If a client stops renewing its lease (for example, if it crashes), +the NFSv4 protocol allows the server to remove the client's open +and lock state after a certain period of time. When a client +restarts, it indicates to servers that open and lock state +associated with its previous leases is no longer valid and can be +destroyed immediately. + +In addition, each NFSv4 server manages a persistent list of client +leases. When the server restarts and clients attempt to recover +their state, the server uses this list to distinguish amongst +clients that held state before the server restarted and clients +sending fresh OPEN and LOCK requests. This enables file locks to +persist safely across server restarts. + +NFSv4 client identifiers +------------------------ + +Each NFSv4 client presents an identifier to NFSv4 servers so that +they can associate the client with its lease. Each client's +identifier consists of two elements: + + - co_ownerid: An arbitrary but fixed string. + + - boot verifier: A 64-bit incarnation verifier that enables a + server to distinguish successive boot epochs of the same client. + +The NFSv4.0 specification refers to these two items as an +"nfs_client_id4". The NFSv4.1 specification refers to these two +items as a "client_owner4". + +NFSv4 servers tie this identifier to the principal and security +flavor that the client used when presenting it. Servers use this +principal to authorize subsequent lease modification operations +sent by the client. Effectively this principal is a third element of +the identifier. + +As part of the identity presented to servers, a good +"co_ownerid" string has several important properties: + + - The "co_ownerid" string identifies the client during reboot + recovery, therefore the string is persistent across client + reboots. + - The "co_ownerid" string helps servers distinguish the client + from others, therefore the string is globally unique. Note + that there is no central authority that assigns "co_ownerid" + strings. + - Because it often appears on the network in the clear, the + "co_ownerid" string does not reveal private information about + the client itself. + - The content of the "co_ownerid" string is set and unchanging + before the client attempts NFSv4 mounts after a restart. + - The NFSv4 protocol places a 1024-byte limit on the size of the + "co_ownerid" string. + +Protecting NFSv4 lease state +---------------------------- + +NFSv4 servers utilize the "client_owner4" as described above to +assign a unique lease to each client. Under this scheme, there are +circumstances where clients can interfere with each other. This is +referred to as "lease stealing". + +If distinct clients present the same "co_ownerid" string and use +the same principal (for example, AUTH_SYS and UID 0), a server is +unable to tell that the clients are not the same. Each distinct +client presents a different boot verifier, so it appears to the +server as if there is one client that is rebooting frequently. +Neither client can maintain open or lock state in this scenario. + +If distinct clients present the same "co_ownerid" string and use +distinct principals, the server is likely to allow the first client +to operate normally but reject subsequent clients with the same +"co_ownerid" string. + +If a client's "co_ownerid" string or principal are not stable, +state recovery after a server or client reboot is not guaranteed. +If a client unexpectedly restarts but presents a different +"co_ownerid" string or principal to the server, the server orphans +the client's previous open and lock state. This blocks access to +locked files until the server removes the orphaned state. + +If the server restarts and a client presents a changed "co_ownerid" +string or principal to the server, the server will not allow the +client to reclaim its open and lock state, and may give those locks +to other clients in the meantime. This is referred to as "lock +stealing". + +Lease stealing and lock stealing increase the potential for denial +of service and in rare cases even data corruption. + +Selecting an appropriate client identifier +------------------------------------------ + +By default, the Linux NFSv4 client implementation constructs its +"co_ownerid" string starting with the words "Linux NFS" followed by +the client's UTS node name (the same node name, incidentally, that +is used as the "machine name" in an AUTH_SYS credential). In small +deployments, this construction is usually adequate. Often, however, +the node name by itself is not adequately unique, and can change +unexpectedly. Problematic situations include: + + - NFS-root (diskless) clients, where the local DCHP server (or + equivalent) does not provide a unique host name. + + - "Containers" within a single Linux host. If each container has + a separate network namespace, but does not use the UTS namespace + to provide a unique host name, then there can be multiple NFS + client instances with the same host name. + + - Clients across multiple administrative domains that access a + common NFS server. If hostnames are not assigned centrally + then uniqueness cannot be guaranteed unless a domain name is + included in the hostname. + +Linux provides two mechanisms to add uniqueness to its "co_ownerid" +string: + + nfs.nfs4_unique_id + This module parameter can set an arbitrary uniquifier string + via the kernel command line, or when the "nfs" module is + loaded. + + /sys/fs/nfs/client/net/identifier + This virtual file, available since Linux 5.3, is local to the + network namespace in which it is accessed and so can provide + distinction between network namespaces (containers) when the + hostname remains uniform. + +Note that this file is empty on name-space creation. If the +container system has access to some sort of per-container identity +then that uniquifier can be used. For example, a uniquifier might +be formed at boot using the container's internal identifier: + + sha256sum /etc/machine-id | awk '{print $1}' \\ + > /sys/fs/nfs/client/net/identifier + +Security considerations +----------------------- + +The use of cryptographic security for lease management operations +is strongly encouraged. + +If NFS with Kerberos is not configured, a Linux NFSv4 client uses +AUTH_SYS and UID 0 as the principal part of its client identity. +This configuration is not only insecure, it increases the risk of +lease and lock stealing. However, it might be the only choice for +client configurations that have no local persistent storage. +"co_ownerid" string uniqueness and persistence is critical in this +case. + +When a Kerberos keytab is present on a Linux NFS client, the client +attempts to use one of the principals in that keytab when +identifying itself to servers. The "sec=" mount option does not +control this behavior. Alternately, a single-user client with a +Kerberos principal can use that principal in place of the client's +host principal. + +Using Kerberos for this purpose enables the client and server to +use the same lease for operations covered by all "sec=" settings. +Additionally, the Linux NFS client uses the RPCSEC_GSS security +flavor with Kerberos and the integrity QOS to prevent in-transit +modification of lease modification requests. + +Additional notes +---------------- +The Linux NFSv4 client establishes a single lease on each NFSv4 +server it accesses. NFSv4 mounts from a Linux NFSv4 client of a +particular server then share that lease. + +Once a client establishes open and lock state, the NFSv4 protocol +enables lease state to transition to other servers, following data +that has been migrated. This hides data migration completely from +running applications. The Linux NFSv4 client facilitates state +migration by presenting the same "client_owner4" to all servers it +encounters. + +======== +See Also +======== + + - nfs(5) + - kerberos(7) + - RFC 7530 for the NFSv4.0 specification + - RFC 8881 for the NFSv4.1 specification. |