[PATCH v3 0/3] kNFSD Signed Filehandles

[PATCH v3 0/3] kNFSD Signed Filehandles

[Benjamin Coddington]

The following series enables the linux NFS server to add a Message
Authentication Code (MAC) to the filehandles it gives to clients.  This
provides additional protection to the exported filesystem against filehandle
guessing attacks.

Filesystems generate their own filehandles through the export_operation
"encode_fh" and a filehandle provides sufficient access to open a file
without needing to perform a lookup.  A trusted NFS client holding a valid
filehandle can remotely access the corresponding file without reference to
access-path restrictions that might be imposed by the ancestor directories
or the server exports.

In order to acquire a filehandle, you must perform lookup operations on the
parent directory(ies), and the permissions on those directories may prohibit
you from walking into them to find the files within.  This would normally be
considered sufficient protection on a local filesystem to prohibit users
from accessing those files, however when the filesystem is exported via NFS
an exported file can be accessed whenever the NFS server is presented with
the correct filehandle, which can be guessed or acquired by means other than
LOOKUP.

Filehandles are easy to guess because they are well-formed.  The
open_by_handle_at(2) man page contains an example C program
(t_name_to_handle_at.c) that can display a filehandle given a path.  Here's
an example filehandle from a fairly modern XFS:

# ./t_name_to_handle_at /exports/foo 
57
12 129    99 00 00 00 00 00 00 00 b4 10 0b 8c

          ^---------  filehandle  ----------^
          ^------- inode -------^ ^-- gen --^

This filehandle consists of a 64-bit inode number and 32-bit generation
number.  Because the handle is well-formed, its easy to fabricate
filehandles that match other files within the same filesystem.  You can
simply insert inode numbers and iterate on the generation number.
Eventually you'll be able to access the file using open_by_handle_at(2).
For a local system, open_by_handle_at(2) requires CAP_DAC_READ_SEARCH, which
protects against guessing attacks by unprivileged users.

Simple testing confirms that the correct generation number can be found
within ~1200 minutes using open_by_handle_at() over NFS on a local system
and it is estimated that adding network delay with genuine NFS calls may
only increase this to around 24 hours.

In contrast to a local user using open_by_handle(2), the NFS server must
permissively allow remote clients to open by filehandle without being able
to check or trust the remote caller's access. Therefore additional
protection against this attack is needed for NFS case.  We propose to sign
filehandles by appending an 8-byte MAC which is the siphash of the
filehandle from a key set from the nfs-utilities.  NFS server can then
ensure that guessing a valid filehandle+MAC is practically impossible
without knowledge of the MAC's key.  The NFS server performs optional
signing by possessing a key set from userspace and having the "sign_fh"
export option.

Because filehandles are long-lived, and there's no method for expiring them,
the server's key should be set once and not changed.  It also should be
persisted across restarts.  The methods to set the key allow only setting it
once, afterward it cannot be changed.  A separate patchset for nfs-utils
contains the userspace changes required to set the server's key.

I had planned on adding additional work to enable the server to check whether the
8-byte MAC will overflow maximum filehandle length for the protocol at
export time.  There could be some filesystems with 40-byte fileid and
24-byte fsid which would break NFSv3's 64-byte filehandle maximum with an
8-byte MAC appended.  The server should refuse to export those filesystems
when "sign_fh" is requested.  However, the way the export caches work (the
server may not even be running when a user sets up the export) its
impossible to do this check at export time.  Instead, the server will refuse
to give out filehandles at mount time and emit a pr_warn().

Thanks for any comments and critique.

Changes from encrypt_fh posting:
https://lore.kernel.org/linux-nfs/510E10A4-11BE-412D-93AF-C4CC969954E7@hammerspace.com
	- sign filehandles instead of encrypt them (Eric Biggers)
	- fix the NFSEXP_ macros, specifically NFSEXP_ALLFLAGS (NeilBrown)
	- rebase onto cel/nfsd-next (Chuck Lever)
	- condensed/clarified problem explantion (thanks Chuck Lever)
	- add nfsctl file "fh_key" for rpc.nfsd to also set the key

Changes from v1 posting:
https://lore.kernel.org/linux-nfs/cover.1768573690.git.bcodding@hammerspace.com
	- remove fh_fileid_offset() (Chuck Lever)
	- fix pr_warns, fix memcmp (Chuck Lever)
	- remove incorrect rootfh comment (NeilBrown)
	- make fh_key setting an optional attr to threads verb (Jeff Layton)
	- drop BIT() EXP_ flag conversion
	- cover-letter tune-ups (NeilBrown, Chuck Lever)
	- fix NFSEXP_ALLFLAGS on 2/3
	- cast fh->fh_size + sizeof(hash) result to int (avoid x86_64 WARNING)
	- move MAC signing into __fh_update() (Chuck Lever)

Changes from v2 posting:
https://lore.kernel.org/linux-nfs/cover.1769026777.git.bcodding@hammerspace.com/
	- more cover-letter detail (NeilBrown)
	- Documentation/filesystems/nfs/exporting.rst section (Jeff Layton)
	- fix key copy (Eric Biggers)
	- use NFSD_A_SERVER_MAX (NeilBrown)
	- remove procfs fh_key interface (Chuck Lever)
	- remove FH_AT_MAC (Chuck Lever)
	- allow fh_key change when server is not running (Chuck/Jeff)
	- accept fh_key as netlink attribute instead of command (Jeff Layton)

Benjamin Coddington (3):
  NFSD: Add a key for signing filehandles
  NFSD/export: Add sign_fh export option
  NFSD: Sign filehandles

 Documentation/filesystems/nfs/exporting.rst | 85 +++++++++++++++++++++
 Documentation/netlink/specs/nfsd.yaml       |  6 ++
 fs/nfsd/export.c                            |  5 +-
 fs/nfsd/netlink.c                           |  5 +-
 fs/nfsd/netns.h                             |  2 +
 fs/nfsd/nfsctl.c                            | 37 ++++++++-
 fs/nfsd/nfsfh.c                             | 64 +++++++++++++++-
 fs/nfsd/trace.h                             | 25 ++++++
 include/uapi/linux/nfsd/export.h            |  4 +-
 include/uapi/linux/nfsd_netlink.h           |  1 +
 10 files changed, 225 insertions(+), 9 deletions(-)


base-commit: dabff11003f9aaf293bd8f907a62f3366bd5e65f (cel/nfsd-testing)
-- 
2.50.1

[PATCH v3 1/3] NFSD: Add a key for signing filehandles

[Benjamin Coddington]

A future patch will enable NFSD to sign filehandles by appending a Message
Authentication Code(MAC).  To do this, NFSD requires a secret 128-bit key
that can persist across reboots.  A persisted key allows the server to
accept filehandles after a restart.  Enable NFSD to be configured with this
key the netlink interface.

Link: https://lore.kernel.org/linux-nfs/cover.1770046529.git.bcodding@hammerspace.com
Signed-off-by: Benjamin Coddington <bcodding@hammerspace.com>
---
 Documentation/netlink/specs/nfsd.yaml |  6 +++++
 fs/nfsd/netlink.c                     |  5 ++--
 fs/nfsd/netns.h                       |  2 ++
 fs/nfsd/nfsctl.c                      | 37 ++++++++++++++++++++++++++-
 fs/nfsd/trace.h                       | 25 ++++++++++++++++++
 include/uapi/linux/nfsd_netlink.h     |  1 +
 6 files changed, 73 insertions(+), 3 deletions(-)

diff --git a/Documentation/netlink/specs/nfsd.yaml b/Documentation/netlink/specs/nfsd.yaml
index badb2fe57c98..d348648033d9 100644
--- a/Documentation/netlink/specs/nfsd.yaml
+++ b/Documentation/netlink/specs/nfsd.yaml
@@ -81,6 +81,11 @@ attribute-sets:
       -
         name: min-threads
         type: u32
+      -
+        name: fh-key
+        type: binary
+        checks:
+            exact-len: 16
   -
     name: version
     attributes:
@@ -163,6 +168,7 @@ operations:
             - leasetime
             - scope
             - min-threads
+            - fh-key
     -
       name: threads-get
       doc: get the number of running threads
diff --git a/fs/nfsd/netlink.c b/fs/nfsd/netlink.c
index 887525964451..4e08c1a6b394 100644
--- a/fs/nfsd/netlink.c
+++ b/fs/nfsd/netlink.c
@@ -24,12 +24,13 @@ const struct nla_policy nfsd_version_nl_policy[NFSD_A_VERSION_ENABLED + 1] = {
 };
 
 /* NFSD_CMD_THREADS_SET - do */
-static const struct nla_policy nfsd_threads_set_nl_policy[NFSD_A_SERVER_MIN_THREADS + 1] = {
+static const struct nla_policy nfsd_threads_set_nl_policy[NFSD_A_SERVER_FH_KEY + 1] = {
 	[NFSD_A_SERVER_THREADS] = { .type = NLA_U32, },
 	[NFSD_A_SERVER_GRACETIME] = { .type = NLA_U32, },
 	[NFSD_A_SERVER_LEASETIME] = { .type = NLA_U32, },
 	[NFSD_A_SERVER_SCOPE] = { .type = NLA_NUL_STRING, },
 	[NFSD_A_SERVER_MIN_THREADS] = { .type = NLA_U32, },
+	[NFSD_A_SERVER_FH_KEY] = NLA_POLICY_EXACT_LEN(16),
 };
 
 /* NFSD_CMD_VERSION_SET - do */
@@ -58,7 +59,7 @@ static const struct genl_split_ops nfsd_nl_ops[] = {
 		.cmd		= NFSD_CMD_THREADS_SET,
 		.doit		= nfsd_nl_threads_set_doit,
 		.policy		= nfsd_threads_set_nl_policy,
-		.maxattr	= NFSD_A_SERVER_MIN_THREADS,
+		.maxattr	= NFSD_A_SERVER_MAX,
 		.flags		= GENL_ADMIN_PERM | GENL_CMD_CAP_DO,
 	},
 	{
diff --git a/fs/nfsd/netns.h b/fs/nfsd/netns.h
index 9fa600602658..c8ed733240a0 100644
--- a/fs/nfsd/netns.h
+++ b/fs/nfsd/netns.h
@@ -16,6 +16,7 @@
 #include <linux/percpu-refcount.h>
 #include <linux/siphash.h>
 #include <linux/sunrpc/stats.h>
+#include <linux/siphash.h>
 
 /* Hash tables for nfs4_clientid state */
 #define CLIENT_HASH_BITS                 4
@@ -224,6 +225,7 @@ struct nfsd_net {
 	spinlock_t              local_clients_lock;
 	struct list_head	local_clients;
 #endif
+	siphash_key_t		*fh_key;
 };
 
 /* Simple check to find out if a given net was properly initialized */
diff --git a/fs/nfsd/nfsctl.c b/fs/nfsd/nfsctl.c
index 4d8e3c1a7be3..590584952bf6 100644
--- a/fs/nfsd/nfsctl.c
+++ b/fs/nfsd/nfsctl.c
@@ -1571,6 +1571,31 @@ int nfsd_nl_rpc_status_get_dumpit(struct sk_buff *skb,
 	return ret;
 }
 
+/**
+ * nfsd_nl_fh_key_set - helper to copy fh_key from userspace
+ * @attr: nlattr NFSD_A_SERVER_FH_KEY
+ * @nn: nfsd_net
+ *
+ * Callers should hold nfsd_mutex, returns 0 on success or negative errno.
+ */
+static int nfsd_nl_fh_key_set(const struct nlattr *attr, struct nfsd_net *nn)
+{
+	siphash_key_t *fh_key;
+
+	if (nla_len(attr) != sizeof(siphash_key_t))
+		return -EINVAL;
+
+	if (!nn->fh_key) {
+		fh_key = kmalloc(sizeof(siphash_key_t), GFP_KERNEL);
+		if (!fh_key)
+			return -ENOMEM;
+	}
+
+	memcpy(fh_key, nla_data(attr), sizeof(siphash_key_t));
+	nn->fh_key = fh_key;
+	return 0;
+}
+
 /**
  * nfsd_nl_threads_set_doit - set the number of running threads
  * @skb: reply buffer
@@ -1612,7 +1637,8 @@ int nfsd_nl_threads_set_doit(struct sk_buff *skb, struct genl_info *info)
 
 	if (info->attrs[NFSD_A_SERVER_GRACETIME] ||
 	    info->attrs[NFSD_A_SERVER_LEASETIME] ||
-	    info->attrs[NFSD_A_SERVER_SCOPE]) {
+	    info->attrs[NFSD_A_SERVER_SCOPE] ||
+	    info->attrs[NFSD_A_SERVER_FH_KEY]) {
 		ret = -EBUSY;
 		if (nn->nfsd_serv && nn->nfsd_serv->sv_nrthreads)
 			goto out_unlock;
@@ -1641,6 +1667,14 @@ int nfsd_nl_threads_set_doit(struct sk_buff *skb, struct genl_info *info)
 		attr = info->attrs[NFSD_A_SERVER_SCOPE];
 		if (attr)
 			scope = nla_data(attr);
+
+		attr = info->attrs[NFSD_A_SERVER_FH_KEY];
+		if (attr) {
+			ret = nfsd_nl_fh_key_set(attr, nn);
+			trace_nfsd_ctl_fh_key_set((const char *)nn->fh_key, ret);
+			if (ret)
+				goto out_unlock;
+		}
 	}
 
 	attr = info->attrs[NFSD_A_SERVER_MIN_THREADS];
@@ -2240,6 +2274,7 @@ static __net_exit void nfsd_net_exit(struct net *net)
 {
 	struct nfsd_net *nn = net_generic(net, nfsd_net_id);
 
+	kfree_sensitive(nn->fh_key);
 	nfsd_proc_stat_shutdown(net);
 	percpu_counter_destroy_many(nn->counter, NFSD_STATS_COUNTERS_NUM);
 	nfsd_idmap_shutdown(net);
diff --git a/fs/nfsd/trace.h b/fs/nfsd/trace.h
index d1d0b0dd0545..c1a5f2fa44ab 100644
--- a/fs/nfsd/trace.h
+++ b/fs/nfsd/trace.h
@@ -2240,6 +2240,31 @@ TRACE_EVENT(nfsd_end_grace,
 	)
 );
 
+TRACE_EVENT(nfsd_ctl_fh_key_set,
+	TP_PROTO(
+		const char *key,
+		int result
+	),
+	TP_ARGS(key, result),
+	TP_STRUCT__entry(
+		__array(unsigned char, key, 16)
+		__field(unsigned long, result)
+		__field(bool, key_set)
+	),
+	TP_fast_assign(
+		__entry->key_set = true;
+		if (!key)
+			__entry->key_set = false;
+		else
+			memcpy(__entry->key, key, 16);
+		__entry->result = result;
+	),
+	TP_printk("key=%s result=%ld",
+		__entry->key_set ? __print_hex_str(__entry->key, 16) : "(null)",
+		__entry->result
+	)
+);
+
 DECLARE_EVENT_CLASS(nfsd_copy_class,
 	TP_PROTO(
 		const struct nfsd4_copy *copy
diff --git a/include/uapi/linux/nfsd_netlink.h b/include/uapi/linux/nfsd_netlink.h
index e9efbc9e63d8..97c7447f4d14 100644
--- a/include/uapi/linux/nfsd_netlink.h
+++ b/include/uapi/linux/nfsd_netlink.h
@@ -36,6 +36,7 @@ enum {
 	NFSD_A_SERVER_LEASETIME,
 	NFSD_A_SERVER_SCOPE,
 	NFSD_A_SERVER_MIN_THREADS,
+	NFSD_A_SERVER_FH_KEY,
 
 	__NFSD_A_SERVER_MAX,
 	NFSD_A_SERVER_MAX = (__NFSD_A_SERVER_MAX - 1)
-- 
2.50.1

[PATCH v3 2/3] NFSD/export: Add sign_fh export option

[Benjamin Coddington]

In order to signal that filehandles on this export should be signed, add a
"sign_fh" export option.  Filehandle signing can help the server defend
against certain filehandle guessing attacks.

Setting the "sign_fh" export option sets NFSEXP_SIGN_FH.  In a future patch
NFSD uses this signal to append a MAC onto filehandles for that export.

While we're in here, tidy a few stray expflags to more closely align to the
export flag order.

Link: https://lore.kernel.org/linux-nfs/cover.1770046529.git.bcodding@hammerspace.com
Signed-off-by: Benjamin Coddington <bcodding@hammerspace.com>
---
 fs/nfsd/export.c                 | 5 +++--
 include/uapi/linux/nfsd/export.h | 4 ++--
 2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/fs/nfsd/export.c b/fs/nfsd/export.c
index 2a1499f2ad19..19c7a91c5373 100644
--- a/fs/nfsd/export.c
+++ b/fs/nfsd/export.c
@@ -1349,13 +1349,14 @@ static struct flags {
 	{ NFSEXP_ASYNC, {"async", "sync"}},
 	{ NFSEXP_GATHERED_WRITES, {"wdelay", "no_wdelay"}},
 	{ NFSEXP_NOREADDIRPLUS, {"nordirplus", ""}},
+	{ NFSEXP_SECURITY_LABEL, {"security_label", ""}},
+	{ NFSEXP_SIGN_FH, {"sign_fh", ""}},
 	{ NFSEXP_NOHIDE, {"nohide", ""}},
-	{ NFSEXP_CROSSMOUNT, {"crossmnt", ""}},
 	{ NFSEXP_NOSUBTREECHECK, {"no_subtree_check", ""}},
 	{ NFSEXP_NOAUTHNLM, {"insecure_locks", ""}},
+	{ NFSEXP_CROSSMOUNT, {"crossmnt", ""}},
 	{ NFSEXP_V4ROOT, {"v4root", ""}},
 	{ NFSEXP_PNFS, {"pnfs", ""}},
-	{ NFSEXP_SECURITY_LABEL, {"security_label", ""}},
 	{ 0, {"", ""}}
 };
 
diff --git a/include/uapi/linux/nfsd/export.h b/include/uapi/linux/nfsd/export.h
index a73ca3703abb..de647cf166c3 100644
--- a/include/uapi/linux/nfsd/export.h
+++ b/include/uapi/linux/nfsd/export.h
@@ -34,7 +34,7 @@
 #define NFSEXP_GATHERED_WRITES	0x0020
 #define NFSEXP_NOREADDIRPLUS    0x0040
 #define NFSEXP_SECURITY_LABEL	0x0080
-/* 0x100 currently unused */
+#define NFSEXP_SIGN_FH		0x0100
 #define NFSEXP_NOHIDE		0x0200
 #define NFSEXP_NOSUBTREECHECK	0x0400
 #define	NFSEXP_NOAUTHNLM	0x0800		/* Don't authenticate NLM requests - just trust */
@@ -55,7 +55,7 @@
 #define NFSEXP_PNFS		0x20000
 
 /* All flags that we claim to support.  (Note we don't support NOACL.) */
-#define NFSEXP_ALLFLAGS		0x3FEFF
+#define NFSEXP_ALLFLAGS		0x3FFFF
 
 /* The flags that may vary depending on security flavor: */
 #define NFSEXP_SECINFO_FLAGS	(NFSEXP_READONLY | NFSEXP_ROOTSQUASH \
-- 
2.50.1

[PATCH v3 3/3] NFSD: Sign filehandles

[Benjamin Coddington]

NFS clients may bypass restrictive directory permissions by using
open_by_handle() (or other available OS system call) to guess the
filehandles for files below that directory.

In order to harden knfsd servers against this attack, create a method to
sign and verify filehandles using siphash as a MAC (Message Authentication
Code).  Filehandles that have been signed cannot be tampered with, nor can
clients reasonably guess correct filehandles and hashes that may exist in
parts of the filesystem they cannot access due to directory permissions.

Append the 8 byte siphash to encoded filehandles for exports that have set
the "sign_fh" export option.  Filehandles received from clients are
verified by comparing the appended hash to the expected hash.  If the MAC
does not match the server responds with NFS error _BADHANDLE.  If unsigned
filehandles are received for an export with "sign_fh" they are rejected
with NFS error _BADHANDLE.

Signed-off-by: Benjamin Coddington <bcodding@hammerspace.com>
---
 Documentation/filesystems/nfs/exporting.rst | 85 +++++++++++++++++++++
 fs/nfsd/nfsfh.c                             | 64 +++++++++++++++-
 2 files changed, 147 insertions(+), 2 deletions(-)

diff --git a/Documentation/filesystems/nfs/exporting.rst b/Documentation/filesystems/nfs/exporting.rst
index de64d2d002a2..9d61e3b4fb37 100644
--- a/Documentation/filesystems/nfs/exporting.rst
+++ b/Documentation/filesystems/nfs/exporting.rst
@@ -238,3 +238,88 @@ following flags are defined:
     all of an inode's dirty data on last close. Exports that behave this
     way should set EXPORT_OP_FLUSH_ON_CLOSE so that NFSD knows to skip
     waiting for writeback when closing such files.
+
+Signed Filehandles
+------------------
+
+To protect against filehandle guessing attacks, the Linux NFS server can be
+configured to sign filehandles with a Message Authentication Code (MAC).
+
+Standard NFS filehandles are often predictable. If an attacker can guess
+a valid filehandle for a file they do not have permission to access via
+directory traversal, they may be able to bypass path-based permissions
+(though they still remain subject to inode-level permissions).
+
+Signed filehandles prevent this by appending a MAC to the filehandle
+before it is sent to the client. Upon receiving a filehandle back from a
+client, the server re-calculates the MAC using its internal key and
+verifies it against the one provided. If the signatures do not match,
+the server treats the filehandle as invalid (returning NFS[34]ERR_STALE).
+
+Note that signing filehandles provides integrity and authenticity but
+not confidentiality. The contents of the filehandle remain visible to
+the client; they simply cannot be forged or modified.
+
+Configuration
+~~~~~~~~~~~~
+
+To enable signed filehandles, the administrator must provide a signing
+key to the kernel and enable the "sign_fh" export option.
+
+1. Providing a Key
+   The signing key is managed via the nfsd netlink interface. This key
+   is per-network-namespace and must be set before any exports using
+   "sign_fh" become active.
+
+2. Export Options
+   The feature is controlled on a per-export basis in /etc/exports:
+
+   sign_fh
+     Enables signing for all filehandles generated under this export.
+
+   no_sign_fh
+     (Default) Disables signing.
+
+Key Management and Rotation
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The security of this mechanism relies entirely on the secrecy of the
+signing key.
+
+Initial Setup:
+  The key should be generated using a high-quality random source and
+  loaded early in the boot process or during the nfs-server startup
+  sequence.
+
+Changing Keys:
+  If a key is changed while clients have active mounts, existing
+  filehandles held by those clients will become invalid, resulting in
+  "Stale file handle" errors on the client side.
+
+Safe Rotation:
+  Currently, there is no mechanism for "graceful" key rotation
+  (maintaining multiple valid keys). Changing the key is an atomic
+  operation that immediately invalidates all previous signatures.
+
+Transitioning Exports
+~~~~~~~~~~~~~~~~~~~~
+
+When adding or removing the "sign_fh" flag from an active export, the
+following behaviors should be expected:
+
++-------------------+---------------------------------------------------+
+| Change            | Result for Existing Clients                       |
++===================+===================================================+
+| Adding sign_fh    | Clients holding unsigned filehandles will find    |
+|                   | them rejected, as the server now expects a        |
+|                   | signature.                                        |
++-------------------+---------------------------------------------------+
+| Removing sign_fh  | Clients holding signed filehandles will find them |
+|                   | rejected, as the server now expects the           |
+|                   | filehandle to end at its traditional boundary     |
+|                   | without a MAC.                                    |
++-------------------+---------------------------------------------------+
+
+Because filehandles are often cached persistently by clients, adding or
+removing this option should generally be done during a scheduled maintenance
+window involving a NFS client unmount/remount.
diff --git a/fs/nfsd/nfsfh.c b/fs/nfsd/nfsfh.c
index ed85dd43da18..998332d93d05 100644
--- a/fs/nfsd/nfsfh.c
+++ b/fs/nfsd/nfsfh.c
@@ -11,6 +11,7 @@
 #include <linux/exportfs.h>
 
 #include <linux/sunrpc/svcauth_gss.h>
+#include <crypto/utils.h>
 #include "nfsd.h"
 #include "vfs.h"
 #include "auth.h"
@@ -137,6 +138,57 @@ static inline __be32 check_pseudo_root(struct dentry *dentry,
 	return nfs_ok;
 }
 
+/*
+ * Append an 8-byte MAC to the filehandle hashed from the server's fh_key:
+ */
+static int fh_append_mac(struct svc_fh *fhp, struct net *net)
+{
+	struct nfsd_net *nn = net_generic(net, nfsd_net_id);
+	struct knfsd_fh *fh = &fhp->fh_handle;
+	siphash_key_t *fh_key = nn->fh_key;
+	u64 hash;
+
+	if (!(fhp->fh_export->ex_flags & NFSEXP_SIGN_FH))
+		return 0;
+
+	if (!fh_key) {
+		pr_warn_ratelimited("NFSD: unable to sign filehandles, fh_key not set.\n");
+		return -EINVAL;
+	}
+
+	if (fh->fh_size + sizeof(hash) > fhp->fh_maxsize) {
+		pr_warn_ratelimited("NFSD: unable to sign filehandles, fh_size %d would be greater"
+			" than fh_maxsize %d.\n", (int)(fh->fh_size + sizeof(hash)), fhp->fh_maxsize);
+		return -EINVAL;
+	}
+
+	hash = siphash(&fh->fh_raw, fh->fh_size, fh_key);
+	memcpy(&fh->fh_raw[fh->fh_size], &hash, sizeof(hash));
+	fh->fh_size += sizeof(hash);
+
+	return 0;
+}
+
+/*
+ * Verify that the the filehandle's MAC was hashed from this filehandle
+ * given the server's fh_key:
+ */
+static int fh_verify_mac(struct svc_fh *fhp, struct net *net)
+{
+	struct nfsd_net *nn = net_generic(net, nfsd_net_id);
+	struct knfsd_fh *fh = &fhp->fh_handle;
+	siphash_key_t *fh_key = nn->fh_key;
+	u64 hash;
+
+	if (!fh_key) {
+		pr_warn_ratelimited("NFSD: unable to verify signed filehandles, fh_key not set.\n");
+		return -EINVAL;
+	}
+
+	hash = siphash(&fh->fh_raw, fh->fh_size - sizeof(hash),  fh_key);
+	return crypto_memneq(&fh->fh_raw[fh->fh_size - sizeof(hash)], &hash, sizeof(hash));
+}
+
 /*
  * Use the given filehandle to look up the corresponding export and
  * dentry.  On success, the results are used to set fh_export and
@@ -237,9 +289,14 @@ static __be32 nfsd_set_fh_dentry(struct svc_rqst *rqstp, struct net *net,
 
 	fileid_type = fh->fh_fileid_type;
 
-	if (fileid_type == FILEID_ROOT)
+	if (fileid_type == FILEID_ROOT) {
 		dentry = dget(exp->ex_path.dentry);
-	else {
+	} else {
+		if (exp->ex_flags & NFSEXP_SIGN_FH && fh_verify_mac(fhp, net)) {
+			trace_nfsd_set_fh_dentry_badhandle(rqstp, fhp, -EKEYREJECTED);
+			goto out;
+		}
+
 		dentry = exportfs_decode_fh_raw(exp->ex_path.mnt, fid,
 						data_left, fileid_type, 0,
 						nfsd_acceptable, exp);
@@ -495,6 +552,9 @@ static void _fh_update(struct svc_fh *fhp, struct svc_export *exp,
 		fhp->fh_handle.fh_fileid_type =
 			fileid_type > 0 ? fileid_type : FILEID_INVALID;
 		fhp->fh_handle.fh_size += maxsize * 4;
+
+		if (fh_append_mac(fhp, exp->cd->net))
+			fhp->fh_handle.fh_fileid_type = FILEID_INVALID;
 	} else {
 		fhp->fh_handle.fh_fileid_type = FILEID_ROOT;
 	}
-- 
2.50.1

Re: [PATCH v3 1/3] NFSD: Add a key for signing filehandles

[Benjamin Coddington]

On 2 Feb 2026, at 11:19, Benjamin Coddington wrote:

> A future patch will enable NFSD to sign filehandles by appending a Message
> Authentication Code(MAC).  To do this, NFSD requires a secret 128-bit key
> that can persist across reboots.  A persisted key allows the server to
> accept filehandles after a restart.  Enable NFSD to be configured with this
> key the netlink interface.
>
> Link: https://lore.kernel.org/linux-nfs/cover.1770046529.git.bcodding@hammerspace.com
> Signed-off-by: Benjamin Coddington <bcodding@hammerspace.com>
> ---
>  Documentation/netlink/specs/nfsd.yaml |  6 +++++
>  fs/nfsd/netlink.c                     |  5 ++--
>  fs/nfsd/netns.h                       |  2 ++
>  fs/nfsd/nfsctl.c                      | 37 ++++++++++++++++++++++++++-
>  fs/nfsd/trace.h                       | 25 ++++++++++++++++++
>  include/uapi/linux/nfsd_netlink.h     |  1 +
>  6 files changed, 73 insertions(+), 3 deletions(-)
>
> diff --git a/Documentation/netlink/specs/nfsd.yaml b/Documentation/netlink/specs/nfsd.yaml
> index badb2fe57c98..d348648033d9 100644
> --- a/Documentation/netlink/specs/nfsd.yaml
> +++ b/Documentation/netlink/specs/nfsd.yaml
> @@ -81,6 +81,11 @@ attribute-sets:
>        -
>          name: min-threads
>          type: u32
> +      -
> +        name: fh-key
> +        type: binary
> +        checks:
> +            exact-len: 16
>    -
>      name: version
>      attributes:
> @@ -163,6 +168,7 @@ operations:
>              - leasetime
>              - scope
>              - min-threads
> +            - fh-key
>      -
>        name: threads-get
>        doc: get the number of running threads
> diff --git a/fs/nfsd/netlink.c b/fs/nfsd/netlink.c
> index 887525964451..4e08c1a6b394 100644
> --- a/fs/nfsd/netlink.c
> +++ b/fs/nfsd/netlink.c
> @@ -24,12 +24,13 @@ const struct nla_policy nfsd_version_nl_policy[NFSD_A_VERSION_ENABLED + 1] = {
>  };
>
>  /* NFSD_CMD_THREADS_SET - do */
> -static const struct nla_policy nfsd_threads_set_nl_policy[NFSD_A_SERVER_MIN_THREADS + 1] = {
> +static const struct nla_policy nfsd_threads_set_nl_policy[NFSD_A_SERVER_FH_KEY + 1] = {
>  	[NFSD_A_SERVER_THREADS] = { .type = NLA_U32, },
>  	[NFSD_A_SERVER_GRACETIME] = { .type = NLA_U32, },
>  	[NFSD_A_SERVER_LEASETIME] = { .type = NLA_U32, },
>  	[NFSD_A_SERVER_SCOPE] = { .type = NLA_NUL_STRING, },
>  	[NFSD_A_SERVER_MIN_THREADS] = { .type = NLA_U32, },
> +	[NFSD_A_SERVER_FH_KEY] = NLA_POLICY_EXACT_LEN(16),
>  };
>
>  /* NFSD_CMD_VERSION_SET - do */
> @@ -58,7 +59,7 @@ static const struct genl_split_ops nfsd_nl_ops[] = {
>  		.cmd		= NFSD_CMD_THREADS_SET,
>  		.doit		= nfsd_nl_threads_set_doit,
>  		.policy		= nfsd_threads_set_nl_policy,
> -		.maxattr	= NFSD_A_SERVER_MIN_THREADS,
> +		.maxattr	= NFSD_A_SERVER_MAX,
>  		.flags		= GENL_ADMIN_PERM | GENL_CMD_CAP_DO,
>  	},
>  	{
> diff --git a/fs/nfsd/netns.h b/fs/nfsd/netns.h
> index 9fa600602658..c8ed733240a0 100644
> --- a/fs/nfsd/netns.h
> +++ b/fs/nfsd/netns.h
> @@ -16,6 +16,7 @@
>  #include <linux/percpu-refcount.h>
>  #include <linux/siphash.h>
>  #include <linux/sunrpc/stats.h>
> +#include <linux/siphash.h>
>
>  /* Hash tables for nfs4_clientid state */
>  #define CLIENT_HASH_BITS                 4
> @@ -224,6 +225,7 @@ struct nfsd_net {
>  	spinlock_t              local_clients_lock;
>  	struct list_head	local_clients;
>  #endif
> +	siphash_key_t		*fh_key;
>  };
>
>  /* Simple check to find out if a given net was properly initialized */
> diff --git a/fs/nfsd/nfsctl.c b/fs/nfsd/nfsctl.c
> index 4d8e3c1a7be3..590584952bf6 100644
> --- a/fs/nfsd/nfsctl.c
> +++ b/fs/nfsd/nfsctl.c
> @@ -1571,6 +1571,31 @@ int nfsd_nl_rpc_status_get_dumpit(struct sk_buff *skb,
>  	return ret;
>  }
>
> +/**
> + * nfsd_nl_fh_key_set - helper to copy fh_key from userspace
> + * @attr: nlattr NFSD_A_SERVER_FH_KEY
> + * @nn: nfsd_net
> + *
> + * Callers should hold nfsd_mutex, returns 0 on success or negative errno.
> + */
> +static int nfsd_nl_fh_key_set(const struct nlattr *attr, struct nfsd_net *nn)
> +{
> +	siphash_key_t *fh_key;
> +
> +	if (nla_len(attr) != sizeof(siphash_key_t))
> +		return -EINVAL;
> +
> +	if (!nn->fh_key) {
> +		fh_key = kmalloc(sizeof(siphash_key_t), GFP_KERNEL);
> +		if (!fh_key)
> +			return -ENOMEM;
> +	}
> +
> +	memcpy(fh_key, nla_data(attr), sizeof(siphash_key_t));
> +	nn->fh_key = fh_key;
> +	return 0;
> +}
> +

Oof, I messed it up right here^^ this hunk needed to be folded in.. can send another version on this one:

diff --git a/fs/nfsd/nfsctl.c b/fs/nfsd/nfsctl.c
index 590584952bf6..9b94e1ed98e0 100644
--- a/fs/nfsd/nfsctl.c
+++ b/fs/nfsd/nfsctl.c
@@ -1580,19 +1580,18 @@ int nfsd_nl_rpc_status_get_dumpit(struct sk_buff *skb,
  */
 static int nfsd_nl_fh_key_set(const struct nlattr *attr, struct nfsd_net *nn)
 {
-       siphash_key_t *fh_key;
+       siphash_key_t *fh_key = nn->fh_key;

        if (nla_len(attr) != sizeof(siphash_key_t))
                return -EINVAL;

-       if (!nn->fh_key) {
+       if (!fh_key) {
                fh_key = kmalloc(sizeof(siphash_key_t), GFP_KERNEL);
                if (!fh_key)
                        return -ENOMEM;
+               nn->fh_key = fh_key;
        }
-
        memcpy(fh_key, nla_data(attr), sizeof(siphash_key_t));
-       nn->fh_key = fh_key;
        return 0;
 }


Ben

Re: [PATCH v3 3/3] NFSD: Sign filehandles

[Eric Biggers]

On Mon, Feb 02, 2026 at 11:19:38AM -0500, Benjamin Coddington wrote:
> +/*
> + * Append an 8-byte MAC to the filehandle hashed from the server's fh_key:
> + */
> +static int fh_append_mac(struct svc_fh *fhp, struct net *net)
> +{
> +	struct nfsd_net *nn = net_generic(net, nfsd_net_id);
> +	struct knfsd_fh *fh = &fhp->fh_handle;
> +	siphash_key_t *fh_key = nn->fh_key;
> +	u64 hash;
> +
> +	if (!(fhp->fh_export->ex_flags & NFSEXP_SIGN_FH))
> +		return 0;
> +
> +	if (!fh_key) {
> +		pr_warn_ratelimited("NFSD: unable to sign filehandles, fh_key not set.\n");
> +		return -EINVAL;
> +	}
> +
> +	if (fh->fh_size + sizeof(hash) > fhp->fh_maxsize) {
> +		pr_warn_ratelimited("NFSD: unable to sign filehandles, fh_size %d would be greater"
> +			" than fh_maxsize %d.\n", (int)(fh->fh_size + sizeof(hash)), fhp->fh_maxsize);
> +		return -EINVAL;
> +	}
> +
> +	hash = siphash(&fh->fh_raw, fh->fh_size, fh_key);
> +	memcpy(&fh->fh_raw[fh->fh_size], &hash, sizeof(hash));
> +	fh->fh_size += sizeof(hash);
> +
> +	return 0;

Note that this is still creating endianness-specific MAC values,
considering that siphash() returns a u64 which is being copied directly
into a byte array.  Maybe not what was intended?

- Eric

Re: [PATCH v3 3/3] NFSD: Sign filehandles

[Benjamin Coddington]

On 2 Feb 2026, at 13:19, Eric Biggers wrote:

> On Mon, Feb 02, 2026 at 11:19:38AM -0500, Benjamin Coddington wrote:
>> +/*
>> + * Append an 8-byte MAC to the filehandle hashed from the server's fh_key:
>> + */
>> +static int fh_append_mac(struct svc_fh *fhp, struct net *net)
>> +{
>> +	struct nfsd_net *nn = net_generic(net, nfsd_net_id);
>> +	struct knfsd_fh *fh = &fhp->fh_handle;
>> +	siphash_key_t *fh_key = nn->fh_key;
>> +	u64 hash;
>> +
>> +	if (!(fhp->fh_export->ex_flags & NFSEXP_SIGN_FH))
>> +		return 0;
>> +
>> +	if (!fh_key) {
>> +		pr_warn_ratelimited("NFSD: unable to sign filehandles, fh_key not set.\n");
>> +		return -EINVAL;
>> +	}
>> +
>> +	if (fh->fh_size + sizeof(hash) > fhp->fh_maxsize) {
>> +		pr_warn_ratelimited("NFSD: unable to sign filehandles, fh_size %d would be greater"
>> +			" than fh_maxsize %d.\n", (int)(fh->fh_size + sizeof(hash)), fhp->fh_maxsize);
>> +		return -EINVAL;
>> +	}
>> +
>> +	hash = siphash(&fh->fh_raw, fh->fh_size, fh_key);
>> +	memcpy(&fh->fh_raw[fh->fh_size], &hash, sizeof(hash));
>> +	fh->fh_size += sizeof(hash);
>> +
>> +	return 0;
>
> Note that this is still creating endianness-specific MAC values,
> considering that siphash() returns a u64 which is being copied directly
> into a byte array.  Maybe not what was intended?

Thanks, yes - not intended.  I only corrected the userspace key copy.

This would break filehandles if the server were migrated onto a different
endian system.  I will fix this in v4.

Ben

Re: [PATCH v3 1/3] NFSD: Add a key for signing filehandles

[kernel test robot]

Hi Benjamin,

kernel test robot noticed the following build warnings:

[auto build test WARNING on dabff11003f9aaf293bd8f907a62f3366bd5e65f]

url:    https://github.com/intel-lab-lkp/linux/commits/Benjamin-Coddington/NFSD-Add-a-key-for-signing-filehandles/20260203-002703
base:   dabff11003f9aaf293bd8f907a62f3366bd5e65f
patch link:    https://lore.kernel.org/r/e3806f53c351c03725ecb12fb7ad100786df04f6.1770046529.git.bcodding%40hammerspace.com
patch subject: [PATCH v3 1/3] NFSD: Add a key for signing filehandles
config: x86_64-buildonly-randconfig-003-20260203 (https://download.01.org/0day-ci/archive/20260203/202602030619.d8NUY35L-lkp@intel.com/config)
compiler: clang version 20.1.8 (https://github.com/llvm/llvm-project 87f0227cb60147a26a1eeb4fb06e3b505e9c7261)
rustc: rustc 1.88.0 (6b00bc388 2025-06-23)
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260203/202602030619.d8NUY35L-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202602030619.d8NUY35L-lkp@intel.com/

All warnings (new ones prefixed by >>):

>> fs/nfsd/nfsctl.c:1588:6: warning: variable 'fh_key' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
    1588 |         if (!nn->fh_key) {
         |             ^~~~~~~~~~~
   fs/nfsd/nfsctl.c:1594:9: note: uninitialized use occurs here
    1594 |         memcpy(fh_key, nla_data(attr), sizeof(siphash_key_t));
         |                ^~~~~~
   fs/nfsd/nfsctl.c:1588:2: note: remove the 'if' if its condition is always true
    1588 |         if (!nn->fh_key) {
         |         ^~~~~~~~~~~~~~~~
   fs/nfsd/nfsctl.c:1583:23: note: initialize the variable 'fh_key' to silence this warning
    1583 |         siphash_key_t *fh_key;
         |                              ^
         |                               = NULL
   1 warning generated.


vim +1588 fs/nfsd/nfsctl.c

  1573	
  1574	/**
  1575	 * nfsd_nl_fh_key_set - helper to copy fh_key from userspace
  1576	 * @attr: nlattr NFSD_A_SERVER_FH_KEY
  1577	 * @nn: nfsd_net
  1578	 *
  1579	 * Callers should hold nfsd_mutex, returns 0 on success or negative errno.
  1580	 */
  1581	static int nfsd_nl_fh_key_set(const struct nlattr *attr, struct nfsd_net *nn)
  1582	{
  1583		siphash_key_t *fh_key;
  1584	
  1585		if (nla_len(attr) != sizeof(siphash_key_t))
  1586			return -EINVAL;
  1587	
> 1588		if (!nn->fh_key) {
  1589			fh_key = kmalloc(sizeof(siphash_key_t), GFP_KERNEL);
  1590			if (!fh_key)
  1591				return -ENOMEM;
  1592		}
  1593	
  1594		memcpy(fh_key, nla_data(attr), sizeof(siphash_key_t));
  1595		nn->fh_key = fh_key;
  1596		return 0;
  1597	}
  1598	

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

Re: [PATCH v3 3/3] NFSD: Sign filehandles

[kernel test robot]

Hi Benjamin,

kernel test robot noticed the following build warnings:

[auto build test WARNING on dabff11003f9aaf293bd8f907a62f3366bd5e65f]

url:    https://github.com/intel-lab-lkp/linux/commits/Benjamin-Coddington/NFSD-Add-a-key-for-signing-filehandles/20260203-002703
base:   dabff11003f9aaf293bd8f907a62f3366bd5e65f
patch link:    https://lore.kernel.org/r/11253ead28024160aaf8abf5c234de6605cc93b5.1770046529.git.bcodding%40hammerspace.com
patch subject: [PATCH v3 3/3] NFSD: Sign filehandles
reproduce: (https://download.01.org/0day-ci/archive/20260203/202602030854.mD7cfzBx-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202602030854.mD7cfzBx-lkp@intel.com/

All warnings (new ones prefixed by >>):

   ERROR: Cannot find file ./include/linux/jbd2.h
   ERROR: Cannot find file ./include/linux/jbd2.h
   WARNING: No kernel-doc for file ./include/linux/jbd2.h
   ERROR: Cannot find file ./include/linux/netfs.h
   WARNING: No kernel-doc for file ./include/linux/netfs.h
>> Documentation/filesystems/nfs/exporting.rst:264: WARNING: Title underline too short.


vim +264 Documentation/filesystems/nfs/exporting.rst

   262	
   263	Configuration
 > 264	~~~~~~~~~~~~
   265	

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki