[PATCH v6 0/3] kNFSD Signed Filehandles

[PATCH v6 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)

Changes from v3 posting:
https://lore.kernel.org/linux-nfs/cover.1770046529.git.bcodding@hammerspace.com
	- /actually/ fix up endianness problems (Eric Biggers)
	- comment typo
	- fix Documentation underline warnings
	- fix possible uninitialized fh_key var

Changes from v4 posting:
https://lore.kernel.org/linux-nfs/cover.1770390036.git.bcodding@hammerspace.com
	- again (!!) fix endian copy from userspace (Chuck Lever)
	- fixup protocol return error for MAC verification failure (Chuck Lever)
	- fix filehandle size after MAC verification (Chuck Lever)
	- fix two sparse errors (LKP)

Changes from v5 posting:
https://lore.kernel.org/linux-nfs/cover.1770660136.git.bcodding@hammerspace.com
	- fixup 3/3 commit message to match code return _STALE (Chuck Lever)
	- convert fh sign functions to return bool (Chuck Lever)
	- comment for FILEID_ROOT always unsigned (Chuck Lever)
	- tracepoint error value match return -ESTALE (Chuck Lever)
	- fix a fh data_left bug (Chuck Lever)
	- symbolize size of signing value in words (Chuck Lever)
	- 3/3 add simple rational for choice of hash (Chuck Lever)
	- fix an incorrect error return leak introduced on v5
	- remove a duplicate include (Chuck Lever)
	- inform callers of nfsd_nl_fh_key_set of shutdown req (Chuck Lever)
	- hash key in tracepoint output (Chuck Lever)

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                             |  1 +
 fs/nfsd/nfsctl.c                            | 41 +++++++++-
 fs/nfsd/nfsfh.c                             | 70 ++++++++++++++++-
 fs/nfsd/trace.h                             | 23 ++++++
 include/uapi/linux/nfsd/export.h            |  4 +-
 include/uapi/linux/nfsd_netlink.h           |  1 +
 10 files changed, 230 insertions(+), 11 deletions(-)


base-commit: e3934bbd57c73b3835a77562ca47b5fbc6f34287
-- 
2.50.1

[PATCH v6 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.1770828956.git.bcodding@hammerspace.com
Signed-off-by: Benjamin Coddington <bcodding@hammerspace.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
---
 Documentation/netlink/specs/nfsd.yaml |  6 ++++
 fs/nfsd/netlink.c                     |  5 ++--
 fs/nfsd/netns.h                       |  1 +
 fs/nfsd/nfsctl.c                      | 41 ++++++++++++++++++++++++++-
 fs/nfsd/trace.h                       | 22 ++++++++++++++
 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 f87b5a05e5e9..8ab43c8253b2 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 maximum 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..0071cc25fbc2 100644
--- a/fs/nfsd/netns.h
+++ b/fs/nfsd/netns.h
@@ -224,6 +224,7 @@ 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 a58eb1adac0f..bdf7311041d6 100644
--- a/fs/nfsd/nfsctl.c
+++ b/fs/nfsd/nfsctl.c
@@ -1571,6 +1571,35 @@ 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.
+ * Callers must ensure the server is shut down (sv_nrthreads == 0),
+ * userspace documentation asserts the key may only be set when the server
+ * is not running.
+ */
+static int nfsd_nl_fh_key_set(const struct nlattr *attr, struct nfsd_net *nn)
+{
+	siphash_key_t *fh_key = nn->fh_key;
+
+	if (nla_len(attr) != sizeof(siphash_key_t))
+		return -EINVAL;
+
+	if (!fh_key) {
+		fh_key = kmalloc(sizeof(siphash_key_t), GFP_KERNEL);
+		if (!fh_key)
+			return -ENOMEM;
+		nn->fh_key = fh_key;
+	}
+
+	fh_key->key[0] = get_unaligned_le64(nla_data(attr));
+	fh_key->key[1] = get_unaligned_le64(nla_data(attr) + 8);
+	return 0;
+}
+
 /**
  * nfsd_nl_threads_set_doit - set the number of running threads
  * @skb: reply buffer
@@ -1612,7 +1641,8 @@ 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 +1671,14 @@ 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 +2278,7 @@ 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..185a998996a0 100644
--- a/fs/nfsd/trace.h
+++ b/fs/nfsd/trace.h
@@ -2240,6 +2240,28 @@ 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(
+		__field(u32, key_hash)
+		__field(int, result)
+	),
+	TP_fast_assign(
+		if (key)
+			__entry->key_hash = ~crc32_le(0xFFFFFFFF, key, 16);
+		else
+			__entry->key_hash = 0;
+		__entry->result = result;
+	),
+	TP_printk("key=0x%08x result=%d",
+		__entry->key_hash, __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 v6 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.1770828956.git.bcodding@hammerspace.com
Signed-off-by: Benjamin Coddington <bcodding@hammerspace.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
---
 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 v6 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-2-4 as a MAC (Message
Authentication Code).  According to
https://cr.yp.to/siphash/siphash-20120918.pdf, SipHash can be used as a
MAC, and our use of SipHash-2-4 provides a low 1 in 2^64 chance of forgery.

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 _STALE.  If unsigned
filehandles are received for an export with "sign_fh" they are rejected
with NFS error _STALE.

Signed-off-by: Benjamin Coddington <bcodding@hammerspace.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
---
 Documentation/filesystems/nfs/exporting.rst | 85 +++++++++++++++++++++
 fs/nfsd/nfsfh.c                             | 70 ++++++++++++++++-
 fs/nfsd/trace.h                             |  1 +
 3 files changed, 152 insertions(+), 4 deletions(-)

diff --git a/Documentation/filesystems/nfs/exporting.rst b/Documentation/filesystems/nfs/exporting.rst
index de64d2d002a2..54343f4cc4fd 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 68b629fbaaeb..3bab2ad0b21f 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"
@@ -140,6 +141,57 @@ 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;
+	__le64 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 = cpu_to_le64(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 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;
+	__le64 hash;
+
+	if (!fh_key) {
+		pr_warn_ratelimited("NFSD: unable to verify signed filehandles, fh_key not set.\n");
+		return -EINVAL;
+	}
+
+	hash = cpu_to_le64(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
@@ -236,13 +288,18 @@ static __be32 nfsd_set_fh_dentry(struct svc_rqst *rqstp, struct net *net,
 	/*
 	 * Look up the dentry using the NFS file handle.
 	 */
-	error = nfserr_badhandle;
-
 	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_badmac(rqstp, fhp, -EKEYREJECTED);
+			goto out;
+		} else {
+			data_left -= sizeof(u64)/4;
+		}
+
 		dentry = exportfs_decode_fh_raw(exp->ex_path.mnt, fid,
 						data_left, fileid_type, 0,
 						nfsd_acceptable, exp);
@@ -258,6 +315,8 @@ static __be32 nfsd_set_fh_dentry(struct svc_rqst *rqstp, struct net *net,
 			}
 		}
 	}
+
+	error = nfserr_badhandle;
 	if (dentry == NULL)
 		goto out;
 	if (IS_ERR(dentry)) {
@@ -498,6 +557,9 @@ 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;
 	}
diff --git a/fs/nfsd/trace.h b/fs/nfsd/trace.h
index 185a998996a0..5ad38f50836d 100644
--- a/fs/nfsd/trace.h
+++ b/fs/nfsd/trace.h
@@ -373,6 +373,7 @@ DEFINE_EVENT_CONDITION(nfsd_fh_err_class, nfsd_##name,	\
 
 DEFINE_NFSD_FH_ERR_EVENT(set_fh_dentry_badexport);
 DEFINE_NFSD_FH_ERR_EVENT(set_fh_dentry_badhandle);
+DEFINE_NFSD_FH_ERR_EVENT(set_fh_dentry_badmac);
 
 TRACE_EVENT(nfsd_exp_find_key,
 	TP_PROTO(const struct svc_expkey *key,
-- 
2.50.1

Re: [PATCH v6 3/3] NFSD: Sign filehandles

[Chuck Lever]

On Wed, Feb 11, 2026, at 12:09 PM, Benjamin Coddington wrote:
> 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.

> diff --git a/fs/nfsd/nfsfh.c b/fs/nfsd/nfsfh.c
> index 68b629fbaaeb..3bab2ad0b21f 100644
> --- a/fs/nfsd/nfsfh.c
> +++ b/fs/nfsd/nfsfh.c

> @@ -236,13 +288,18 @@ static __be32 nfsd_set_fh_dentry(struct svc_rqst 
> *rqstp, struct net *net,
>  	/*
>  	 * Look up the dentry using the NFS file handle.
>  	 */
> -	error = nfserr_badhandle;
> -
>  	fileid_type = fh->fh_fileid_type;
> 
> -	if (fileid_type == FILEID_ROOT)
> +	if (fileid_type == FILEID_ROOT) {

Still need a comment here explaining why ROOT is exempt from
file handle signing checks.


>  		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_badmac(rqstp, fhp, -EKEYREJECTED);
> +			goto out;
> +		} else {
> +			data_left -= sizeof(u64)/4;
> +		}

The data_left bug identified during v5 review:
https://lore.kernel.org/linux-nfs/8574c412-31fb-4810-a675-edf72240ae29@oracle.com/

does not appear to be addressed in v6 3/3. Likewise, the
"sizeof(u64)/4" has not been replaced by a symbolic constant.

Additionally, the goto out path when MAC verification fails has an
error value problem in the !NOSUBTREECHECK case:

    error = nfsd_setuser_and_check_port(rqstp, cred, exp);
    if (error)
        goto out;
    /* error is now nfs_ok (0) */
    ...
    if (exp->ex_flags & NFSEXP_SIGN_FH && fh_verify_mac(fhp, net)) {
        trace_nfsd_set_fh_dentry_badmac(...);
        goto out;   /* returns nfs_ok */
    }

nfsd_set_fh_dentry() returns 0 without setting fhp->fh_dentry or
fhp->fh_export. The caller __fh_verify() then proceeds with:

    dentry = fhp->fh_dentry;   /* NULL */
    exp = fhp->fh_export;      /* NULL */
    check_pseudo_root(dentry, exp);

check_pseudo_root() dereferences exp on its first line:

    if (!(exp->ex_flags & NFSEXP_V4ROOT))

Can this NULL dereference occur for signed exports that also have
subtree checking enabled?


> +
>  		dentry = exportfs_decode_fh_raw(exp->ex_path.mnt, fid,
>  						data_left, fileid_type, 0,
>  						nfsd_acceptable, exp);


-- 
Chuck Lever

Re: [PATCH v6 3/3] NFSD: Sign filehandles

[Benjamin Coddington]

On 11 Feb 2026, at 17:05, Chuck Lever wrote:

> On Wed, Feb 11, 2026, at 12:09 PM, Benjamin Coddington wrote:
>> 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.
>
>> diff --git a/fs/nfsd/nfsfh.c b/fs/nfsd/nfsfh.c
>> index 68b629fbaaeb..3bab2ad0b21f 100644
>> --- a/fs/nfsd/nfsfh.c
>> +++ b/fs/nfsd/nfsfh.c
>
>> @@ -236,13 +288,18 @@ static __be32 nfsd_set_fh_dentry(struct svc_rqst
>> *rqstp, struct net *net,
>>  	/*
>>  	 * Look up the dentry using the NFS file handle.
>>  	 */
>> -	error = nfserr_badhandle;
>> -
>>  	fileid_type = fh->fh_fileid_type;
>>
>> -	if (fileid_type == FILEID_ROOT)
>> +	if (fileid_type == FILEID_ROOT) {
>
> Still need a comment here explaining why ROOT is exempt from
> file handle signing checks.

Right on all points.

I'm really sorry Chuck, I sent the wrong patch here.  I am going to resend
v6 with the correct 3/3 patch.

Ben

[PATCH v6 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/8C67F451-980D-4739-B044-F8562B2A8B74@hammerspace.com
Signed-off-by: Benjamin Coddington <bcodding@hammerspace.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
---
 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

Re: [PATCH v6 3/3] NFSD: Sign filehandles

[Chuck Lever]

On 2/12/26 12:24 AM, Benjamin Coddington wrote:
> On 11 Feb 2026, at 17:05, Chuck Lever wrote:
> 
>> On Wed, Feb 11, 2026, at 12:09 PM, Benjamin Coddington wrote:
>>> 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.
>>
>>> diff --git a/fs/nfsd/nfsfh.c b/fs/nfsd/nfsfh.c
>>> index 68b629fbaaeb..3bab2ad0b21f 100644
>>> --- a/fs/nfsd/nfsfh.c
>>> +++ b/fs/nfsd/nfsfh.c
>>
>>> @@ -236,13 +288,18 @@ static __be32 nfsd_set_fh_dentry(struct svc_rqst
>>> *rqstp, struct net *net,
>>>  	/*
>>>  	 * Look up the dentry using the NFS file handle.
>>>  	 */
>>> -	error = nfserr_badhandle;
>>> -
>>>  	fileid_type = fh->fh_fileid_type;
>>>
>>> -	if (fileid_type == FILEID_ROOT)
>>> +	if (fileid_type == FILEID_ROOT) {
>>
>> Still need a comment here explaining why ROOT is exempt from
>> file handle signing checks.
> 
> Right on all points.
> 
> I'm really sorry Chuck, I sent the wrong patch here.  I am going to resend
> v6 with the correct 3/3 patch.

No worries, it looked to me like a tooling issue because the v6 3/3
diff was exactly the same as v5's. The other two v6 patches in the
series are ready to apply IMO, so I'll have a look at the RESEND.

-- 
Chuck Lever

[PATCH v6 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/8C67F451-980D-4739-B044-F8562B2A8B74@hammerspace.com
Signed-off-by: Benjamin Coddington <bcodding@hammerspace.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
---
 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