kNFSD Signed Filehandles

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/[email protected]
- 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/[email protected]
- 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/[email protected]/
- 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

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.

---
 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(-)

index badb2fe57c98..d348648033d9 100644
@@ -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

index 887525964451..4e08c1a6b394 100644
@@ -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,
 	},
 	{

index 9fa600602658..c8ed733240a0 100644
@@ -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 */

index 4d8e3c1a7be3..590584952bf6 100644
@@ -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);

index d1d0b0dd0545..c1a5f2fa44ab 100644
@@ -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

index e9efbc9e63d8..97c7447f4d14 100644
@@ -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

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.

---
 fs/nfsd/export.c                 | 5 +++--
 include/uapi/linux/nfsd/export.h | 4 ++--
 2 files changed, 5 insertions(+), 4 deletions(-)

index 2a1499f2ad19..19c7a91c5373 100644
@@ -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, {"", ""}}
 };
 

index a73ca3703abb..de647cf166c3 100644
@@ -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

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.

---
 Documentation/filesystems/nfs/exporting.rst | 85 +++++++++++++++++++++
 fs/nfsd/nfsfh.c                             | 64 +++++++++++++++-
 2 files changed, 147 insertions(+), 2 deletions(-)

index de64d2d002a2..9d61e3b4fb37 100644
@@ -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.

2.50.1

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