* [PATCH v3 1/3] NFSD: Add a key for signing filehandles
2026-02-02 16:19 [PATCH v3 0/3] kNFSD Signed Filehandles Benjamin Coddington
@ 2026-02-02 16:19 ` Benjamin Coddington
2026-02-02 16:51 ` Benjamin Coddington
2026-02-02 22:31 ` kernel test robot
2026-02-02 16:19 ` [PATCH v3 2/3] NFSD/export: Add sign_fh export option Benjamin Coddington
2026-02-02 16:19 ` [PATCH v3 3/3] NFSD: Sign filehandles Benjamin Coddington
2 siblings, 2 replies; 9+ messages in thread
From: Benjamin Coddington @ 2026-02-02 16:19 UTC (permalink / raw)
To: Chuck Lever, Jeff Layton, NeilBrown, Trond Myklebust,
Anna Schumaker, Benjamin Coddington, Eric Biggers, Rick Macklem
Cc: linux-nfs, linux-fsdevel, linux-crypto
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
^ permalink raw reply related [flat|nested] 9+ messages in thread* Re: [PATCH v3 1/3] NFSD: Add a key for signing filehandles
2026-02-02 16:19 ` [PATCH v3 1/3] NFSD: Add a key for signing filehandles Benjamin Coddington
@ 2026-02-02 16:51 ` Benjamin Coddington
2026-02-02 22:31 ` kernel test robot
1 sibling, 0 replies; 9+ messages in thread
From: Benjamin Coddington @ 2026-02-02 16:51 UTC (permalink / raw)
To: Chuck Lever, Jeff Layton, NeilBrown, Trond Myklebust,
Anna Schumaker, Benjamin Coddington, Eric Biggers, Rick Macklem
Cc: linux-nfs, linux-fsdevel, linux-crypto
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
^ permalink raw reply related [flat|nested] 9+ messages in thread* Re: [PATCH v3 1/3] NFSD: Add a key for signing filehandles
2026-02-02 16:19 ` [PATCH v3 1/3] NFSD: Add a key for signing filehandles Benjamin Coddington
2026-02-02 16:51 ` Benjamin Coddington
@ 2026-02-02 22:31 ` kernel test robot
1 sibling, 0 replies; 9+ messages in thread
From: kernel test robot @ 2026-02-02 22:31 UTC (permalink / raw)
To: Benjamin Coddington, Chuck Lever, Jeff Layton, NeilBrown,
Trond Myklebust, Anna Schumaker, Eric Biggers, Rick Macklem
Cc: llvm, oe-kbuild-all, linux-nfs, linux-fsdevel, linux-crypto
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
^ permalink raw reply [flat|nested] 9+ messages in thread
* [PATCH v3 2/3] NFSD/export: Add sign_fh export option
2026-02-02 16:19 [PATCH v3 0/3] kNFSD Signed Filehandles Benjamin Coddington
2026-02-02 16:19 ` [PATCH v3 1/3] NFSD: Add a key for signing filehandles Benjamin Coddington
@ 2026-02-02 16:19 ` Benjamin Coddington
2026-02-02 16:19 ` [PATCH v3 3/3] NFSD: Sign filehandles Benjamin Coddington
2 siblings, 0 replies; 9+ messages in thread
From: Benjamin Coddington @ 2026-02-02 16:19 UTC (permalink / raw)
To: Chuck Lever, Jeff Layton, NeilBrown, Trond Myklebust,
Anna Schumaker, Benjamin Coddington, Eric Biggers, Rick Macklem
Cc: linux-nfs, linux-fsdevel, linux-crypto
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
^ permalink raw reply related [flat|nested] 9+ messages in thread* [PATCH v3 3/3] NFSD: Sign filehandles
2026-02-02 16:19 [PATCH v3 0/3] kNFSD Signed Filehandles Benjamin Coddington
2026-02-02 16:19 ` [PATCH v3 1/3] NFSD: Add a key for signing filehandles Benjamin Coddington
2026-02-02 16:19 ` [PATCH v3 2/3] NFSD/export: Add sign_fh export option Benjamin Coddington
@ 2026-02-02 16:19 ` Benjamin Coddington
2026-02-02 18:19 ` Eric Biggers
2026-02-03 7:11 ` kernel test robot
2 siblings, 2 replies; 9+ messages in thread
From: Benjamin Coddington @ 2026-02-02 16:19 UTC (permalink / raw)
To: Chuck Lever, Jeff Layton, NeilBrown, Trond Myklebust,
Anna Schumaker, Benjamin Coddington, Eric Biggers, Rick Macklem
Cc: linux-nfs, linux-fsdevel, linux-crypto
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
^ permalink raw reply related [flat|nested] 9+ messages in thread* Re: [PATCH v3 3/3] NFSD: Sign filehandles
2026-02-02 16:19 ` [PATCH v3 3/3] NFSD: Sign filehandles Benjamin Coddington
@ 2026-02-02 18:19 ` Eric Biggers
2026-02-02 18:25 ` Benjamin Coddington
2026-02-03 7:11 ` kernel test robot
1 sibling, 1 reply; 9+ messages in thread
From: Eric Biggers @ 2026-02-02 18:19 UTC (permalink / raw)
To: Benjamin Coddington
Cc: Chuck Lever, Jeff Layton, NeilBrown, Trond Myklebust,
Anna Schumaker, Rick Macklem, linux-nfs, linux-fsdevel,
linux-crypto
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
^ permalink raw reply [flat|nested] 9+ messages in thread* Re: [PATCH v3 3/3] NFSD: Sign filehandles
2026-02-02 18:19 ` Eric Biggers
@ 2026-02-02 18:25 ` Benjamin Coddington
0 siblings, 0 replies; 9+ messages in thread
From: Benjamin Coddington @ 2026-02-02 18:25 UTC (permalink / raw)
To: Eric Biggers
Cc: Chuck Lever, Jeff Layton, NeilBrown, Trond Myklebust,
Anna Schumaker, Rick Macklem, linux-nfs, linux-fsdevel,
linux-crypto
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
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: [PATCH v3 3/3] NFSD: Sign filehandles
2026-02-02 16:19 ` [PATCH v3 3/3] NFSD: Sign filehandles Benjamin Coddington
2026-02-02 18:19 ` Eric Biggers
@ 2026-02-03 7:11 ` kernel test robot
1 sibling, 0 replies; 9+ messages in thread
From: kernel test robot @ 2026-02-03 7:11 UTC (permalink / raw)
To: Benjamin Coddington, Chuck Lever, Jeff Layton, NeilBrown,
Trond Myklebust, Anna Schumaker, Eric Biggers, Rick Macklem
Cc: oe-kbuild-all, linux-nfs, linux-fsdevel, linux-crypto
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
^ permalink raw reply [flat|nested] 9+ messages in thread