[PATCH 6.6.y] nfsd: decouple the xprtsec policy check from check_nfsd_access()

Scott Mayhew
2025-10-20T20:50:10Z

[ Upstream commit e4f574ca9c6dfa66695bb054ff5df43ecea873ec ]

This is a backport of e4f574ca9c6d specifically for the 6.6-stable
kernel. It differs from the upstream version mainly in that it's
working around the absence of some 6.12-era commits:
- 1459ad57673b nfsd: Move error code mapping to per-version proc code.
- 0a183f24a7ae NFSD: Handle @rqstp == NULL in check_nfsd_access()
- 5e66d2d92a1c nfsd: factor out __fh_verify to allow NULL rqstp to be
passed

A while back I had reported that an NFSv3 client could successfully
mount using '-o xprtsec=none' an export that had been exported with
'xprtsec=tls:mtls'. By "successfully" I mean that the mount command
would succeed and the mount would show up in /proc/mount. Attempting
to do anything futher with the mount would be met with NFS3ERR_ACCES.

Transport Layer Security isn't an RPC security flavor or pseudo-flavor,
so we shouldn't be conflating them when determining whether the access
checks can be bypassed. Split check_nfsd_access() into two helpers, and
have fh_verify() call the helpers directly since fh_verify() has
logic that allows one or both of the checks to be skipped. All other
sites will continue to call check_nfsd_access().

Link: https://lore.kernel.org/linux-nfs/ZjO3Qwf_G87yNXb2@aion/
Fixes: 9280c5774314 ("NFSD: Handle new xprtsec= export option")
Signed-off-by: Scott Mayhew <smayhew@redhat.com>
---
fs/nfsd/export.c | 60 +++++++++++++++++++++++++++++++++++++++++-------
fs/nfsd/export.h | 2 ++
fs/nfsd/nfsfh.c | 12 +++++++++-
3 files changed, 65 insertions(+), 9 deletions(-)

diff --git a/fs/nfsd/export.c b/fs/nfsd/export.c
index 4b5d998cbc2f..f4e77859aa85 100644
--- a/fs/nfsd/export.c
+++ b/fs/nfsd/export.c
@@ -1071,28 +1071,62 @@ static struct svc_export *exp_find(struct cache_detail *cd,
return exp;
}

-__be32 check_nfsd_access(struct svc_export *exp, struct svc_rqst *rqstp)
+/**
+ * check_xprtsec_policy - check if access to export is allowed by the
+ * xprtsec policy
+ * @exp: svc_export that is being accessed.
+ * @rqstp: svc_rqst attempting to access @exp.
+ *
+ * Helper function for check_nfsd_access(). Note that callers should be
+ * using check_nfsd_access() instead of calling this function directly. The
+ * one exception is fh_verify() since it has logic that may result in one
+ * or both of the helpers being skipped.
+ *
+ * Return values:
+ * %nfs_ok if access is granted, or
+ * %nfserr_acces or %nfserr_wrongsec if access is denied
+ */
+__be32 check_xprtsec_policy(struct svc_export *exp, struct svc_rqst *rqstp)
{
- struct exp_flavor_info *f, *end = exp->ex_flavors + exp->ex_nflavors;
struct svc_xprt *xprt = rqstp->rq_xprt;

if (exp->ex_xprtsec_modes & NFSEXP_XPRTSEC_NONE) {
if (!test_bit(XPT_TLS_SESSION, &xprt->xpt_flags))
- goto ok;
+ return nfs_ok;
}
if (exp->ex_xprtsec_modes & NFSEXP_XPRTSEC_TLS) {
if (test_bit(XPT_TLS_SESSION, &xprt->xpt_flags) &&
!test_bit(XPT_PEER_AUTH, &xprt->xpt_flags))
- goto ok;
+ return nfs_ok;
}
if (exp->ex_xprtsec_modes & NFSEXP_XPRTSEC_MTLS) {
if (test_bit(XPT_TLS_SESSION, &xprt->xpt_flags) &&
test_bit(XPT_PEER_AUTH, &xprt->xpt_flags))
- goto ok;
+ return nfs_ok;
}
- goto denied;

-ok:
+ return rqstp->rq_vers < 4 ? nfserr_acces : nfserr_wrongsec;
+}
+
+/**
+ * check_security_flavor - check if access to export is allowed by the
+ * xprtsec policy
+ * @exp: svc_export that is being accessed.
+ * @rqstp: svc_rqst attempting to access @exp.
+ *
+ * Helper function for check_nfsd_access(). Note that callers should be
+ * using check_nfsd_access() instead of calling this function directly. The
+ * one exception is fh_verify() since it has logic that may result in one
+ * or both of the helpers being skipped.
+ *
+ * Return values:
+ * %nfs_ok if access is granted, or
+ * %nfserr_acces or %nfserr_wrongsec if access is denied
+ */
+__be32 check_security_flavor(struct svc_export *exp, struct svc_rqst *rqstp)
+{
+ struct exp_flavor_info *f, *end = exp->ex_flavors + exp->ex_nflavors;
+
/* legacy gss-only clients are always OK: */
if (exp->ex_client == rqstp->rq_gssclient)
return 0;
@@ -1117,10 +1151,20 @@ __be32 check_nfsd_access(struct svc_export *exp, struct svc_rqst *rqstp)
if (nfsd4_spo_must_allow(rqstp))
return 0;

-denied:
return rqstp->rq_vers < 4 ? nfserr_acces : nfserr_wrongsec;
}

+__be32 check_nfsd_access(struct svc_export *exp, struct svc_rqst *rqstp)
+{
+ __be32 status;
+
+ status = check_xprtsec_policy(exp, rqstp);
+ if (status != nfs_ok)
+ return status;
+
+ return check_security_flavor(exp, rqstp);
+}
+
/*
* Uses rq_client and rq_gssclient to find an export; uses rq_client (an
* auth_unix client) if it's available and has secinfo information;
diff --git a/fs/nfsd/export.h b/fs/nfsd/export.h
index ca9dc230ae3d..4a48b2ad5606 100644
--- a/fs/nfsd/export.h
+++ b/fs/nfsd/export.h
@@ -100,6 +100,8 @@ struct svc_expkey {
#define EX_WGATHER(exp) ((exp)->ex_flags & NFSEXP_GATHERED_WRITES)

int nfsexp_flags(struct svc_rqst *rqstp, struct svc_export *exp);
+__be32 check_xprtsec_policy(struct svc_export *exp, struct svc_rqst *rqstp);
+__be32 check_security_flavor(struct svc_export *exp, struct svc_rqst *rqstp);
__be32 check_nfsd_access(struct svc_export *exp, struct svc_rqst *rqstp);

/*
diff --git a/fs/nfsd/nfsfh.c b/fs/nfsd/nfsfh.c
index c2495d98c189..283c1a60c846 100644
--- a/fs/nfsd/nfsfh.c
+++ b/fs/nfsd/nfsfh.c
@@ -370,6 +370,16 @@ fh_verify(struct svc_rqst *rqstp, struct svc_fh *fhp, umode_t type, int access)
if (error)
goto out;

+ /*
+ * NLM is allowed to bypass the xprtsec policy check because lockd
+ * doesn't support xprtsec.
+ */
+ if (!(access & NFSD_MAY_LOCK)) {
+ error = check_xprtsec_policy(exp, rqstp);
+ if (error)
+ goto out;
+ }
+
/*
* pseudoflavor restrictions are not enforced on NLM,
* which clients virtually always use auth_sys for,
@@ -386,7 +396,7 @@ fh_verify(struct svc_rqst *rqstp, struct svc_fh *fhp, umode_t type, int access)
&& exp->ex_path.dentry == dentry)
goto skip_pseudoflavor_check;

- error = check_nfsd_access(exp, rqstp);
+ error = check_security_flavor(exp, rqstp);
if (error)
goto out;

--
2.47.3