Currently NFSD_MAY_BYPASS_GSS and NFSD_MAY_BYPASS_GSS_ON_ROOT do not bypass
only GSS, but bypass any method. This is a problem specially for NFS3
AUTH_NULL-only exports.

The purpose of NFSD_MAY_BYPASS_GSS_ON_ROOT is described in RFC 2623,
section 2.3.2, to allow mounting NFS2/3 GSS-only export without
authentication. So few procedures which do not expose security risk used
during mount time can be called also with AUTH_NONE or AUTH_SYS, to allow
client mount operation to finish successfully.

The problem with current implementation is that for AUTH_NULL-only exports,
the NFSD_MAY_BYPASS_GSS_ON_ROOT is active also for NFS3 AUTH_UNIX mount
attempts which confuse NFS3 clients, and make them think that AUTH_UNIX is
enabled and is working. Linux NFS3 client never switches from AUTH_UNIX to
AUTH_NONE on active mount, which makes the mount inaccessible.

Fix the NFSD_MAY_BYPASS_GSS and NFSD_MAY_BYPASS_GSS_ON_ROOT implementation
and really allow to bypass only exports which have enabled some real
authentication (GSS, TLS, or any other).

The result would be: For AUTH_NULL-only export if client attempts to do
mount with AUTH_UNIX flavor then it will receive access errors, which
instruct client that AUTH_UNIX flavor is not usable and will either try
other auth flavor (AUTH_NULL if enabled) or fails mount procedure.
Similarly if client attempt to do mount with AUTH_NULL flavor and only
AUTH_UNIX flavor is enabled then the client will receive access error.

This should fix problems with AUTH_NULL-only or AUTH_UNIX-only exports if
client attempts to mount it with other auth flavor (e.g. with AUTH_NULL for
AUTH_UNIX-only export, or with AUTH_UNIX for AUTH_NULL-only export).

Signed-off-by: Pali Rohár <pali@kernel.org>

---
Changes in v2:
* Apply whole NFSD_MAY_BYPASS_GSS and NFSD_MAY_BYPASS_GSS_ON_ROOT logic not
  only for GSS, but also for any method with the real authentication
  (anything except RPC_AUTH_NULL, RPC_AUTH_UNIX and RPC_AUTH_SHORT).
---
 fs/nfsd/export.c   | 19 ++++++++++++++++++-
 fs/nfsd/export.h   |  2 +-
 fs/nfsd/nfs4proc.c |  2 +-
 fs/nfsd/nfs4xdr.c  |  2 +-
 fs/nfsd/nfsfh.c    | 12 +++++++++---
 fs/nfsd/vfs.c      |  2 +-
 6 files changed, 31 insertions(+), 8 deletions(-)

diff --git a/fs/nfsd/export.c b/fs/nfsd/export.c
index 50b3135d07ac..8a12876b9335 100644
--- a/fs/nfsd/export.c
+++ b/fs/nfsd/export.c
@@ -1074,7 +1074,7 @@ static struct svc_export *exp_find(struct cache_detail *cd,
 	return exp;
 }
 
-__be32 check_nfsd_access(struct svc_export *exp, struct svc_rqst *rqstp)
+__be32 check_nfsd_access(struct svc_export *exp, struct svc_rqst *rqstp, bool may_bypass_gss)
 {
 	struct exp_flavor_info *f, *end = exp->ex_flavors + exp->ex_nflavors;
 	struct svc_xprt *xprt = rqstp->rq_xprt;
@@ -1120,6 +1120,23 @@ __be32 check_nfsd_access(struct svc_export *exp, struct svc_rqst *rqstp)
 	if (nfsd4_spo_must_allow(rqstp))
 		return 0;
 
+	/* Some calls may be processed without authentication
+	 * on GSS exports. For example NFS2/3 calls on root
+	 * directory, see section 2.3.2 of rfc 2623.
+	 * For "may_bypass_gss" check that export has really
+	 * enabled some flavor with authentication (GSS or any
+	 * other) and also check that the used auth flavor is
+	 * without authentication (none or sys).
+	 */
+	if (may_bypass_gss && (
+	     rqstp->rq_cred.cr_flavor == RPC_AUTH_NULL ||
+	     rqstp->rq_cred.cr_flavor == RPC_AUTH_UNIX)) {
+		for (f = exp->ex_flavors; f < end; f++) {
+			if (f->pseudoflavor >= RPC_AUTH_DES)
+				return 0;
+		}
+	}
+
 denied:
 	return rqstp->rq_vers < 4 ? nfserr_acces : nfserr_wrongsec;
 }
diff --git a/fs/nfsd/export.h b/fs/nfsd/export.h
index ca9dc230ae3d..dc7cf4f6ac53 100644
--- a/fs/nfsd/export.h
+++ b/fs/nfsd/export.h
@@ -100,7 +100,7 @@ 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_nfsd_access(struct svc_export *exp, struct svc_rqst *rqstp);
+__be32 check_nfsd_access(struct svc_export *exp, struct svc_rqst *rqstp, bool may_bypass_gss);
 
 /*
  * Function declarations
diff --git a/fs/nfsd/nfs4proc.c b/fs/nfsd/nfs4proc.c
index 2e39cf2e502a..0f67f4a7b8b2 100644
--- a/fs/nfsd/nfs4proc.c
+++ b/fs/nfsd/nfs4proc.c
@@ -2791,7 +2791,7 @@ nfsd4_proc_compound(struct svc_rqst *rqstp)
 
 			if (current_fh->fh_export &&
 					need_wrongsec_check(rqstp))
-				op->status = check_nfsd_access(current_fh->fh_export, rqstp);
+				op->status = check_nfsd_access(current_fh->fh_export, rqstp, false);
 		}
 encode_op:
 		if (op->status == nfserr_replay_me) {
diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c
index 97f583777972..b45ea5757652 100644
--- a/fs/nfsd/nfs4xdr.c
+++ b/fs/nfsd/nfs4xdr.c
@@ -3775,7 +3775,7 @@ nfsd4_encode_entry4_fattr(struct nfsd4_readdir *cd, const char *name,
 			nfserr = nfserrno(err);
 			goto out_put;
 		}
-		nfserr = check_nfsd_access(exp, cd->rd_rqstp);
+		nfserr = check_nfsd_access(exp, cd->rd_rqstp, false);
 		if (nfserr)
 			goto out_put;
 
diff --git a/fs/nfsd/nfsfh.c b/fs/nfsd/nfsfh.c
index dd4e11a703aa..ed0eabfa3cb0 100644
--- a/fs/nfsd/nfsfh.c
+++ b/fs/nfsd/nfsfh.c
@@ -329,6 +329,7 @@ fh_verify(struct svc_rqst *rqstp, struct svc_fh *fhp, umode_t type, int access)
 {
 	struct nfsd_net *nn = net_generic(SVC_NET(rqstp), nfsd_net_id);
 	struct svc_export *exp = NULL;
+	bool may_bypass_gss = false;
 	struct dentry	*dentry;
 	__be32		error;
 
@@ -375,8 +376,13 @@ fh_verify(struct svc_rqst *rqstp, struct svc_fh *fhp, umode_t type, int access)
 	 * which clients virtually always use auth_sys for,
 	 * even while using RPCSEC_GSS for NFS.
 	 */
-	if (access & NFSD_MAY_LOCK || access & NFSD_MAY_BYPASS_GSS)
+	if (access & NFSD_MAY_LOCK)
 		goto skip_pseudoflavor_check;
+	/*
+	 * NFS4 PUTFH may bypass GSS (see nfsd4_putfh() in nfs4proc.c).
+	 */
+	if (access & NFSD_MAY_BYPASS_GSS)
+		may_bypass_gss = true;
 	/*
 	 * Clients may expect to be able to use auth_sys during mount,
 	 * even if they use gss for everything else; see section 2.3.2
@@ -384,9 +390,9 @@ fh_verify(struct svc_rqst *rqstp, struct svc_fh *fhp, umode_t type, int access)
 	 */
 	if (access & NFSD_MAY_BYPASS_GSS_ON_ROOT
 			&& exp->ex_path.dentry == dentry)
-		goto skip_pseudoflavor_check;
+		may_bypass_gss = true;
 
-	error = check_nfsd_access(exp, rqstp);
+	error = check_nfsd_access(exp, rqstp, may_bypass_gss);
 	if (error)
 		goto out;
 
diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index 29b1f3613800..b2f5ea7c2187 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -320,7 +320,7 @@ nfsd_lookup(struct svc_rqst *rqstp, struct svc_fh *fhp, const char *name,
 	err = nfsd_lookup_dentry(rqstp, fhp, name, len, &exp, &dentry);
 	if (err)
 		return err;
-	err = check_nfsd_access(exp, rqstp);
+	err = check_nfsd_access(exp, rqstp, false);
 	if (err)
 		goto out;
 	/*
-- 
2.20.1

From: Chuck Lever <chuck.lever@oracle.com>

On Sat, 05 Oct 2024 18:40:39 +0200, Pali Rohár wrote:                                              
> Currently NFSD_MAY_BYPASS_GSS and NFSD_MAY_BYPASS_GSS_ON_ROOT do not bypass
> only GSS, but bypass any method. This is a problem specially for NFS3
> AUTH_NULL-only exports.
> 
> The purpose of NFSD_MAY_BYPASS_GSS_ON_ROOT is described in RFC 2623,
> section 2.3.2, to allow mounting NFS2/3 GSS-only export without
> authentication. So few procedures which do not expose security risk used
> during mount time can be called also with AUTH_NONE or AUTH_SYS, to allow
> client mount operation to finish successfully.
> 
> [...]                                                                        

Applied to nfsd-next for v6.13, thanks!                                                                

[1/1] nfsd: Fix NFSD_MAY_BYPASS_GSS and NFSD_MAY_BYPASS_GSS_ON_ROOT
      commit: fa3d3ae84c5a6e9bd406c9ef75d3128a46cf1109                                                                      

--                                                                              
Chuck Lever