nfsd: decouple the xprtsec policy check from check_nfsd_access()

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.

This was fixed (albeit accidentally) by commit bb4f07f2409c ("nfsd:
Fix NFSD_MAY_BYPASS_GSS and NFSD_MAY_BYPASS_GSS_ON_ROOT") and was
subsequently re-broken by commit 0813c5f01249 ("nfsd: fix access
checking for NLM under XPRTSEC policies").

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")
Cc: [e-mail]
Signed-off-by: Scott Mayhew <[e-mail]>
Signed-off-by: Chuck Lever <[e-mail]>

fs/nfsd/export.c

···
 }
 
 /**
- * check_nfsd_access - check if access to export is allowed.
+ * 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 (will be NULL for LOCALIO).
- * @may_bypass_gss: reduce strictness of authorization check
+ * @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_wrongsec if access is denied
  */
-__be32 check_nfsd_access(struct svc_export *exp, struct svc_rqst *rqstp,
-			 bool may_bypass_gss)
+__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;
-
-	/*
-	 * If rqstp is NULL, this is a LOCALIO request which will only
-	 * ever use a filehandle/credential pair for which access has
-	 * been affirmed (by ACCESS or OPEN NFS requests) over the
-	 * wire. So there is no need for further checks here.
-	 */
-	if (!rqstp)
-		return nfs_ok;
-
-	xprt = rqstp->rq_xprt;
+	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;
 	}
-	if (!may_bypass_gss)
-		goto denied;
+	return nfserr_wrongsec;
+}
 
-ok:
+/**
+ * check_security_flavor - check if access to export is allowed by the
+ *			   security flavor
+ * @exp: svc_export that is being accessed.
+ * @rqstp: svc_rqst attempting to access @exp.
+ * @may_bypass_gss: reduce strictness of authorization check
+ *
+ * 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_wrongsec if access is denied
+ */
+__be32 check_security_flavor(struct svc_export *exp, struct svc_rqst *rqstp,
+			     bool may_bypass_gss)
+{
+	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 nfs_ok;
···
 		}
 	}
 
-denied:
 	return nfserr_wrongsec;
+}
+
+/**
+ * check_nfsd_access - check if access to export is allowed.
+ * @exp: svc_export that is being accessed.
+ * @rqstp: svc_rqst attempting to access @exp.
+ * @may_bypass_gss: reduce strictness of authorization check
+ *
+ * Return values:
+ *   %nfs_ok if access is granted, or
+ *   %nfserr_wrongsec if access is denied
+ */
+__be32 check_nfsd_access(struct svc_export *exp, struct svc_rqst *rqstp,
+			 bool may_bypass_gss)
+{
+	__be32 status;
+
+	status = check_xprtsec_policy(exp, rqstp);
+	if (status != nfs_ok)
+		return status;
+	return check_security_flavor(exp, rqstp, may_bypass_gss);
 }
 
 /*

fs/nfsd/export.h

···
 
 struct svc_cred;
 int nfsexp_flags(struct svc_cred *cred, 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,
+			     bool may_bypass_gss);
 __be32 check_nfsd_access(struct svc_export *exp, struct svc_rqst *rqstp,
 			 bool may_bypass_gss);
 

fs/nfsd/nfsfh.c

···
 	if (error)
 		goto out;
 
+	/*
+	 * If rqstp is NULL, this is a LOCALIO request which will only
+	 * ever use a filehandle/credential pair for which access has
+	 * been affirmed (by ACCESS or OPEN NFS requests) over the
+	 * wire.  Skip both the xprtsec policy and the security flavor
+	 * checks.
+	 */
+	if (!rqstp)
+		goto check_permissions;
+
 	if ((access & NFSD_MAY_NLM) && (exp->ex_flags & NFSEXP_NOAUTHNLM))
 		/* NLM is allowed to fully bypass authentication */
 		goto out;
+
+	/*
+	 * NLM is allowed to bypass the xprtsec policy check because lockd
+	 * doesn't support xprtsec.
+	 */
+	if (!(access & NFSD_MAY_NLM)) {
+		error = check_xprtsec_policy(exp, rqstp);
+		if (error)
+			goto out;
+	}
 
 	if (access & NFSD_MAY_BYPASS_GSS)
 		may_bypass_gss = true;
···
 			&& exp->ex_path.dentry == dentry)
 		may_bypass_gss = true;
 
-	error = check_nfsd_access(exp, rqstp, may_bypass_gss);
+	error = check_security_flavor(exp, rqstp, may_bypass_gss);
 	if (error)
 		goto out;
+
 	/* During LOCALIO call to fh_verify will be called with a NULL rqstp */
 	if (rqstp)
 		svc_xprt_set_valid(rqstp->rq_xprt);
 
+check_permissions:
 	/* Finally, check access permissions. */
 	error = nfsd_permission(cred, exp, dentry, access);
 out: