nfsd: permit unauthenticated stat of export root

RFC 2623 section 2.3.2 permits the server to bypass gss authentication
checks for certain operations that a client may perform when mounting.
In the case of a client that doesn't have some form of credentials
available to it on boot, this allows it to perform the mount unattended.
(Presumably real file access won't be needed until a user with
credentials logs in.)

Being slightly more lenient allows lots of old clients to access
krb5-only exports, with the only loss being a small amount of
information leaked about the root directory of the export.

This affects only v2 and v3; v4 still requires authentication for all
access.

Thanks to Peter Staubach testing against a Solaris client, which
suggesting addition of v3 getattr, to the list, and to Trond for noting
that doing so exposes no additional information.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Cc: Peter Staubach <staubach@redhat.com>
Cc: Trond Myklebust <trond.myklebust@fys.uio.no>

1 files changed, 20 insertions, 10 deletions

diff --git a/fs/nfsd/nfsfh.c b/fs/nfsd/nfsfh.c
index ea37c96f0445..cd25d91895a1 100644
--- a/fs/nfsd/nfsfh.c
+++ b/fs/nfsd/nfsfh.c
@@ -302,17 +302,27 @@ fh_verify(struct svc_rqst *rqstp, struct svc_fh *fhp, int type, int access)
         if (error)
                 goto out;
 
-        if (!(access & NFSD_MAY_LOCK)) {
-                /*
-                 * pseudoflavor restrictions are not enforced on NLM,
-                 * which clients virtually always use auth_sys for,
-                 * even while using RPCSEC_GSS for NFS.
-                 */
-                error = check_nfsd_access(exp, rqstp);
-                if (error)
-                        goto out;
-        }
+        /*
+         * pseudoflavor restrictions are not enforced on NLM,
+         * which clients virtually always use auth_sys for,
+         * even while using RPCSEC_GSS for NFS.
+         */
+        if (access & NFSD_MAY_LOCK)
+                goto skip_pseudoflavor_check;
+        /*
+         * 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
+         * of rfc 2623.
+         */
+        if (access & NFSD_MAY_BYPASS_GSS_ON_ROOT
+                        && exp->ex_path.dentry == dentry)
+                goto skip_pseudoflavor_check;
+
+        error = check_nfsd_access(exp, rqstp);
+        if (error)
+                goto out;
 
+skip_pseudoflavor_check:
         /* Finally, check access permissions. */
         error = nfsd_permission(rqstp, exp, dentry, access);