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 <[e-mail]>
Cc: Peter Staubach <[e-mail]>
Cc: Trond Myklebust <[e-mail]>

fs/nfsd/nfs3proc.c

···
 		SVCFH_fmt(&argp->fh));
 
 	fh_copy(&resp->fh, &argp->fh);
-	nfserr = fh_verify(rqstp, &resp->fh, 0, NFSD_MAY_NOP);
+	nfserr = fh_verify(rqstp, &resp->fh, 0,
+			NFSD_MAY_NOP | NFSD_MAY_BYPASS_GSS_ON_ROOT);
 	if (nfserr)
 		RETURN_STATUS(nfserr);
 
···
 	dprintk("nfsd: FSSTAT(3)   %s\n",
 				SVCFH_fmt(&argp->fh));
 
-	nfserr = nfsd_statfs(rqstp, &argp->fh, &resp->stats);
+	nfserr = nfsd_statfs(rqstp, &argp->fh, &resp->stats, 0);
 	fh_put(&argp->fh);
 	RETURN_STATUS(nfserr);
 }
···
 	resp->f_maxfilesize = ~(u32) 0;
 	resp->f_properties = NFS3_FSF_DEFAULT;
 
-	nfserr = fh_verify(rqstp, &argp->fh, 0, NFSD_MAY_NOP);
+	nfserr = fh_verify(rqstp, &argp->fh, 0,
+			NFSD_MAY_NOP | NFSD_MAY_BYPASS_GSS_ON_ROOT);
 
 	/* Check special features of the file system. May request
 	 * different read/write sizes for file systems known to have

fs/nfsd/nfsfh.c

···
 	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);
 

fs/nfsd/nfsproc.c

···
 	dprintk("nfsd: GETATTR  %s\n", SVCFH_fmt(&argp->fh));
 
 	fh_copy(&resp->fh, &argp->fh);
-	nfserr = fh_verify(rqstp, &resp->fh, 0, NFSD_MAY_NOP);
+	nfserr = fh_verify(rqstp, &resp->fh, 0,
+			NFSD_MAY_NOP | NFSD_MAY_BYPASS_GSS_ON_ROOT);
 	return nfsd_return_attrs(nfserr, resp);
 }
 
···
 
 	dprintk("nfsd: STATFS   %s\n", SVCFH_fmt(&argp->fh));
 
-	nfserr = nfsd_statfs(rqstp, &argp->fh, &resp->stats);
+	nfserr = nfsd_statfs(rqstp, &argp->fh, &resp->stats,
+			NFSD_MAY_BYPASS_GSS_ON_ROOT);
 	fh_put(&argp->fh);
 	return nfserr;
 }

fs/nfsd/vfs.c

···
  * N.B. After this call fhp needs an fh_put
  */
 __be32
-nfsd_statfs(struct svc_rqst *rqstp, struct svc_fh *fhp, struct kstatfs *stat)
+nfsd_statfs(struct svc_rqst *rqstp, struct svc_fh *fhp, struct kstatfs *stat, int access)
 {
-	__be32 err = fh_verify(rqstp, fhp, 0, NFSD_MAY_NOP);
+	__be32 err = fh_verify(rqstp, fhp, 0, NFSD_MAY_NOP | access);
 	if (!err && vfs_statfs(fhp->fh_dentry,stat))
 		err = nfserr_io;
 	return err;

include/linux/nfsd/nfsd.h

···
 #define NFSD_MAY_LOCK		32
 #define NFSD_MAY_OWNER_OVERRIDE	64
 #define NFSD_MAY_LOCAL_ACCESS	128 /* IRIX doing local access check on device special file*/
+#define NFSD_MAY_BYPASS_GSS_ON_ROOT 256
 
 #define NFSD_MAY_CREATE		(NFSD_MAY_EXEC|NFSD_MAY_WRITE)
 #define NFSD_MAY_REMOVE		(NFSD_MAY_EXEC|NFSD_MAY_WRITE|NFSD_MAY_TRUNC)
···
 __be32		nfsd_readdir(struct svc_rqst *, struct svc_fh *,
 			     loff_t *, struct readdir_cd *, filldir_t);
 __be32		nfsd_statfs(struct svc_rqst *, struct svc_fh *,
-				struct kstatfs *);
+				struct kstatfs *, int access);
 
 int		nfsd_notify_change(struct inode *, struct iattr *);
 __be32		nfsd_permission(struct svc_rqst *, struct svc_export *,