nfsd: fix EXDEV checking in rename

We again check for the EXDEV a little later on, so the first check is
redundant.  This check is also slightly racier, since a badly timed
eviction from the export cache could leave us with the two fh_export
pointers pointing to two different cache entries which each refer to the
same underlying export.

It's better to compare vfsmounts as the later check does, but that
leaves a minor security hole in the case where the two exports refer to
two different directories especially if (for example) they have
different root-squashing options.

So, compare ex_path.dentry too.
Reported-by: Joe Habermann <joe.habermann@gmail.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/vfs.c

@@ -1758,10 +1758,6 @@ nfsd_rename(struct svc_rqst *rqstp, struct svc_fh *ffhp, char *fname, int flen,
 	tdentry = tfhp->fh_dentry;
 	tdir = tdentry->d_inode;
 
-	err = (rqstp->rq_vers == 2) ? nfserr_acces : nfserr_xdev;
-	if (ffhp->fh_export != tfhp->fh_export)
-		goto out;
-
 	err = nfserr_perm;
 	if (!flen || isdotent(fname, flen) || !tlen || isdotent(tname, tlen))
 		goto out;
@@ -1802,6 +1798,8 @@ nfsd_rename(struct svc_rqst *rqstp, struct svc_fh *ffhp, char *fname, int flen,
 	host_err = -EXDEV;
 	if (ffhp->fh_export->ex_path.mnt != tfhp->fh_export->ex_path.mnt)
 		goto out_dput_new;
+	if (ffhp->fh_export->ex_path.dentry != tfhp->fh_export->ex_path.dentry)
+		goto out_dput_new;
 
 	host_err = nfsd_break_lease(odentry->d_inode);
 	if (host_err)