nfsd4: fix READ permission checking

In the case we already have a struct file (derived from a stateid), we
still need to do permission-checking; otherwise an unauthorized user
could gain access to a file by sniffing or guessing somebody else's
stateid.

Cc: stable@vger.kernel.org
Fixes: dc97618d "nfsd4: separate splice and readv cases"
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4xdr.c

@@ -3422,6 +3422,7 @@ nfsd4_encode_read(struct nfsd4_compoundres *resp, __be32 nfserr,
 	unsigned long maxcount;
 	struct xdr_stream *xdr = &resp->xdr;
 	struct file *file = read->rd_filp;
+	struct svc_fh *fhp = read->rd_fhp;
 	int starting_len = xdr->buf->len;
 	struct raparms *ra;
 	__be32 *p;
@@ -3445,12 +3446,15 @@ nfsd4_encode_read(struct nfsd4_compoundres *resp, __be32 nfserr,
 	maxcount = min_t(unsigned long, maxcount, (xdr->buf->buflen - xdr->buf->len));
 	maxcount = min_t(unsigned long, maxcount, read->rd_length);
 
-	if (!read->rd_filp) {
+	if (read->rd_filp)
+		err = nfsd_permission(resp->rqstp, fhp->fh_export,
+				fhp->fh_dentry,
+				NFSD_MAY_READ|NFSD_MAY_OWNER_OVERRIDE);
+	else
 		err = nfsd_get_tmp_read_open(resp->rqstp, read->rd_fhp,
 						&file, &ra);
 	if (err)
 		goto err_truncate;
-	}
 
 	if (file->f_op->splice_read && test_bit(RQ_SPLICE_OK, &resp->rqstp->rq_flags))
 		err = nfsd4_encode_splice_read(resp, read, file, maxcount);