[CIFS] use krb5 session key from first SMB session after a NegProt

Currently, any new kerberos SMB session overwrites the server's session
key. The session key should only be set by the first SMB session set up
on the socket.
Signed-off-by: NJeff Layton <jlayton@tupile.poochiereds.net>
Signed-off-by: NSteve French <sfrench@us.ibm.com>

fs/cifs/sess.c

@@ -528,9 +528,11 @@ CIFS_SessSetup(unsigned int xid, struct cifsSesInfo *ses, int first_time,
 			rc = -EOVERFLOW;
 			goto ssetup_exit;
 		}
-		ses->server->mac_signing_key.len = msg->sesskey_len;
-		memcpy(ses->server->mac_signing_key.data.krb5, msg->data,
-			msg->sesskey_len);
+		if (first_time) {
+			ses->server->mac_signing_key.len = msg->sesskey_len;
+			memcpy(ses->server->mac_signing_key.data.krb5,
+				msg->data, msg->sesskey_len);
+		}
 		pSMB->req.hdr.Flags2 |= SMBFLG2_EXT_SEC;
 		capabilities |= CAP_EXTENDED_SECURITY;
 		pSMB->req.Capabilities = cpu_to_le32(capabilities);