From mboxrd@z Thu Jan 1 00:00:00 1970 From: William Allen Simpson Subject: query: redundant tcp header length checks? Date: Sun, 10 Jan 2010 08:02:57 -0500 Message-ID: <4B49D001.4000302@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Linux Kernel Network Developers , =?ISO-8859-1?Q?Ilpo_J=E4rvinen?= To: Linux Kernel Developers Return-path: Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On a related note, back on Nov 10th, Ilpo brought to my attention -- *hidden* inside the pskb_may_pull() -- the tcp header length is range checked for being too short (skb->len < th->doff * 4). tcp_v4_rcv() ... 1585 if (!pskb_may_pull(skb, th->doff * 4)) 1586 goto discard_it; 1587 But the next comment in tcp_ipv4.c (missing from tcp_ipv6.c) says: 1588 /* An explanation is required here, I think. 1589 * Packet length and doff are validated by header prediction, 1590 * provided case of th->doff==0 is eliminated. 1591 * So, we defer the checks. */ That's not correct anymore, as the checks aren't deferred! But the redundant deferred checks _are_ still present in tcp_input.c: tcp_rcv_established() ... 5210 if (len <= tcp_header_len) { 5211 /* Bulk data transfer: sender */ 5212 if (len == tcp_header_len) { ... 5229 } else { /* Header too small */ 5230 TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_INERRS); 5231 goto discard; 5232 } Also: 5325 slow_path: 5326 if (len < (th->doff << 2) || tcp_checksum_complete_user(sk, skb)) 5327 goto csum_error; Apparently tcp_rcv_established() might be called through some other path that requires this extra header length checking? Its callers also have another reference, for example in tcp_ipv4.c: 2432 .backlog_rcv = tcp_v4_do_rcv, And tcp_ipv6.c: 213 sk->sk_backlog_rcv = tcp_v4_do_rcv; 223 sk->sk_backlog_rcv = tcp_v6_do_rcv; 1302 newsk->sk_backlog_rcv = tcp_v4_do_rcv; 2073 .backlog_rcv = tcp_v6_do_rcv, Can anybody explain these?