kern/49788: vlan(4) extra padding

[ozaki-r%netbsd.org@localhost]

>Number:         49788
>Category:       kern
>Synopsis:       vlan(4) extra padding
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Thu Mar 26 08:15:00 +0000 2015
>Originator:     Ryota Ozaki
>Release:        current
>Organization:
IIJ
>Environment:
NetBSD kvm 7.99.7 NetBSD 7.99.7 (KVM) #141: Thu Mar 26 15:16:00 JST 2015  ozaki-r@rangeley:(hidden) amd64

>Description:
http://nxr.netbsd.org/xref/src/sys/net/if_vlan.c#808

Here vlan pads a frame with zeros up to 68 bytes
(ETHER_MIN_LEN + ETHER_VLAN_ENCAP_LEN). It expects
that if the frame is untagged, it keeps 64 bytes
at least. However, it lacks concern about CRC
(4 bytes). So a sending frame will be 72 (68 + 4) bytes.
>How-To-Repeat:
Create and set up a vlan interface, ping to somewhere via it and see packet size.
>Fix:
diff --git a/sys/net/if_vlan.c b/sys/net/if_vlan.c
index 2d5b861..02b6686 100644
--- a/sys/net/if_vlan.c
+++ b/sys/net/if_vlan.c
@@ -806,9 +806,10 @@ vlan_start(struct ifnet *ifp)
                                 * after deleting a tag.
                                 */
                                if (m->m_pkthdr.len <
-                                   (ETHER_MIN_LEN + ETHER_VLAN_ENCAP_LEN)) {
+                                   (ETHER_MIN_LEN - ETHER_CRC_LEN +
+                                    ETHER_VLAN_ENCAP_LEN)) {
                                        m_copyback(m, m->m_pkthdr.len,
-                                           (ETHER_MIN_LEN +
+                                           (ETHER_MIN_LEN - ETHER_CRC_LEN +
                                             ETHER_VLAN_ENCAP_LEN) -
                                             m->m_pkthdr.len,
                                            vlan_zero_pad_buff);