kern/109815: wrong interface identifier at pfil_hooks for vlans + if_bridge

Eygene Ryabinkin rea-fbsd at
Sat Mar 3 21:20:05 UTC 2007

>Number:         109815
>Category:       kern
>Synopsis:       wrong interface identifier at pfil_hooks for vlans + if_bridge
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    freebsd-bugs
>State:          open
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Sat Mar 03 21:20:04 GMT 2007
>Originator:     Eygene Ryabinkin
>Release:        FreeBSD 6.2-STABLE i386
Code Labs
System: FreeBSD XXX 6.2-STABLE FreeBSD 6.2-STABLE #13: Mon Feb 12 15:59:07 MSK 2007 root at XXX:/usr/obj/usr/src/sys/XXX i386
When if_bridge is used to gather multiple vlan interfaces that have
the same physical parent (I will call such vlan interfaces the 'vlan
group') the interface identifier that will be passed to the
pfil_hooks will be sometimes wrong. For all packets coming from the
'vlan group' and destined to some local interface the incoming interface
passed to the pfil_hooks will be the last interface in that group
regardless of the actual incoming interface. The 'last interface' is
the interface that was added to the if_bridge at the very last 'addm'

The problem is lying in the fact that MAC addresses of the 'vlan group'
are just the same and they are equal to the MAC address of the parent
interface. And the check for the unicat packet that is destined for
'us' in the if_bridge.c:bridge_input() is walking by the list of the
if_bridge-attached interfaces and compares the MAC addresses to the
packet's one. Once match is found the interface in the packet header
will be rewritten to the found list entry's one. Apparently such
code flow will select the last added interface from the 'vlan group'
because FreeBSD list macros are adding list members to the beginning
of the linked list.

BPF will receive the right interface, because it is tapped before
bridge_input (in if_ethersubr.c).

Set up three vlan interfaces on the same physical one, and add them
to the same if_bridge. Give one VLAN interface IP address and send
packets from the other VLAN segment to that IP. See that tcpdump
is seeing the correct interface, but ipfw/pf are seeing the same
packet having the last added interface as the rcvif, not the right one.
For the last added interface the result will be correct, but for
others -- not.
Since bridge_input always knows about the incoming interface as
it is passed as the first argument my fix is just to set the
right interface using the passed parameter and not the interface
found by the list traversal. The patch is below. It should not
break the ordinary situation without CARP and I see no troubles
even with the CARP code. But I am not expirienced with CARP, so
I am CCing Gleb, Andrew and Andre. We'd found this bug with rik@
so he gots the CC too.

Shutting up ;))

--- if_bridge.c.orig	Fri Mar  2 18:23:56 2007
+++ if_bridge.c	Sat Mar  3 05:04:38 2007
@@ -2122,7 +2122,11 @@
 	LIST_FOREACH(bif2, &sc->sc_iflist, bif_next) {
 		if (bif2->bif_ifp->if_type == IFT_GIF)
-		/* It is destined for us. */
+		/* It is destined for us. We should not rely on the
+		 * found interface's MAC as the interface identifier,
+		 * because vlanX interfaces are sharing their MAC
+		 * with the parent. Moreover, we do know the interface
+		 * the packet is coming from. So we're using it. */
 		if (memcmp(IF_LLADDR(bif2->bif_ifp), eh->ether_dhost,
 		    ETHER_ADDR_LEN) == 0
 #ifdef DEV_CARP
@@ -2133,7 +2137,7 @@
 			if (bif->bif_flags & IFBIF_LEARNING)
 				(void) bridge_rtupdate(sc,
 				    eh->ether_shost, bif, 0, IFBAF_DYNAMIC);
-			m->m_pkthdr.rcvif = bif2->bif_ifp;
+			m->m_pkthdr.rcvif = ifp;
 			return (m);

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