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SubjectPossible bug in net/ipv4/route.c?
Found Linux: 2.6.28
Arch: Ubicom32 <not yet pushed>
Project: uCLinux based Router
Test: Bit torrent Stress Test

Note: The top of Linus git net/ipv4/route.c appears to have the same issue.

The following is a patch for clearing out IP options area in an input skb during link failure processing.  Without this patch, the icmp_send() can result in a call to ip_options_echo() where the common buffer area of the skb is incorrectly interpreted.  Depending on the previous use of the skb->cb[], the interpreted option length values can cause stack corruption by copying more than 40 bytes to the output options.

In our case, a driver is using the skb->cb[] area to hold driver specific data. The driver is not zeroing out the area after use. I can see three basic solutions:

1) Drivers are not allowed to use the skb->cb[] area at all. Ubicom should modify the driver to use a different approach.

2) The layer using skb->cb[] should clear this area after use and before handing the skb to another layer. Ubicom should modify the driver to clear the skb->cb[] area before sending it up the line.

3) Any layer that "uses" the skb->cb[] area must clear the area before use. In which case, the proposed patch would fix the problem for the ipv4_link_failure(). I believe that this is the correct fix because I see ip_rcv() clears the skb->cb[] before using it.

Can someone confirm that this is the appropriate fix?  If this is documented somewhere, please direct me to the documentation.

Please send email to in addition to posting your response.


Sol Kavy/Murat Sezgin
Ubicom, Inc.


diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index 125ee64..d13805f 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -1606,6 +1606,14 @@ static void ipv4_link_failure(struct sk_buff *skb)
        struct rtable *rt;

+       /*
+         * Since link failure can be called with skbs from many layers (see arp)
+         * the cb area of the skb must be cleared before use. Because the cb area
+         * can be formatted according to the caller layer's cb area format and it may cause
+         * corruptions when it is handled in a different network layer.
+         */
+       memset(&(IPCB(skb)->opt), 0, sizeof(IPCB(skb)->opt));
        icmp_send(skb, ICMP_DEST_UNREACH, ICMP_HOST_UNREACH, 0);
        rt = skb->rtable;

The packet is enqueud by:

The packet is then dequeued by:
do_IRQ() -> irq_exit() -> do_softirq() -> run_timer_softirq() -> neigh_timer_handler() -> arp_error_report() -> ipv4_link_failure() -> icmp_send() -> ip_options_echo().

Because the Ubicom Ethernet driver overwrites the common buffer area, the enqueued packet contains garbage when casted as an IP options data structure. This results in ip_options_echo() miss reading the option length information and overwriting memory.  By clearing the skb->cb[] before processing the icmp_send() against the packet, we ensure that ip_options_echo() does not corrupt memory.  

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