Possible bug in net/ipv4/route.c?

From: Sol Kavy
Date: Thu Jul 01 2010 - 20:23:10 EST

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 sol@xxxxxxxxxx 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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