Re: Fwd: Possible bug in net/ipv4/route.c?
From: Eric Dumazet <hidden>
Date: 2010-07-02 05:44:52
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Le vendredi 02 juillet 2010 à 11:49 +0900, YOSHIFUJI Hideaki a écrit :
Switch to netdev.
thanks ;)
--yoshfuji -------- Original Message -------- Subject: Possible bug in net/ipv4/route.c? Date: Thu, 1 Jul 2010 16:00:29 -0700 From: Sol Kavy <redacted> To: <redacted> CC: Greg Ren <redacted>, Guojun Jin <redacted>, Murat Sezgin <redacted>, Sener Ilgen <redacted> 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.
Please use < 72 char lines
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.
This is the right option. If you use one word in cb[], only your driver knows how to clear it efficiently.
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.
No : ip_rcv clears() skb->cb when leaving ip_rcv, not entering. skb allocation clears whole cb[], and each layer is responsible to clear the part it eventually dirtied.
quoted hunk ↗ jump to hunk
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@ubicom.com in addition to posting your response. Thanks, Sol Kavy/Murat Sezgin Ubicom, Inc. Patch: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: do_IRQ()->do_softirq()->__do_softirq()->net_rx_action()->ubi32_eth_napi_poll()->ubi32_eth_receive()->__vlan_hwaccel_rx()->netif_receive_skb()->br_handle_frame()->nf_hook_slow()->br_nf_pre_routing_finish()->br_nfr_pre_routing_finish_bridge()->neight_resolve_output()->__neigh_event_send(). 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.