* RE: Possible bug in net/ipv4/route.c?
2010-07-05 12:04 ` Possible bug in net/ipv4/route.c? Herbert Xu
@ 2010-07-05 16:37 ` Sol Kavy
2010-07-05 18:03 ` Stephen Hemminger
1 sibling, 0 replies; 6+ messages in thread
From: Sol Kavy @ 2010-07-05 16:37 UTC (permalink / raw)
To: Herbert Xu
Cc: Greg Ren, Guojun Jin, Murat Sezgin, Sener Ilgen, David S. Miller,
Stephen Hemminger, netdev
So is there some place else that should have the clear instead of ipv4_link_failure()?
Sol
-----Original Message-----
From: Herbert Xu [mailto:herbert@gondor.apana.org.au]
Sent: Monday, July 05, 2010 5:04 AM
To: Sol Kavy
Cc: linux-kernel@vger.kernel.org; Greg Ren; Guojun Jin; Murat Sezgin; Sener Ilgen; David S. Miller; Stephen Hemminger; netdev@vger.kernel.org
Subject: Re: Possible bug in net/ipv4/route.c?
Sol Kavy <skavy@ubicom.com> wrote:
> 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.
Thanks for the report!
> 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.
Generally this area should be cleared on entry to each stack intending on using it. So in this case, I'd point the finger of blame at the bridge stack for letting this packet into the IP stack through the back entrance without taking the proper precautions.
Cheers,
--
Email: Herbert Xu <herbert@gondor.apana.org.au> Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
^ permalink raw reply [flat|nested] 6+ messages in thread* Re: Possible bug in net/ipv4/route.c?
2010-07-05 12:04 ` Possible bug in net/ipv4/route.c? Herbert Xu
2010-07-05 16:37 ` Sol Kavy
@ 2010-07-05 18:03 ` Stephen Hemminger
2010-07-06 0:28 ` Herbert Xu
1 sibling, 1 reply; 6+ messages in thread
From: Stephen Hemminger @ 2010-07-05 18:03 UTC (permalink / raw)
To: Herbert Xu
Cc: Sol Kavy, linux-kernel, gren, gjin, msezgin, silgen,
David S. Miller, netdev
On Mon, 5 Jul 2010 20:04:13 +0800
Herbert Xu <herbert@gondor.apana.org.au> wrote:
> Sol Kavy <skavy@ubicom.com> wrote:
> > 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.
>
> Thanks for the report!
>
> > 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.
>
> Generally this area should be cleared on entry to each stack
> intending on using it. So in this case, I'd point the finger
> of blame at the bridge stack for letting this packet into the
> IP stack through the back entrance without taking the proper
> precautions.
The CB is used in two places in the bridge code.
1) The recent multicast changes (IGMP)
2) Netfilter / ebtables to store header.
Ebtables is okay because the only part of the cb[] it uses
is the incoming device (brdev) which is always initialized coming into
the bridge: br_dev_xmit and br_handle_frame_finish
The IGMP code looks buggy.
/* net device transmit always called with no BH (preempt_disabled) */
netdev_tx_t br_dev_xmit(struct sk_buff *skb, struct net_device *dev)
{
... [A]
BR_INPUT_SKB_CB(skb)->brdev = dev;
skb_reset_mac_header(skb);
skb_pull(skb, ETH_HLEN);
if (is_multicast_ether_addr(dest)) {
if (br_multicast_rcv(br, NULL, skb))
goto out;
mdst = br_mdb_get(br, skb);
if (mdst || BR_INPUT_SKB_CB_MROUTERS_ONLY(skb))
... [B]
The problem is that br_dev_xmit is looking at flags in the CB[] that
are uninitialized.
if br_dev_xmit cleared the CB at [A] the mrouters_only would always be zero
at [B].
Where should the mrouters and igmp_only fields in skb be initialized?
^ permalink raw reply [flat|nested] 6+ messages in thread