Re: [PATCH] bonding: Fix corrupted queue_mapping

[Eric Dumazet <eric.dumazet@gmail.com>]

On Thu, 2012-06-07 at 23:02 -0700, David Miller wrote:
> From: Eric Dumazet <eric.dumazet@gmail.com>
> Date: Fri, 08 Jun 2012 07:57:37 +0200
> 
> > On Thu, 2012-06-07 at 22:05 -0700, Tom Herbert wrote:
> >> In the transmit path of the bonding driver, skb->cb is used to
> >> stash the skb->queue_mapping so that the bonding device can set its
> >> own queue mapping.  This value becomes corrupted since the skb->cb is
> >> also used in __dev_xmit_skb.
> >> 
> >> When transmitting through bonding driver, bond_select_queue is
> >> called from dev_queue_xmit.  In bond_select_queue the original
> >> skb->queue_mapping is copied into skb->cb (via bond_queue_mapping)
> >> and skb->queue_mapping is overwritten with the bond driver queue.
> >> Subsequently in dev_queue_xmit, __dev_xmit_skb is called which writes
> >> the packet length into skb->cb, thereby overwriting the stashed
> >> queue mappping.  In bond_dev_queue_xmit (called from hard_start_xmit),
> >> the queue mapping for the skb is set to the stashed value which is now
> >> the skb length and hence is an invalid queue for the slave device.
> >> 
> >> Fix is to set bond_queue_mapping to skb->cb +
> >> sizeof((struct qdisc_skb_cb)
> >> 
> >> Signed-off-by: Tom Herbert <therbert@google.com>
> >> ---
> >>  drivers/net/bonding/bond_main.c |    4 +++-
> >>  1 files changed, 3 insertions(+), 1 deletions(-)
> >> 
> >> diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
> >> index 2ee8cf9..044c1c0 100644
> >> --- a/drivers/net/bonding/bond_main.c
> >> +++ b/drivers/net/bonding/bond_main.c
> >> @@ -76,6 +76,7 @@
> >>  #include <net/route.h>
> >>  #include <net/net_namespace.h>
> >>  #include <net/netns/generic.h>
> >> +#include <net/sch_generic.h>
> >>  #include "bonding.h"
> >>  #include "bond_3ad.h"
> >>  #include "bond_alb.h"
> >> @@ -381,7 +382,8 @@ struct vlan_entry *bond_next_vlan(struct bonding *bond, struct vlan_entry *curr)
> >>  	return next;
> >>  }
> >>  
> >> -#define bond_queue_mapping(skb) (*(u16 *)((skb)->cb))
> >> +#define bond_queue_mapping(skb) (*(u16 *)((skb)->cb + \
> >> +    sizeof(struct qdisc_skb_cb)))
> >>  
> >>  /**
> >>   * bond_dev_queue_xmit - Prepare skb for xmit.
> > 
> > 
> > Sorry this wont work in all cases.
> > 
> > Some qdisc also use skb->cb[]
> 
> Hmmm, isn't that what qdisc_skb_cb is for?  And even private data is
> explicitly allocated:
> 
> >  	unsigned char		data[24];
> 
> there. :-)
> 

Yes, but some other layers can use the same trick so it might collide.

Inserting the bond field in qdisc_skb_cb (level0) is safer.