net: Fix skb_under_panic oops in neigh_resolve_output

From: Ramesh Nagappa <ramesh.nagappa@ericsson.com>

The retry loop in neigh_resolve_output() and neigh_connected_output()
call dev_hard_header() with out reseting the skb to network_header.
This causes the retry to fail with skb_under_panic. The fix is to
reset the network_header within the retry loop.

Signed-off-by: Ramesh Nagappa <ramesh.nagappa@ericsson.com>
Reviewed-by: Shawn Lu <shawn.lu@ericsson.com>
Reviewed-by: Robert Coulson <robert.coulson@ericsson.com>
Reviewed-by: Billie Alsup <billie.alsup@ericsson.com>
---
 net/core/neighbour.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

On Thu, 2012-10-04 at 20:05 -0700, ramesh.nagappa@gmail.com wrote:
> From: Ramesh Nagappa <ramesh.nagappa@ericsson.com>
> 
> The retry loop in neigh_resolve_output() and neigh_connected_output()
> call dev_hard_header() with out reseting the skb to network_header.
> This causes the retry to fail with skb_under_panic. The fix is to
> reset the network_header within the retry loop.
> 
> Signed-off-by: Ramesh Nagappa <ramesh.nagappa@ericsson.com>
> Reviewed-by: Shawn Lu <shawn.lu@ericsson.com>
> Reviewed-by: Robert Coulson <robert.coulson@ericsson.com>
> Reviewed-by: Billie Alsup <billie.alsup@ericsson.com>
> ---
>  net/core/neighbour.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/net/core/neighbour.c b/net/core/neighbour.c
> index 117afaf..f50c542 100644
> --- a/net/core/neighbour.c
> +++ b/net/core/neighbour.c
> @@ -1313,6 +1313,7 @@ int neigh_resolve_output(struct neighbour *neigh, struct sk_buff *skb)
>  
>  		do {
>  			seq = read_seqbegin(&neigh->ha_lock);
> +			__skb_pull(skb, skb_network_offset(skb));

This __skb_pull() could be done before the read_seqbegin() to keep the
protected section short.

>  			err = dev_hard_header(skb, dev, ntohs(skb->protocol),
>  					      neigh->ha, NULL, skb->len);
>  		} while (read_seqretry(&neigh->ha_lock, seq));
> @@ -1342,10 +1343,9 @@ int neigh_connected_output(struct neighbour *neigh, struct sk_buff *skb)
>  	unsigned int seq;
>  	int err;
>  
> -	__skb_pull(skb, skb_network_offset(skb));
> -
>  	do {
>  		seq = read_seqbegin(&neigh->ha_lock);
> +		__skb_pull(skb, skb_network_offset(skb));

This __skb_pull() could be done before the read_seqbegin() to keep the
protected section short.

>  		err = dev_hard_header(skb, dev, ntohs(skb->protocol),
>  				      neigh->ha, NULL, skb->len);
>  	} while (read_seqretry(&neigh->ha_lock, seq));

Thanks


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Hi Eric,

Yes, that is a good optimization. neigh_resolve_output() also has the
__skb_pull() outside the loop, is that required ? The changes would be
like ...

neigh_resolve_output()
...
-        __skb_pull(skb, skb_network_offset(skb));


        if (!neigh_event_send(neigh, skb)) {
                int err;
                struct net_device *dev = neigh->dev;
                unsigned int seq;

                if (dev->header_ops->cache && !neigh->hh.hh_len)
                        neigh_hh_init(neigh, dst);

                do {                        
+			      __skb_pull(skb, skb_network_offset(skb));
                        seq = read_seqbegin(&neigh->ha_lock);
                        err = dev_hard_header(skb, dev, ntohs(skb->protocol),
                                              neigh->ha, NULL, skb->len);
                } while (read_seqretry(&neigh->ha_lock, seq));

Thanks,
Ramesh
 

> -----Original Message-----
> From: Eric Dumazet [mailto:eric.dumazet@gmail.com] 
> >  	do {
> >  		seq = read_seqbegin(&neigh->ha_lock);
> > +		__skb_pull(skb, skb_network_offset(skb));
> 
> This __skb_pull() could be done before the read_seqbegin() to 
> keep the protected section short.
> 
> >  		err = dev_hard_header(skb, dev, ntohs(skb->protocol),
> >  				      neigh->ha, NULL, skb->len);
> >  	} while (read_seqretry(&neigh->ha_lock, seq));
> 
> Thanks
> 
> 
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On Fri, 2012-10-05 at 12:33 -0400, Ramesh Nagappa wrote:
> Hi Eric,
> 
> Yes, that is a good optimization. neigh_resolve_output() also has the
> __skb_pull() outside the loop, is that required ? The changes would be
> like ...
> 
> neigh_resolve_output()
> ...
> -        __skb_pull(skb, skb_network_offset(skb));
> 
> 
>         if (!neigh_event_send(neigh, skb)) {
>                 int err;
>                 struct net_device *dev = neigh->dev;
>                 unsigned int seq;
> 
>                 if (dev->header_ops->cache && !neigh->hh.hh_len)
>                         neigh_hh_init(neigh, dst);
> 
>                 do {                        
> +			      __skb_pull(skb, skb_network_offset(skb));
>                         seq = read_seqbegin(&neigh->ha_lock);
>                         err = dev_hard_header(skb, dev, ntohs(skb->protocol),
>                                               neigh->ha, NULL, skb->len);
>                 } while (read_seqretry(&neigh->ha_lock, seq));

All similar constructions should be audited.

For example in net/decnet/dn_neigh.c , dn_neigh_output_packet()



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