u32: negative offset fix

It was possible to use a negative offset in a u32 match to reference
the ethernet header or other parts of the link layer header.
This fixes the regression caused by:

commit fbc2e7d9cf49e0bf89b9e91fd60a06851a855c5d
Author: Changli Gao <xiaosuo@gmail.com>
Date:   Wed Jun 2 07:32:42 2010 -0700

    cls_u32: use skb_header_pointer() to dereference data safely

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 4 insertions, 2 deletions

diff --git a/net/sched/cls_u32.c b/net/sched/cls_u32.c
index 4f522143811e..7416a5c73b2a 100644
--- a/net/sched/cls_u32.c
+++ b/net/sched/cls_u32.c
@@ -134,10 +134,12 @@ next_knode:
 #endif
 
                 for (i = n->sel.nkeys; i>0; i--, key++) {
-                        unsigned int toff;
+                        int toff = off + key->off + (off2 & key->offmask);
                         __be32 *data, _data;
 
-                        toff = off + key->off + (off2 & key->offmask);
+                        if (skb_headroom(skb) + toff < 0)
+                                goto out;
+
                         data = skb_header_pointer(skb, toff, 4, &_data);
                         if (!data)
                                 goto out;