[VLAN/BRIDGE]: Fix "skb_pull_rcsum - Fatal exception in interrupt"

I tried to preserve bridging code as it was before, but logic is quite
strange - I think we should free skb on error, since it is already
unshared and thus will just leak.

Herbert Xu states:

> +	if ((skb = skb_share_check(skb, GFP_ATOMIC)) == NULL)
> +		goto out;

If this happens it'll be a double-free on skb since we'll
return NF_DROP which makes the caller free it too.

We could return NF_STOLEN to prevent that but I'm not sure
whether that's correct netfilter semantics.  Patrick, could
you please make a call on this?

Patrick McHardy states:

NF_STOLEN should work fine here.

Signed-off-by: Evgeniy Polyakov <johnpol@2ka.mipt.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 7 insertions, 5 deletions

diff --git a/net/bridge/br_netfilter.c b/net/bridge/br_netfilter.c
index fa779874b9dd..3ee2022928e3 100644
--- a/net/bridge/br_netfilter.c
+++ b/net/bridge/br_netfilter.c
@@ -509,8 +509,14 @@ static unsigned int br_nf_pre_routing(unsigned int hook, struct sk_buff **pskb,
                                       int (*okfn)(struct sk_buff *))
 {
         struct iphdr *iph;
-        __u32 len;
         struct sk_buff *skb = *pskb;
+        __u32 len = nf_bridge_encap_header_len(skb);
+
+        if ((skb = skb_share_check(skb, GFP_ATOMIC)) == NULL)
+                return NF_STOLEN;
+
+        if (unlikely(!pskb_may_pull(skb, len)))
+                goto out;
 
         if (skb->protocol == htons(ETH_P_IPV6) || IS_VLAN_IPV6(skb) ||
             IS_PPPOE_IPV6(skb)) {
@@ -518,8 +524,6 @@ static unsigned int br_nf_pre_routing(unsigned int hook, struct sk_buff **pskb,
                 if (!brnf_call_ip6tables)
                         return NF_ACCEPT;
 #endif
-                if ((skb = skb_share_check(*pskb, GFP_ATOMIC)) == NULL)
-                        goto out;
                 nf_bridge_pull_encap_header_rcsum(skb);
                 return br_nf_pre_routing_ipv6(hook, skb, in, out, okfn);
         }
@@ -532,8 +536,6 @@ static unsigned int br_nf_pre_routing(unsigned int hook, struct sk_buff **pskb,
             !IS_PPPOE_IP(skb))
                 return NF_ACCEPT;
 
-        if ((skb = skb_share_check(*pskb, GFP_ATOMIC)) == NULL)
-                goto out;
         nf_bridge_pull_encap_header_rcsum(skb);
 
         if (!pskb_may_pull(skb, sizeof(struct iphdr)))