[PATCH v14 nf-next 1/3] netfilter: utils: nf_checksum(_partial) correct data!=networkheader

[Eric Woudstra]

In the conntrack hook it may not always be the case that:
skb_network_header(skb) == skb->data.

This is problematic when L4 function nf_conntrack_handle_packet()
is accessing L3 data. This function uses thoff and ip_hdr()
to finds it's data. But it also calculates the checksum.
nf_checksum() and nf_checksum_partial() both use lower skb-checksum
functions that are based on using skb->data.

When skb_network_header(skb) != skb->data, adjust accordingly,
so that the checksum is calculated correctly.

Signed-off-by: Eric Woudstra <ericwo...@gmail.com>
---
 net/netfilter/utils.c | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/net/netfilter/utils.c b/net/netfilter/utils.c
index 008419db815a..9ba822983bc0 100644
--- a/net/netfilter/utils.c
+++ b/net/netfilter/utils.c
@@ -124,16 +124,20 @@ __sum16 nf_checksum(struct sk_buff *skb, unsigned int 
hook,
                    unsigned int dataoff, u8 protocol,
                    unsigned short family)
 {
+       unsigned int nhpull = skb_network_header(skb) - skb->data;
        __sum16 csum = 0;
 
+       DEBUG_NET_WARN_ON_ONCE(!skb_pointer_if_linear(skb, nhpull, 0));
+       __skb_pull(skb, nhpull);
        switch (family) {
        case AF_INET:
-               csum = nf_ip_checksum(skb, hook, dataoff, protocol);
+               csum = nf_ip_checksum(skb, hook, dataoff - nhpull, protocol);
                break;
        case AF_INET6:
-               csum = nf_ip6_checksum(skb, hook, dataoff, protocol);
+               csum = nf_ip6_checksum(skb, hook, dataoff - nhpull, protocol);
                break;
        }
+       __skb_push(skb, nhpull);
 
        return csum;
 }
@@ -143,18 +147,22 @@ __sum16 nf_checksum_partial(struct sk_buff *skb, unsigned 
int hook,
                            unsigned int dataoff, unsigned int len,
                            u8 protocol, unsigned short family)
 {
+       unsigned int nhpull = skb_network_header(skb) - skb->data;
        __sum16 csum = 0;
 
+       DEBUG_NET_WARN_ON_ONCE(!skb_pointer_if_linear(skb, nhpull, 0));
+       __skb_pull(skb, nhpull);
        switch (family) {
        case AF_INET:
-               csum = nf_ip_checksum_partial(skb, hook, dataoff, len,
-                                             protocol);
+               csum = nf_ip_checksum_partial(skb, hook, dataoff - nhpull,
+                                             len, protocol);
                break;
        case AF_INET6:
-               csum = nf_ip6_checksum_partial(skb, hook, dataoff, len,
-                                              protocol);
+               csum = nf_ip6_checksum_partial(skb, hook, dataoff - nhpull,
+                                              len, protocol);
                break;
        }
+       __skb_push(skb, nhpull);
 
        return csum;
 }
-- 
2.47.1