Subject: [PATCH v17 nf-next 1/4] netfilter: utils: nf_checksum(_partial) correct data!=networkheader

From: Eric Woudstra <[email-masked]>
Date: Sun, 9 Nov 2025 20:24:24 +0100

In the conntrack hook it may not always be the case that:
skb_network_header(skb) == skb->data, i.e. skb_network_offset(skb)
is zero.

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.

Adjust for skb_network_offset(skb), so that the checksum is calculated
correctly.

Signed-off-by: Eric Woudstra <[email-masked]>
---
 net/netfilter/utils.c | 28 ++++++++++++++++++++++------
 1 file changed, 22 insertions(+), 6 deletions(-)

diff --git a/net/netfilter/utils.c b/net/netfilter/utils.c
index 008419db815a..7b33fe63c5fa 100644
--- a/net/netfilter/utils.c
+++ b/net/netfilter/utils.c
@@ -124,16 +124,25 @@ __sum16 nf_checksum(struct sk_buff *skb, unsigned int hook,
 		    unsigned int dataoff, u8 protocol,
 		    unsigned short family)
 {
+	unsigned int nhpull = skb_network_offset(skb);
 	__sum16 csum = 0;
 
+	if (WARN_ON(!skb_pointer_if_linear(skb, nhpull, 0)))
+		return 0;
+
+	/* pull/push because the lower csum functions assume that
+	 * skb_network_offset(skb) is zero.
+	 */
+	__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 +152,25 @@ __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_offset(skb);
 	__sum16 csum = 0;
 
+	if (WARN_ON(!skb_pointer_if_linear(skb, nhpull, 0)))
+		return 0;
+
+	/* See nf_checksum() */
+	__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;
 }
-- 
2.50.0