[PATCH net] sctp: get sctphdr by offset in sctp_compute_cksum

[Xin Long]

sctp_hdr(skb) only works when skb->transport_header is set properly.

But in the path of nf_conntrack_in: sctp_packet() -> sctp_error()

skb->transport_header is not guaranteed to be right value for sctp.
It will cause to fail to check the checksum for sctp packets.

So fix it by using offset, which is always right in all places.

Fixes: e6d8b64b34aa ("net: sctp: fix and consolidate SCTP checksumming code")
Reported-by: Li Shuang <shuali@xxxxxxxxxx>
Signed-off-by: Xin Long <lucien.xin@xxxxxxxxx>
---
 include/net/sctp/checksum.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/net/sctp/checksum.h b/include/net/sctp/checksum.h
index 32ee65a..1c6e6c0 100644
--- a/include/net/sctp/checksum.h
+++ b/include/net/sctp/checksum.h
@@ -61,7 +61,7 @@ static inline __wsum sctp_csum_combine(__wsum csum, __wsum csum2,
 static inline __le32 sctp_compute_cksum(const struct sk_buff *skb,
 					unsigned int offset)
 {
-	struct sctphdr *sh = sctp_hdr(skb);
+	struct sctphdr *sh = (struct sctphdr *)(skb->data + offset);
 	const struct skb_checksum_ops ops = {
 		.update  = sctp_csum_update,
 		.combine = sctp_csum_combine,
-- 
2.1.0