netdev - [Patch net] ip: fix csum_sub() with csum_block

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Message-Id: <20181108220407.17691-1-xiyou.wangcong@gmail.com>
Date:   Thu,  8 Nov 2018 14:04:07 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     netdev@...r.kernel.org
Cc:     Cong Wang <xiyou.wangcong@...il.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: [Patch net] ip: fix csum_sub() with csum_block_sub()

When subtracting the checksum of a block of data,
csum_block_sub() must be used to respect the offset.

We learned this lesson from both commit d55bef5059dd
("net: fix pskb_trim_rcsum_slow() with odd trim offset") and
commit d48051c5b837 ("net/mlx5e: fix csum adjustments caused by RXFCS").

Fixes: ca4ef4574f1e ("ip: fix IP_CHECKSUM handling")
Cc: Paolo Abeni <pabeni@...hat.com>
Cc: Eric Dumazet <edumazet@...gle.com>
Cc: Willem de Bruijn <willemb@...gle.com>
Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
---
 net/ipv4/ip_sockglue.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
index fffcc130900e..0d69f0823c08 100644
--- a/net/ipv4/ip_sockglue.c
+++ b/net/ipv4/ip_sockglue.c
@@ -122,7 +122,10 @@ static void ip_cmsg_recv_checksum(struct msghdr *msg, struct sk_buff *skb,
 
 	if (offset != 0) {
 		int tend_off = skb_transport_offset(skb) + tlen;
-		csum = csum_sub(csum, skb_checksum(skb, tend_off, offset, 0));
+
+		csum = csum_block_sub(csum,
+				      skb_checksum(skb, tend_off, offset, 0),
+				      tend_off);
 	}
 
 	put_cmsg(msg, SOL_IP, IP_CHECKSUM, sizeof(__wsum), &csum);
-- 
2.19.1