[PATCH AUTOSEL 4.14 18/19] net: fix pskb_trim_rcsum_slow() with odd trim offset

[Sasha Levin]

From: Dimitris Michailidis <dmichail@xxxxxxxxxx>

[ Upstream commit d55bef5059dd057bd077155375c581b49d25be7e ]

We've been getting checksum errors involving small UDP packets, usually
59B packets with 1 extra non-zero padding byte. netdev_rx_csum_fault()
has been complaining that HW is providing bad checksums. Turns out the
problem is in pskb_trim_rcsum_slow(), introduced in commit 88078d98d1bb
("net: pskb_trim_rcsum() and CHECKSUM_COMPLETE are friends").

The source of the problem is that when the bytes we are trimming start
at an odd address, as in the case of the 1 padding byte above,
skb_checksum() returns a byte-swapped value. We cannot just combine this
with skb->csum using csum_sub(). We need to use csum_block_sub() here
that takes into account the parity of the start address and handles the
swapping.

Matches existing code in __skb_postpull_rcsum() and esp_remove_trailer().

Fixes: 88078d98d1bb ("net: pskb_trim_rcsum() and CHECKSUM_COMPLETE are friends")
Signed-off-by: Dimitris Michailidis <dmichail@xxxxxxxxxx>
Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 net/core/skbuff.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 9f80b947f53b..c19a118f9f82 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -1843,8 +1843,9 @@ int pskb_trim_rcsum_slow(struct sk_buff *skb, unsigned int len)
 	if (skb->ip_summed == CHECKSUM_COMPLETE) {
 		int delta = skb->len - len;
 
-		skb->csum = csum_sub(skb->csum,
-				     skb_checksum(skb, len, delta, 0));
+		skb->csum = csum_block_sub(skb->csum,
+					   skb_checksum(skb, len, delta, 0),
+					   len);
 	}
 	return __pskb_trim(skb, len);
 }
-- 
2.17.1