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Subject[PATCH 4.9 150/171] net: fix pskb_trim_rcsum_slow() with odd trim offset
4.9-stable review patch.  If anyone has any objections, please let me know.


From: Dimitris Michailidis <>

[ 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

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 <>
Reviewed-by: Eric Dumazet <>
Signed-off-by: David S. Miller <>
Signed-off-by: Greg Kroah-Hartman <>
net/core/skbuff.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -1585,8 +1585,9 @@ int pskb_trim_rcsum_slow(struct sk_buff
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);

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