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net: fix pskb_trim_rcsum_slow() with odd trim offset 

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 88078d98d1
("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: 88078d98d1 ("net: pskb_trim_rcsum() and CHECKSUM_COMPLETE are friends")
Signed-off-by: Dimitris Michailidis <dmichail@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Dimitris Michailidis 2018-10-19 17:07:13 -07:00 committed by David S. Miller
parent 4899542314
commit d55bef5059
1 changed files with 3 additions and 2 deletions
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5
net/core/skbuff.c
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@ -1846,8 +1846,9 @@ 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);
}