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 7943627623eb
("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: 7943627623eb ("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>

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 428094b577fc96f5f93ab3f93d27997935de35e3..f817f336595db61588425aa50a396ee06be48dda 100644 (file)
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -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);
 }