net: fix pskb_trim_rcsum_slow() with odd trim offset

dimichail · davem330 · commit d55bef5059dd · 2018-10-20T01:13:42.000-07:00
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 88078d9 ("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: 88078d9 ("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
@@ -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);
 }

Original file line number	Diff line number	Diff line change
`@@ -1846,8 +1846,9 @@ int pskb_trim_rcsum_slow(struct sk_buff *skb, unsigned int len)`
`1846`	`1846`	`if (skb->ip_summed == CHECKSUM_COMPLETE) {`
`1847`	`1847`	`int delta = skb->len - len;`
`1848`	`1848`
`1849`		`- skb->csum = csum_sub(skb->csum,`
`1850`		`- skb_checksum(skb, len, delta, 0));`
	`1849`	`+ skb->csum = csum_block_sub(skb->csum,`
	`1850`	`+ skb_checksum(skb, len, delta, 0),`
	`1851`	`+ len);`
`1851`	`1852`	`}`
`1852`	`1853`	`return __pskb_trim(skb, len);`
`1853`	`1854`	`}`