netdev - Re: [PATCH v3] net: Do not include padding in TCP GRO checksum

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Message-ID: <20131116064611.GA12146@gondor.apana.org.au>
Date:	Sat, 16 Nov 2013 14:46:11 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Alexander Duyck <alexander.duyck@...il.com>
Cc:	Alexander Duyck <alexander.h.duyck@...el.com>, davem@...emloft.net,
	netdev@...r.kernel.org, edumazet@...gle.com
Subject: Re: [PATCH v3] net: Do not include padding in TCP GRO checksum

On Fri, Nov 15, 2013 at 08:10:00PM -0800, Alexander Duyck wrote:
>
> The case I am addressing is padding added by the remote side. 
> Specifically in my case I was seeing TCP frames without options that
> were padded up to 60 bytes from a netperf TCP_RR test.  I messed up the
> padding/checksum logic so it was making the same mistake in the Tx
> checksum logic in the driver that I caught here in GRO.  As a result I
> was seeing checksum errors errors in wireshark, but noticed they were
> being accepted by the stack as valid.

OK great.  So this isn't normal data that we expect to aggregate.

In that case the simplest solution is to skip the checksum check
altogether.  We only require it if the packet is going to be merged.

So how about something like this?

gro: Only verify TCP checksums for candidates

In some cases we may receive IP packets that are longer than
their stated lengths.  Such packets are never merged in GRO.
However, we may end up computing their checksums incorrectly
and end up allowing packets with a bogus checksum enter our
stack with the checksum status set as verified.

Since such packets are rare and not performance-critical, this
patch simply skips the checksum verification for them.

Reported-by: Alexander Duyck <alexander.h.duyck@...el.com>
Signed-off-by: Herbert Xu <herbert@...dor.apana.org.au>

diff --git a/net/ipv4/tcp_offload.c b/net/ipv4/tcp_offload.c
index a2b68a1..55aeec9 100644
--- a/net/ipv4/tcp_offload.c
+++ b/net/ipv4/tcp_offload.c
@@ -276,6 +276,10 @@ static struct sk_buff **tcp4_gro_receive(struct sk_buff **head, struct sk_buff *
 	__wsum wsum;
 	__sum16 sum;
 
+	/* Don't bother verifying checksum if we're going to flush anyway. */
+	if (NAPI_GRO_CB(skb)->flush)
+		goto skip_csum;
+
 	switch (skb->ip_summed) {
 	case CHECKSUM_COMPLETE:
 		if (!tcp_v4_check(skb_gro_len(skb), iph->saddr, iph->daddr,
@@ -301,6 +305,7 @@ flush:
 		break;
 	}
 
+skip_csum:
 	return tcp_gro_receive(head, skb);
 }
 
diff --git a/net/ipv6/tcpv6_offload.c b/net/ipv6/tcpv6_offload.c
index c1097c7..71923d1 100644
--- a/net/ipv6/tcpv6_offload.c
+++ b/net/ipv6/tcpv6_offload.c
@@ -39,6 +39,10 @@ static struct sk_buff **tcp6_gro_receive(struct sk_buff **head,
 	__wsum wsum;
 	__sum16 sum;
 
+	/* Don't bother verifying checksum if we're going to flush anyway. */
+	if (NAPI_GRO_CB(skb)->flush)
+		goto skip_csum;
+
 	switch (skb->ip_summed) {
 	case CHECKSUM_COMPLETE:
 		if (!tcp_v6_check(skb_gro_len(skb), &iph->saddr, &iph->daddr,
@@ -65,6 +69,7 @@ flush:
 		break;
 	}
 
+skip_csum:
 	return tcp_gro_receive(head, skb);
 }

Thanks,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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