netdev - [PATCH net] ip_gre: validate csum_start only if CHECKSUM

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Date:   Thu,  2 Sep 2021 15:34:47 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     netdev@...r.kernel.org
Cc:     davem@...emloft.net, kuba@...nel.org, idosch@...sch.org,
        chouhan.shreyansh630@...il.com,
        Willem de Bruijn <willemb@...gle.com>
Subject: [PATCH net] ip_gre: validate csum_start only if CHECKSUM_PARTIAL

From: Willem de Bruijn <willemb@...gle.com>

Only test integrity of csum_start if checksum offload is configured.

With checksum offload and GRE tunnel checksum, gre_build_header will
cheaply build the GRE checksum using local checksum offload. This
depends on inner packet csum offload, and thus that csum_start points
behind GRE. But validate this condition only with checksum offload.

Link: https://lore.kernel.org/netdev/YS+h%2FtqCJJiQei+W@shredder/
Fixes: 1d011c4803c7 ("ip_gre: add validation for csum_start")
Signed-off-by: Willem de Bruijn <willemb@...gle.com>
---
 net/ipv4/ip_gre.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c
index 177d26d8fb9c..09311992a617 100644
--- a/net/ipv4/ip_gre.c
+++ b/net/ipv4/ip_gre.c
@@ -473,8 +473,11 @@ static void __gre_xmit(struct sk_buff *skb, struct net_device *dev,
 
 static int gre_handle_offloads(struct sk_buff *skb, bool csum)
 {
-	if (csum && skb_checksum_start(skb) < skb->data)
+	/* Local checksum offload requires csum offload of the inner packet */
+	if (csum && skb->ip_summed == CHECKSUM_PARTIAL &&
+	    skb_checksum_start(skb) < skb->data)
 		return -EINVAL;
+
 	return iptunnel_handle_offloads(skb, csum ? SKB_GSO_GRE_CSUM : SKB_GSO_GRE);
 }
 
-- 
2.33.0.153.gba50c8fa24-goog