Subject: [PATCH 13/18] net: gre: recompute gre csum for sctp over gre tunnels

From: Lorenzo Bianconi <[email-masked]>
Date: Fri, 31 Jul 2020 20:12:05 +0200

[ Upstream commit 622e32b7d4a6492cf5c1f759ef833f817418f7b3 ]

The GRE tunnel can be used to transport traffic that does not rely on a
Internet checksum (e.g. SCTP). The issue can be triggered creating a GRE
or GRETAP tunnel and transmitting SCTP traffic ontop of it where CRC
offload has been disabled. In order to fix the issue we need to
recompute the GRE csum in gre_gso_segment() not relying on the inner
checksum.
The issue is still present when we have the CRC offload enabled.
In this case we need to disable the CRC offload if we require GRE
checksum since otherwise skb_checksum() will report a wrong value.

Fixes: 90017accff61 ("sctp: Add GSO support")
Signed-off-by: Lorenzo Bianconi <[email-masked]>
Reviewed-by: Marcelo Ricardo Leitner <[email-masked]>
Signed-off-by: David S. Miller <[email-masked]>
---
 net/ipv4/gre_offload.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/gre_offload.c b/net/ipv4/gre_offload.c
index 4de7e962d3da..c840141876bc 100644
--- a/net/ipv4/gre_offload.c
+++ b/net/ipv4/gre_offload.c
@@ -15,12 +15,12 @@ static struct sk_buff *gre_gso_segment(struct sk_buff *skb,
 				       netdev_features_t features)
 {
 	int tnl_hlen = skb_inner_mac_header(skb) - skb_transport_header(skb);
+	bool need_csum, need_recompute_csum, gso_partial;
 	struct sk_buff *segs = ERR_PTR(-EINVAL);
 	u16 mac_offset = skb->mac_header;
 	__be16 protocol = skb->protocol;
 	u16 mac_len = skb->mac_len;
 	int gre_offset, outer_hlen;
-	bool need_csum, gso_partial;
 
 	if (!skb->encapsulation)
 		goto out;
@@ -41,6 +41,7 @@ static struct sk_buff *gre_gso_segment(struct sk_buff *skb,
 	skb->protocol = skb->inner_protocol;
 
 	need_csum = !!(skb_shinfo(skb)->gso_type & SKB_GSO_GRE_CSUM);
+	need_recompute_csum = skb->csum_not_inet;
 	skb->encap_hdr_csum = need_csum;
 
 	features &= skb->dev->hw_enc_features;
@@ -98,7 +99,15 @@ static struct sk_buff *gre_gso_segment(struct sk_buff *skb,
 		}
 
 		*(pcsum + 1) = 0;
-		*pcsum = gso_make_checksum(skb, 0);
+		if (need_recompute_csum && !skb_is_gso(skb)) {
+			__wsum csum;
+
+			csum = skb_checksum(skb, gre_offset,
+					    skb->len - gre_offset, 0);
+			*pcsum = csum_fold(csum);
+		} else {
+			*pcsum = gso_make_checksum(skb, 0);
+		}
 	} while ((skb = skb->next));
 out:
 	return segs;
-- 
2.26.2