[PATCH net 3/6] seg6: update skb checksum when SRv6 behaviors advance to the next SID

[Andrea Mayer <andrea.mayer@uniroma2.it>]

advance_nextseg() decrements Segments Left and writes the next segment into
the destination address, and seg6_next_csid_advance_arg() advances the
NEXT-C-SID argument in the destination address.
With CHECKSUM_COMPLETE, neither updates skb->csum for these changes. The
stale skb->csum is read when the packet is delivered on the same node, e.g.
through End.B6.Encaps towards a local End.DT6 SID.
As a consequence, when skb->csum is computed in software, for instance by
conntrack, UDP can drop the packet as a checksum error. With skb->csum
computed by the device, TCP can instead log a "hw csum failure", as if the
device had computed a wrong skb->csum.

Update skb->csum in both functions with update_csum_diff16() for the
destination address. advance_nextseg() also uses update_csum_diff4() for
Segments Left.

Depends on patch "seg6: fix skb checksum update helpers".

Found by manual code inspection while working on
commit 8ae3f4067ba4 ("seg6: ensure packet data is writable before modifying
SRH and IPv6 DA"), and reproduced in a VM without kernel changes or error
injection. A following commit adds a selftest that reproduces the problem
with a tc ct action. With this patch, the selftest passes for the
End.B6.Encaps and next-csid paths. Also tested against the srv6 selftests,
and with a ConnectX-6 Lx NIC (mlx5) that provides CHECKSUM_COMPLETE.

Fixes: 140f04c33bbc ("ipv6: sr: implement several seg6local actions")
Fixes: 848f3c0d4769 ("seg6: add NEXT-C-SID support for SRv6 End behavior")
Signed-off-by: Andrea Mayer <andrea.mayer@uniroma2.it>
---
 net/ipv6/seg6_local.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/seg6_local.c b/net/ipv6/seg6_local.c
index 0c3d1be7f129..57a82477ce3f 100644
--- a/net/ipv6/seg6_local.c
+++ b/net/ipv6/seg6_local.c
@@ -291,7 +291,9 @@ static struct ipv6_sr_hdr *advance_nextseg(struct sk_buff *skb,
 					   struct ipv6_sr_hdr *srh,
 					   enum skb_drop_reason *reason)
 {
+	struct in6_addr *daddr;
 	struct in6_addr *addr;
+	__be32 old;
 	int srhoff;
 	int wlen;
 
@@ -309,10 +311,19 @@ static struct ipv6_sr_hdr *advance_nextseg(struct sk_buff *skb,
 
 	/* skb_ensure_writable() may change skb pointers; evaluate srh again */
 	srh = (struct ipv6_sr_hdr *)(skb->data + srhoff);
+	daddr = &ipv6_hdr(skb)->daddr;
+	/* nexthdr, hdrlen, type and segments_left */
+	old = *(__be32 *)srh;
 
 	srh->segments_left--;
 	addr = srh->segments + srh->segments_left;
-	ipv6_hdr(skb)->daddr = *addr;
+
+	if (skb->ip_summed == CHECKSUM_COMPLETE) {
+		update_csum_diff4(skb, old, *(__be32 *)srh);
+		update_csum_diff16(skb, (__be32 *)daddr, (__be32 *)addr);
+	}
+
+	*daddr = *addr;
 
 	return srh;
 }
@@ -427,11 +438,13 @@ seg6_next_csid_advance_arg(struct sk_buff *skb,
 	__u8 fnc_octects = seg6_flv_lcnode_func_octects(finfo);
 	__u8 blk_octects = seg6_flv_lcblock_octects(finfo);
 	struct in6_addr *addr;
+	struct in6_addr old;
 
 	if (unlikely(skb_ensure_writable(skb, sizeof(struct ipv6hdr))))
 		return SKB_DROP_REASON_NOMEM;
 
 	addr = &ipv6_hdr(skb)->daddr;
+	old = *addr;
 
 	/* advance DA.Argument */
 	memmove(&addr->s6_addr[blk_octects],
@@ -440,6 +453,9 @@ seg6_next_csid_advance_arg(struct sk_buff *skb,
 
 	memset(&addr->s6_addr[16 - fnc_octects], 0x00, fnc_octects);
 
+	if (skb->ip_summed == CHECKSUM_COMPLETE)
+		update_csum_diff16(skb, (__be32 *)&old, (__be32 *)addr);
+
 	return SKB_NOT_DROPPED_YET;
 }
 
-- 
2.43.0