[PATCH stable] ipvs: sctp: fix checksumming on snat and dnat handlers

[Daniel Borkmann <dborkman@xxxxxxxxxx>]

In our test lab, we have a simple SCTP client connecting to a SCTP
server via an IPVS load balancer. On some machines, load balancing
works, but on others the initial handshake just fails, thus no
SCTP connection whatsoever can be established!

We observed that the SCTP INIT-ACK handshake reply from the IPVS
machine to the client had a correct IP checksum, but corrupt SCTP
checksum when forwarded, thus on the client-side the packet was
dropped and an intial handshake retriggered until all attempts
run into the void.

To fix this issue, this patch i) adds a missing CHECKSUM_UNNECESSARY
after the full checksum (re-)calculation (as done in IPVS TCP and UDP
code as well), and ii) calculates the checksum in little-endian format
(as fixed with the SCTP code in commit 4458f04c: sctp: Clean up sctp
checksumming code). Stable backport of upstream commit 4b47bc9a.

Cc: Julian Anastasov <ja@xxxxxx>
Cc: Simon Horman <horms@xxxxxxxxxxxx>
Cc: Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>
Signed-off-by: Daniel Borkmann <dborkman@xxxxxxxxxx>
---
 net/netfilter/ipvs/ip_vs_proto_sctp.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/net/netfilter/ipvs/ip_vs_proto_sctp.c 
b/net/netfilter/ipvs/ip_vs_proto_sctp.c
index 9f3fb75..94bb367 100644
--- a/net/netfilter/ipvs/ip_vs_proto_sctp.c
+++ b/net/netfilter/ipvs/ip_vs_proto_sctp.c
@@ -70,7 +70,7 @@ sctp_snat_handler(struct sk_buff *skb,
        sctp_sctphdr_t *sctph;
        unsigned int sctphoff;
        struct sk_buff *iter;
-       __be32 crc32;
+       __u32 crc32;
 
 #ifdef CONFIG_IP_VS_IPV6
        if (cp->af == AF_INET6)
@@ -101,8 +101,9 @@ sctp_snat_handler(struct sk_buff *skb,
        skb_walk_frags(skb, iter)
                crc32 = sctp_update_cksum((u8 *) iter->data, skb_headlen(iter),
                                          crc32);
-       crc32 = sctp_end_cksum(crc32);
-       sctph->checksum = crc32;
+       sctph->checksum = sctp_end_cksum(crc32);
+
+       skb->ip_summed = CHECKSUM_UNNECESSARY;
 
        return 1;
 }
@@ -114,7 +115,7 @@ sctp_dnat_handler(struct sk_buff *skb,
        sctp_sctphdr_t *sctph;
        unsigned int sctphoff;
        struct sk_buff *iter;
-       __be32 crc32;
+       __u32 crc32;
 
 #ifdef CONFIG_IP_VS_IPV6
        if (cp->af == AF_INET6)
@@ -145,8 +146,9 @@ sctp_dnat_handler(struct sk_buff *skb,
        skb_walk_frags(skb, iter)
                crc32 = sctp_update_cksum((u8 *) iter->data, skb_headlen(iter),
                                          crc32);
-       crc32 = sctp_end_cksum(crc32);
-       sctph->checksum = crc32;
+       sctph->checksum = sctp_end_cksum(crc32);
+
+       skb->ip_summed = CHECKSUM_UNNECESSARY;
 
        return 1;
 }
-- 
1.7.11.7

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