[PATCH net] ipv6: avoid atomic fragment on GSO packets

[Yan Zhai <yan@cloudflare.com>]

GSO packets can contain a trailing segment that is smaller than
gso_size. When examining the dst MTU for such packet, if its gso_size
is too large, then all segments would be fragmented. However, there is a
good chance the trailing segment has smaller actual size than both
gso_size as well as the MTU, which leads to an "atomic fragment".
RFC-8021 explicitly recommend to deprecate such use case. An Existing
report from APNIC also shows that atomic fragments can be dropped
unexpectedly along the path [1].

Add an extra check in ip6_fragment to catch all possible generation of
atomic fragments. Skip atomic header if it is called on a packet no
larger than MTU.

Link: https://www.potaroo.net/presentations/2022-03-01-ipv6-frag.pdf [1]
Fixes: b210de4f8c97 ("net: ipv6: Validate GSO SKB before finish IPv6 processing")
Reported-by: David Wragg <dwragg@cloudflare.com>
Signed-off-by: Yan Zhai <yan@cloudflare.com>
---
 net/ipv6/ip6_output.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 951ba8089b5b..42f5f68a6e24 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -854,6 +854,13 @@ int ip6_fragment(struct net *net, struct sock *sk, struct sk_buff *skb,
 	__be32 frag_id;
 	u8 *prevhdr, nexthdr = 0;
 
+	/* RFC-8021 recommended atomic fragments to be deprecated. Double check
+	 * the actual packet size before fragment it.
+	 */
+	mtu = ip6_skb_dst_mtu(skb);
+	if (unlikely(skb->len <= mtu))
+		return output(net, sk, skb);
+
 	err = ip6_find_1stfragopt(skb, &prevhdr);
 	if (err < 0)
 		goto fail;
@@ -861,7 +868,6 @@ int ip6_fragment(struct net *net, struct sock *sk, struct sk_buff *skb,
 	nexthdr = *prevhdr;
 	nexthdr_offset = prevhdr - skb_network_header(skb);
 
-	mtu = ip6_skb_dst_mtu(skb);
 
 	/* We must not fragment if the socket is set to force MTU discovery
 	 * or if the skb it not generated by a local socket.
-- 
2.30.2