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Message-ID: <ZRcOXJ0pkuph6fko@debian.debian> Date: Fri, 29 Sep 2023 10:50:20 -0700 From: Yan Zhai <yan@...udflare.com> To: netdev@...r.kernel.org Cc: "David S. Miller" <davem@...emloft.net>, David Ahern <dsahern@...nel.org>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Aya Levin <ayal@...dia.com>, Tariq Toukan <tariqt@...dia.com>, linux-kernel@...r.kernel.org, kernel-team@...udflare.com Subject: [PATCH net] ipv6: avoid atomic fragment on GSO packets 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@...udflare.com> Signed-off-by: Yan Zhai <yan@...udflare.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
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