Messages in this thread Patch in this message | ![/](/images/icornerl.gif) | | Date | Thu, 19 Oct 2023 22:32:51 -0700 | From | Yan Zhai <> | Subject | [PATCH v3 net-next 3/3] ipv6: avoid atomic fragment on GSO packets |
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When the ipv6 stack output a GSO packet, if its gso_size is larger than dst MTU, then all segments would be fragmented. However, it is possible for a GSO packet to have a trailing segment with smaller actual size than both gso_size as well as the MTU, which leads to an "atomic fragment". Atomic fragments are considered harmful in RFC-8021. An Existing report from APNIC also shows that atomic fragments are more likely to be dropped even it is equivalent to a no-op [1].
Add an extra check in the GSO slow output path. For each segment from the original over-sized packet, if it fits with the path MTU, then avoid generating an atomic fragment.
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 3270d56b5c37..3d4e8edaa10b 100644 --- a/net/ipv6/ip6_output.c +++ b/net/ipv6/ip6_output.c @@ -162,7 +162,13 @@ ip6_finish_output_gso_slowpath_drop(struct net *net, struct sock *sk, int err; skb_mark_not_on_list(segs); - err = ip6_fragment(net, sk, segs, ip6_finish_output2); + /* Last GSO segment can be smaller than gso_size (and MTU). + * Adding a fragment header would produce an "atomic fragment", + * which is considered harmful (RFC-8021). Avoid that. + */ + err = segs->len > mtu ? + ip6_fragment(net, sk, segs, ip6_finish_output2) : + ip6_finish_output2(net, sk, segs); if (err && ret == 0) ret = err; } -- 2.30.2
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