Messages in this thread | | | Date | Sat, 30 Sep 2023 13:08:54 +0200 | From | Florian Westphal <> | Subject | Re: [PATCH net] ipv6: avoid atomic fragment on GSO packets |
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Yan Zhai <yan@cloudflare.com> wrote: > 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); > +
This helper is also called for skbs where IP6CB(skb)->frag_max_size exceeds the MTU, so this check looks wrong to me.
Same remark for dst_allfrag() check in __ip6_finish_output(), after this patch, it would be ignored.
I think you should consider to first refactor __ip6_finish_output to make the existing checks more readable (e.g. handle gso vs. non-gso in separate branches) and then add the check to last seg in ip6_finish_output_gso_slowpath_drop().
Alternatively you might be able to pass more info down to ip6_fragment and move decisions there.
In any case we should make same frag-or-no-frag decisions, regardless of this being the orig skb or a segmented one,
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