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ipv6: avoid atomic fragment on GSO packets

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[ Upstream commit 03d6c848bf ]

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: b210de4f8c ("net: ipv6: Validate GSO SKB before finish IPv6 processing")
Reported-by: David Wragg <dwragg@cloudflare.com>
Signed-off-by: Yan Zhai <yan@cloudflare.com>
Link: https://lore.kernel.org/r/90912e3503a242dca0bc36958b11ed03a2696e5e.1698156966.git.yan@cloudflare.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
This commit is contained in:
Yan Zhai 2023-10-24 07:26:40 -07:00 committed by Greg Kroah-Hartman
parent cd524da2cb
commit 1c3b6106b6
1 changed files with 7 additions and 1 deletions
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8
net/ipv6/ip6_output.c
View File

@ -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;
}