This is a note to let you know that I've just added the patch titled
ipv6: avoid atomic fragment on GSO packets
to the 6.1-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
ipv6-avoid-atomic-fragment-on-gso-packets.patch
and it can be found in the queue-6.1 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 29217349ab547d0bf119a85eaaefcb6881cf11ef
Author: Yan Zhai <yan@xxxxxxxxxxxxxx>
Date: Tue Oct 24 07:26:40 2023 -0700
ipv6: avoid atomic fragment on GSO packets
[ Upstream commit 03d6c848bfb406e9ef6d9846d759e97beaeea113 ]
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@xxxxxxxxxxxxxx>
Signed-off-by: Yan Zhai <yan@xxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/90912e3503a242dca0bc36958b11ed03a2696e5e.1698156966.git.yan@xxxxxxxxxxxxxx
Signed-off-by: Jakub Kicinski <kuba@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index ce2c5e728745f..3c2b2a85de367 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -161,7 +161,13 @@ 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;
}
