6.5-stable review patch. If anyone has any objections, please let me know. ------------------ From: Yan Zhai <yan@xxxxxxxxxxxxxx> [ 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> --- 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 9270ef7f8e98b..0ac1d4595f0f0 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; } -- 2.42.0
