Patch "ipv6: avoid atomic fragment on GSO packets" has been added to the 5.15-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

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 5.15-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-5.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit b1b2a0fb684cbeeca74f27767135240a722cccba
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 2207acd7108c1..5045b479c2a95 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -159,7 +159,13 @@ 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;
 	}