linux-cve-announce - CVE-2026-23016: inet: frags: drop fraglist conntrack references

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Message-ID: <2026013135-CVE-2026-23016-5869@gregkh>
Date: Sat, 31 Jan 2026 12:39:37 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...nel.org>
Subject: CVE-2026-23016: inet: frags: drop fraglist conntrack references

From: Greg Kroah-Hartman <gregkh@...nel.org>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

inet: frags: drop fraglist conntrack references

Jakub added a warning in nf_conntrack_cleanup_net_list() to make debugging
leaked skbs/conntrack references more obvious.

syzbot reports this as triggering, and I can also reproduce this via
ip_defrag.sh selftest:

 conntrack cleanup blocked for 60s
 WARNING: net/netfilter/nf_conntrack_core.c:2512
 [..]

conntrack clenups gets stuck because there are skbs with still hold nf_conn
references via their frag_list.

   net.core.skb_defer_max=0 makes the hang disappear.

Eric Dumazet points out that skb_release_head_state() doesn't follow the
fraglist.

ip_defrag.sh can only reproduce this problem since
commit 6471658dc66c ("udp: use skb_attempt_defer_free()"), but AFAICS this
problem could happen with TCP as well if pmtu discovery is off.

The relevant problem path for udp is:
1. netns emits fragmented packets
2. nf_defrag_v6_hook reassembles them (in output hook)
3. reassembled skb is tracked (skb owns nf_conn reference)
4. ip6_output refragments
5. refragmented packets also own nf_conn reference (ip6_fragment
   calls ip6_copy_metadata())
6. on input path, nf_defrag_v6_hook skips defragmentation: the
   fragments already have skb->nf_conn attached
7. skbs are reassembled via ipv6_frag_rcv()
8. skb_consume_udp -> skb_attempt_defer_free() -> skb ends up
   in pcpu freelist, but still has nf_conn reference.

Possible solutions:
 1 let defrag engine drop nf_conn entry, OR
 2 export kick_defer_list_purge() and call it from the conntrack
   netns exit callback, OR
 3 add skb_has_frag_list() check to skb_attempt_defer_free()

2 & 3 also solve ip_defrag.sh hang but share same drawback:

Such reassembled skbs, queued to socket, can prevent conntrack module
removal until userspace has consumed the packet. While both tcp and udp
stack do call nf_reset_ct() before placing skb on socket queue, that
function doesn't iterate frag_list skbs.

Therefore drop nf_conn entries when they are placed in defrag queue.
Keep the nf_conn entry of the first (offset 0) skb so that reassembled
skb retains nf_conn entry for sake of TX path.

Note that fixes tag is incorrect; it points to the commit introducing the
'ip_defrag.sh reproducible problem': no need to backport this patch to
every stable kernel.

The Linux kernel CVE team has assigned CVE-2026-23016 to this issue.

Affected and fixed versions
===========================

	Issue introduced in 6.18 with commit 6471658dc66c670580a7616e75f51b52917e7883 and fixed in 6.18.6 with commit 088ca99dbb039c444c3ff987c5412a73f4f0cbf8
	Issue introduced in 6.18 with commit 6471658dc66c670580a7616e75f51b52917e7883 and fixed in 6.19-rc5 with commit 2ef02ac38d3c17f34a00c4b267d961a8d4b45d1a

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-23016
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	net/ipv4/inet_fragment.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/088ca99dbb039c444c3ff987c5412a73f4f0cbf8
	https://git.kernel.org/stable/c/2ef02ac38d3c17f34a00c4b267d961a8d4b45d1a