CVE-2026-89564: ip: orphan prefetched skbs before multicast forwarding

CVE-2026-89564: ip: orphan prefetched skbs before multicast forwarding

[Greg Kroah-Hartman]

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

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

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

ip: orphan prefetched skbs before multicast forwarding

IPv4 and IPv6 input preserve an skb->sk association installed by
bpf_sk_assign() so that local delivery can use the selected socket under
RCU. Both address families can also prefetch a socket in UDP early demux.
In both paths (BPF and UDP early demux) a reference is not guaranteed to
be held on the socket.

When a multicast packet is not locally deliverable, IPv6 hands the
original skb to ip6_mr_input(). IPv4's ip_mr_input() similarly keeps the
original skb when local delivery is not needed. Either path can put the
skb on an unresolved multicast route queue or forward it after the
receive-side RCU section ends.

After the prefetched socket is destroyed, a later skb free invokes
sock_pfree() and dereferences the stale skb->sk. Orphan the skb before
each non-local multicast forwarding path. Local delivery retains the
original skb; the existing skb_clone() calls provide multicast forwarding
with a socket-free clone.

The Linux kernel CVE team has assigned CVE-2026-89564 to this issue.


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

	Issue introduced in 5.7 with commit cf7fbe660f2dbd738ab58aea8e9b0ca6ad232449 and fixed in 6.18.50 with commit a8af6fbac895f057c4b8ff8a2e3fb4c5827fe4ce
	Issue introduced in 5.7 with commit cf7fbe660f2dbd738ab58aea8e9b0ca6ad232449 and fixed in 7.2.4 with commit f1281d4b99089fbaf0d3579bba62a814ebb4de4d
	Issue introduced in 5.7 with commit cf7fbe660f2dbd738ab58aea8e9b0ca6ad232449 and fixed in 7.3-rc1 with commit e36ce6e78fe3fc3c071a26750783b7ba081ce10d

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-2026-89564
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/ipmr.c
	net/ipv6/ip6_input.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/a8af6fbac895f057c4b8ff8a2e3fb4c5827fe4ce
	https://git.kernel.org/stable/c/f1281d4b99089fbaf0d3579bba62a814ebb4de4d
	https://git.kernel.org/stable/c/e36ce6e78fe3fc3c071a26750783b7ba081ce10d