CVE-2026-98558: net: flush skb_defer_nodes in dev_cpu_dead()

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

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

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

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

net: flush skb_defer_nodes in dev_cpu_dead()

When a CPU goes offline, dev_cpu_dead() drains its softnet queues
(completion_queue, output_queue, poll_list, process_queue, and
input_pkt_queue), but leaves net_hotdata.skb_defer_nodes untouched.

If oldcpu goes offline while holding pending skbs in its
skb_defer_nodes lists (e.g. below the sysctl_skb_defer_max >> 1 IPI
threshold, or if the IPI races with CPU teardown), those skbs remain
stranded until oldcpu is brought back online. If any of these skbs
hold page_pool fragments, page_pool_destroy() will stall indefinitely
waiting for inflight pages to be returned when a netdev or driver is
torn down while oldcpu is offline.

Additionally, if smp_call_function_single_async() fails in
kick_defer_list_purge() because the target CPU went offline, reset
defer_ipi_scheduled to 0 so future IPI kicks are not blocked when the
CPU comes back online.

Also, if oldcpu was the last online CPU on its NUMA node, drain that
node's slot across all CPUs so no skbs deferred from that node remain
stranded on idle remote CPUs (or if the node itself is subsequently
offlined).

Finally, in skb_attempt_defer_free(), re-check cpu_online(cpu) and
whether the caller migrated CPUs after llist_add(), flushing the node
list if so, to close the preemption TOCTOU race against CPU/node
teardown.

The Linux kernel CVE team has assigned CVE-2026-98558 to this issue.


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

	Issue introduced in 5.19 with commit 68822bdf76f10c3dc80609d4e2cdc1e847429086 and fixed in 7.2.9 with commit c68f4157463448a76893223e117170be8374d773
	Issue introduced in 5.19 with commit 68822bdf76f10c3dc80609d4e2cdc1e847429086 and fixed in 7.3-rc5 with commit 06e3f54e8b22040ada01a28343badf1990b4dd7d

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-2026-98558
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/core/dev.c
	net/core/dev.h
	net/core/skbuff.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/c68f4157463448a76893223e117170be8374d773
	https://git.kernel.org/stable/c/06e3f54e8b22040ada01a28343badf1990b4dd7d