CVE-2026-43374: net: nexthop: fix percpu use-after-free in remove_nh_grp_entry

CVE-2026-43374: net: nexthop: fix percpu use-after-free in remove_nh_grp_entry

[Greg Kroah-Hartman]

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

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

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

net: nexthop: fix percpu use-after-free in remove_nh_grp_entry

When removing a nexthop from a group, remove_nh_grp_entry() publishes
the new group via rcu_assign_pointer() then immediately frees the
removed entry's percpu stats with free_percpu(). However, the
synchronize_net() grace period in the caller remove_nexthop_from_groups()
runs after the free. RCU readers that entered before the publish still
see the old group and can dereference the freed stats via
nh_grp_entry_stats_inc() -> get_cpu_ptr(nhge->stats), causing a
use-after-free on percpu memory.

Fix by deferring the free_percpu() until after synchronize_net() in the
caller. Removed entries are chained via nh_list onto a local deferred
free list. After the grace period completes and all RCU readers have
finished, the percpu stats are safely freed.

The Linux kernel CVE team has assigned CVE-2026-43374 to this issue.


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

	Issue introduced in 6.9 with commit f4676ea74b8549cd88dbfe2a592ce4530039e61f and fixed in 6.12.78 with commit abf4feaee6405f1441929c6ebe7a250f2cd170a7
	Issue introduced in 6.9 with commit f4676ea74b8549cd88dbfe2a592ce4530039e61f and fixed in 6.18.19 with commit ab5ebab9664214ba41a7633cb4e72f128204f924
	Issue introduced in 6.9 with commit f4676ea74b8549cd88dbfe2a592ce4530039e61f and fixed in 6.19.9 with commit 9e08ad731862b22a87cc55f752e16d66cdc9e231
	Issue introduced in 6.9 with commit f4676ea74b8549cd88dbfe2a592ce4530039e61f and fixed in 7.0 with commit b2662e7593e94ae09b1cf7ee5f09160a3612bcb2

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-2026-43374
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/nexthop.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/abf4feaee6405f1441929c6ebe7a250f2cd170a7
	https://git.kernel.org/stable/c/ab5ebab9664214ba41a7633cb4e72f128204f924
	https://git.kernel.org/stable/c/9e08ad731862b22a87cc55f752e16d66cdc9e231
	https://git.kernel.org/stable/c/b2662e7593e94ae09b1cf7ee5f09160a3612bcb2