CVE-2026-64576: nexthop: initialize extack in nh_res_bucket_migrate()

From: Greg Kroah-Hartman <[email protected]>

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

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

nexthop: initialize extack in nh_res_bucket_migrate()

nh_res_bucket_migrate() passes an uninitialized netlink_ext_ack to
call_nexthop_res_bucket_notifiers(). When
nh_notifier_res_bucket_info_init() fails (e.g. the kzalloc returns
-ENOMEM), the error is propagated back before any notifier sets
extack._msg, and the error path formats the stale pointer with
pr_err_ratelimited("%s\n", extack._msg). With CONFIG_INIT_STACK_NONE
this dereferences uninitialized stack memory:

  Oops: general protection fault, probably for non-canonical address ...
  KASAN: maybe wild-memory-access in range [...]
  RIP: 0010:string (lib/vsprintf.c:730)
   vsnprintf (lib/vsprintf.c:2945)
   _printk (kernel/printk/printk.c:2504)
   nh_res_bucket_migrate (net/ipv4/nexthop.c:1816)
   nh_res_table_upkeep (net/ipv4/nexthop.c:1866)
   rtm_new_nexthop (net/ipv4/nexthop.c:3323)
   rtnetlink_rcv_msg (net/core/rtnetlink.c:7076)
   netlink_sendmsg (net/netlink/af_netlink.c:1900)
  Kernel panic - not syncing: Fatal exception

Zero-initialize extack so _msg is NULL on error paths that never set it.

The Linux kernel CVE team has assigned CVE-2026-64576 to this issue.


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

	Issue introduced in 5.13 with commit 7c37c7e00411b3d1e0c5292368317aca69d1f324 and fixed in 6.6.148 with commit c0936c131a71657afc635d0db2ab096d15d473e1
	Issue introduced in 5.13 with commit 7c37c7e00411b3d1e0c5292368317aca69d1f324 and fixed in 6.12.101 with commit 3081702ea5aca0aeed9c1ade8eadf6cde8db6b7d
	Issue introduced in 5.13 with commit 7c37c7e00411b3d1e0c5292368317aca69d1f324 and fixed in 6.18.42 with commit d536bf205c71f700f6de2086038c3e1d77724715
	Issue introduced in 5.13 with commit 7c37c7e00411b3d1e0c5292368317aca69d1f324 and fixed in 7.1.6 with commit 18506d7263768d76ac8e057ba55a4d9da50aad66
	Issue introduced in 5.13 with commit 7c37c7e00411b3d1e0c5292368317aca69d1f324 and fixed in 7.2-rc5 with commit 6347c5314cee49f364aaf2e40ff15415a57a116e

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-2026-64576
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/c0936c131a71657afc635d0db2ab096d15d473e1
	https://git.kernel.org/stable/c/3081702ea5aca0aeed9c1ade8eadf6cde8db6b7d
	https://git.kernel.org/stable/c/d536bf205c71f700f6de2086038c3e1d77724715
	https://git.kernel.org/stable/c/18506d7263768d76ac8e057ba55a4d9da50aad66
	https://git.kernel.org/stable/c/6347c5314cee49f364aaf2e40ff15415a57a116e