CVE-2026-90232: amt: Don't support cross-netns setup.

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

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

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

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

amt: Don't support cross-netns setup.

When a lower device is unregistered, amt_device_event() tries
to unregister its upper AMT device, but it has two problems.

  1. amt_lookup_upper_dev() looks up an upper device in the
      lower device's netns only

  2. amt_device_event() unregisters a single upper device only

If AMT device is created on a lower device in another netns,
removing the lower device triggers the splat below and gets
stuck until all upper devices are removed. [0]

The cross-netns setup seems unintentional considering 1. and
the following points:

  * amt_link_setup() sets dev->netns_immutable to true
  * skb_scrub_packet() is not called in the fast path
  * iproute2 binary fails to find cross-netns lower device via
    link-netns:
      # ip -n ns1 link add amt0 link-netns ns2 type amt dev veth1
      Cannot find device "veth1"

Instead of supporting it properly and preparing for per-netns
netdev unreg, let's forbid cross-netns setup.

Note that the problem 2. needs a separate fix.

[0]:
WARNING: net/core/dev.c:12518 at unregister_netdevice_many_notify+0x1cce/0x2250, CPU#48: ip/2031
Modules linked in:
CPU: 48 UID: 0 PID: 2031 Comm: ip Not tainted 7.2.0-rc5+ #27 PREEMPT(full)
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-debian-1.17.0-1 04/01/2014
RIP: 0010:unregister_netdevice_many_notify (net/core/dev.c:12518)
Code: 89 ef e8 d5 52 ae fe e9 d0 f4 ff ff 48 8d 3d f9 3b 9c 02 48 c7 c6 c0 0b 63 84 ba ab 1f 00 00 67 48 0f b9 3a e9 65 ff ff ff 90 <0f> 0b 90 eb 81 48 8d 3d f6 3b 9c 02 48 c7 c6 c0 0b 63 84 ba e2 1f
RSP: 0018:ffffc90004abf160 EFLAGS: 00010212
RAX: ffff888104d38260 RBX: ffff88800b0911b8 RCX: dffffc0000000000
RDX: 0000000000000000 RSI: 0000000000000008 RDI: ffffffff85b9f880
RBP: ffffc90004abf2d0 R08: ffffffff85b9f887 R09: 1ffffffff0b73f10
R10: dffffc0000000000 R11: fffffbfff0b73f11 R12: ffff88800b091d08
R13: ffff88800b091178 R14: dffffc0000000000 R15: ffff88800b091000
FS:  00007f555b86c600(0000) GS:ffff8881942a0000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000562107d489c0 CR3: 0000000109a40002 CR4: 0000000000372ef0
Call Trace:
 <TASK>
 rtnl_dellink (net/core/rtnetlink.c:3632 net/core/rtnetlink.c:3674)
 rtnetlink_rcv_msg (net/core/rtnetlink.c:7112)
 netlink_rcv_skb (net/netlink/af_netlink.c:2556)
 netlink_unicast (net/netlink/af_netlink.c:1319)
 netlink_sendmsg (net/netlink/af_netlink.c:1900)
 ____sys_sendmsg (net/socket.c:775)
 __sys_sendmsg (net/socket.c:2738)
 do_syscall_64 (arch/x86/entry/syscall_64.c:63)
 entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
...
unregister_netdevice: waiting for veth0 to become free. Usage count = 7
ref_tracker: netdev@ffff88800d7496d8 has 3/3 users at
     __netdev_adjacent_dev_insert (./include/linux/netdevice.h:4525 ./include/linux/netdevice.h:4554 net/core/dev.c:8791)
     __netdev_upper_dev_link (net/core/dev.c:8879 net/core/dev.c:8963)
     netdev_upper_dev_link (net/core/dev.c:9009)
     amt_newlink (drivers/net/amt.c:3321)

The Linux kernel CVE team has assigned CVE-2026-90232 to this issue.


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

	Issue introduced in 5.16 with commit b9022b53adad88fd6cf2b9718c9e498504f3e1dd and fixed in 6.18.52 with commit 569eb11f0990849ba48706a2bfa743273707e254
	Issue introduced in 5.16 with commit b9022b53adad88fd6cf2b9718c9e498504f3e1dd and fixed in 7.2.6 with commit e1dc0af719a2566ae7ccb82c0a11f70aa54908e9
	Issue introduced in 5.16 with commit b9022b53adad88fd6cf2b9718c9e498504f3e1dd and fixed in 7.3-rc1 with commit e99ecc3046ea5f5c6b5e1f8b4ef854c6c6998e06

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-2026-90232
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:
	drivers/net/amt.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/569eb11f0990849ba48706a2bfa743273707e254
	https://git.kernel.org/stable/c/e1dc0af719a2566ae7ccb82c0a11f70aa54908e9
	https://git.kernel.org/stable/c/e99ecc3046ea5f5c6b5e1f8b4ef854c6c6998e06