[PATCH v1 net] amt: Don't support cross-netns setup.

[PATCH v1 net] amt: Don't support cross-netns setup.

[Kuniyuki Iwashima]

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)

Fixes: b9022b53adad ("amt: add control plane of amt interface")
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 drivers/net/amt.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/net/amt.c b/drivers/net/amt.c
index 182a41d59a75..7645ce3a29ad 100644
--- a/drivers/net/amt.c
+++ b/drivers/net/amt.c
@@ -3221,6 +3221,9 @@ static int amt_newlink(struct net_device *dev,
 	struct nlattr **tb = params->tb;
 	int err = -EINVAL;
 
+	if (!net_eq(link_net, dev_net(dev)))
+		return err;
+
 	amt->net = link_net;
 	amt->mode = nla_get_u32(data[IFLA_AMT_MODE]);
 
-- 
2.55.0.679.g6767b8d81c-goog

Re: [PATCH v1 net] amt: Don't support cross-netns setup.

[Taehee Yoo]

On Fri, Aug 7, 2026 at 11:03 AM Kuniyuki Iwashima <kuniyu@google.com> wrote:
>

Hi Kuniyuki,
Thanks a lot for your work!

> 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)
>

Reviewed-by: Taehee Yoo <ap420073@gmail.com>

Thanks a lot!
Taehee Yoo

> Fixes: b9022b53adad ("amt: add control plane of amt interface")
> Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
> ---
>  drivers/net/amt.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/drivers/net/amt.c b/drivers/net/amt.c
> index 182a41d59a75..7645ce3a29ad 100644
> --- a/drivers/net/amt.c
> +++ b/drivers/net/amt.c
> @@ -3221,6 +3221,9 @@ static int amt_newlink(struct net_device *dev,
>         struct nlattr **tb = params->tb;
>         int err = -EINVAL;
>
> +       if (!net_eq(link_net, dev_net(dev)))
> +               return err;
> +
>         amt->net = link_net;
>         amt->mode = nla_get_u32(data[IFLA_AMT_MODE]);
>
> --
> 2.55.0.679.g6767b8d81c-goog
>

Re: [PATCH v1 net] amt: Don't support cross-netns setup.

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net-next.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Fri,  7 Aug 2026 02:03:23 +0000 you wrote:
> 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
> 
> [...]

Here is the summary with links:
  - [v1,net] amt: Don't support cross-netns setup.
    https://git.kernel.org/netdev/net-next/c/e99ecc3046ea

You are awesome, thank you!
-- 
Deet-doot-dot, I am a bot.
https://korg.docs.kernel.org/patchwork/pwbot.html