netpoll: normalize skb->dev to the netpoll device

netpoll: normalize skb->dev to the netpoll device

__netpoll_send_skb() always transmits through np->dev and queues busy
packets on np->dev->npinfo->txq, but it leaves skb->dev unchanged.
Stacked callers such as DSA and macvlan can reach netpoll with skb->dev
still naming the upper device while np->dev is the lower device that
owns the netpoll state.

If the skb has to be deferred, queue_process() later dequeues it from
the lower device's txq but retries it through skb->dev. That can
re-enter the upper ndo_start_xmit path on an already transformed skb,
and if the upper device disappears before the lower txq drains the
workqueue can dereference a stale skb->dev pointer.

The buggy scenario involves two paths, with each column showing the
order within that path:

path A label: netpoll enqueue path   path B label: upper-device teardown
1. Stacked xmit calls netpoll        1. Teardown unregisters the upper
   with lower np->dev and upper         net_device while lower npinfo
   skb->dev.                            stays alive.
2. __netpoll_send_skb() uses         2. netdev_release() runs for the
   np->dev->npinfo as the txq           upper net_device.
   owner.
3. Busy transmit queues the skb      3. The lower txq still owns the
   on that lower txq with upper         deferred skb.
   skb->dev.
4. queue_process() drains the        4. queue_process() dereferences
   lower txq and reads skb->dev.        that stale upper skb->dev.

Normalize skb->dev to np->dev after loading np->dev from the netpoll
instance, before either the direct transmit path or the fallback enqueue.
This keeps the queued skb in the same device and txq domain as the
netpoll state that owns it.

KASAN report as below:

KASAN slab-use-after-free in queue_process+0x7c/0x480
Workqueue: events queue_process
The buggy address belongs to the object at ffff88810906c000 which belongs
to the cache kmalloc-4k of size 4096
The buggy address is located 168 bytes inside of freed 4096-byte region
[ffff88810906c000, ffff88810906d000)
Read of size 8
Call trace:
  dump_stack_lvl+0x73/0xb0 (?:?)
  print_report+0xd1/0x620 (?:?)
  srso_alias_return_thunk+0x5/0xfbef5 (?:?)
  __virt_addr_valid+0x215/0x420 (?:?)
  kasan_complete_mode_report_info+0x64/0x200 (?:?)
  kasan_report+0xf7/0x130 (?:?)
  queue_process+0x7c/0x480 (net/core/netpoll.c:88)
  kasan_check_range+0x10c/0x1c0 (?:?)
  __kasan_check_read+0x15/0x20 (?:?)
  process_one_work+0x8b7/0x1af0 (kernel/workqueue.c:3200)
  assign_work+0x170/0x3f0 (?:?)
  worker_thread+0x574/0xf10 (?:?)
  _raw_spin_unlock_irqrestore+0x4b/0x60 (?:?)
  trace_hardirqs_on+0x2a/0x180 (?:?)
  kthread+0x2fc/0x3f0 (?:?)
  ret_from_fork+0x58b/0x830 (?:?)
  __switch_to+0x58e/0xe90 (?:?)
  __switch_to_asm+0x39/0x70 (?:?)
  ret_from_fork_asm+0x1a/0x30 (?:?)
Freed by task stack:
  kasan_save_stack+0x3d/0x60 (?:?)
  kasan_save_track+0x18/0x40 (?:?)
  kasan_save_free_info+0x3f/0x60 (?:?)
  __kasan_slab_free+0x48/0x70 (?:?)
  kfree+0x20e/0x4e0 (?:?)
  kvfree+0x31/0x40 (?:?)
  netdev_release+0x71/0x90 (net/core/net-sysfs.c:2227)
  device_release+0xd2/0x250 (?:?)
  kobject_put+0x181/0x4c0 (lib/kobject.c:730)
  netdev_run_todo+0x700/0x1000 (net/core/dev.c:11666)
  rtnl_dellink+0x396/0xc00 (net/core/rtnetlink.c:3558)
  rtnetlink_rcv_msg+0x740/0xc20 (net/core/rtnetlink.c:6897)
  netlink_rcv_skb+0x147/0x3a0 (?:?)
  rtnetlink_rcv+0x19/0x20 (net/core/rtnetlink.c:7021)
  netlink_unicast+0x4d1/0x830 (net/netlink/af_netlink.c:1327)
  netlink_sendmsg+0x840/0xe10 (net/netlink/af_netlink.c:1812)
  ____sys_sendmsg+0x8a7/0xb50 (?:?)
  ___sys_sendmsg+0x104/0x190 (?:?)
  __sys_sendmsg+0x135/0x1d0 (?:?)
  __x64_sys_sendmsg+0x7b/0xc0 (?:?)
  x64_sys_call+0x205c/0x2130 (?:?)
  do_syscall_64+0x115/0x6a0 (arch/x86/entry/syscall_64.c:87)
  entry_SYSCALL_64_after_hwframe+0x77/0x7f (?:?)

Fixes: 5de4a473bda4 ("netpoll queue cleanup")
Signed-off-by: Zhang Cen <rollkingzzc@gmail.com>
Link: https://patch.msgid.link/20260519104647.3517990-1-rollkingzzc@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>