Subject: [PATCH net v1 0/2] bonding: fix missing XDP compat check on xmit_hash_policy change

From: Jiayuan Chen <[email-masked]>
Date: Thu, 26 Feb 2026 16:03:00 +0800

syzkaller reported a bug https://syzkaller.appspot.com/bug?extid=5a287bcdc08104bc3132

When a bond device is in 802.3ad or balance-xor mode, XDP is supported
only when xmit_hash_policy != vlan+srcmac.  This constraint is enforced
in bond_option_mode_set() via bond_xdp_check(), which prevents switching
to an XDP-incompatible mode while a program is loaded.  However, the
symmetric path -- changing xmit_hash_policy while XDP is loaded -- had
no such guard in bond_option_xmit_hash_policy_set().

This means the following sequence silently creates an inconsistent state:

  1. Create a bond in 802.3ad mode with xmit_hash_policy=layer2+3.
  2. Attach a native XDP program to the bond.
  3. Change xmit_hash_policy to vlan+srcmac (no error, not checked).

Now bond->xdp_prog is set but bond_xdp_check() returns false for the
same device.  When the bond is later torn down (e.g. netns deletion),
dev_xdp_uninstall() calls bond_xdp_set(dev, NULL) to remove the
program, which hits the bond_xdp_check() guard and returns -EOPNOTSUPP,
triggering a kernel WARNING:

  bond1 (unregistering): Error: No native XDP support for the current bonding mode
  ------------[ cut here ]------------
  dev_xdp_install(dev, mode, bpf_op, NULL, 0, NULL)
  WARNING: net/core/dev.c:10361 at dev_xdp_uninstall net/core/dev.c:10361 [inline], CPU#0: kworker/u8:22/11031
  Modules linked in:
  CPU: 0 UID: 0 PID: 11031 Comm: kworker/u8:22 Not tainted syzkaller #0 PREEMPT(full)
  Workqueue: netns cleanup_net
  RIP: 0010:dev_xdp_uninstall net/core/dev.c:10361 [inline]
  RIP: 0010:unregister_netdevice_many_notify+0x1efd/0x2370 net/core/dev.c:12393
  RSP: 0018:ffffc90003b2f7c0 EFLAGS: 00010293
  RAX: ffffffff8971e99c RBX: ffff888052f84c40 RCX: ffff88807896bc80
  RDX: 0000000000000000 RSI: 00000000ffffffa1 RDI: 0000000000000000
  RBP: ffffc90003b2f930 R08: ffffc90003b2f207 R09: 1ffff92000765e40
  R10: dffffc0000000000 R11: fffff52000765e41 R12: 00000000ffffffa1
  R13: ffff888052f84c38 R14: 1ffff1100a5f0988 R15: ffffc9000df67000
  FS:  0000000000000000(0000) GS:ffff8881254ae000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 00007f60871d5d58 CR3: 000000006c41c000 CR4: 00000000003526f0
  Call Trace:
   <TASK>
   ops_exit_rtnl_list net/core/net_namespace.c:187 [inline]
   ops_undo_list+0x3d3/0x940 net/core/net_namespace.c:248
   cleanup_net+0x56b/0x800 net/core/net_namespace.c:704
   process_one_work kernel/workqueue.c:3275 [inline]
   process_scheduled_works+0xaec/0x17a0 kernel/workqueue.c:3358
   worker_thread+0xa50/0xfc0 kernel/workqueue.c:3439
   kthread+0x388/0x470 kernel/kthread.c:467
   ret_from_fork+0x51e/0xb90 arch/x86/kernel/process.c:158
   ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
   </TASK>

Beyond the WARNING itself, when dev_xdp_install() fails during
dev_xdp_uninstall(), bond_xdp_set() returns early without calling
bpf_prog_put() on the old program.  dev_xdp_uninstall() then releases
only the reference held by dev->xdp_state[], while the reference held
by bond->xdp_prog is never dropped, leaking the struct bpf_prog.

The fix refactors the core logic of bond_xdp_check() into a new helper
__bond_xdp_check_mode(mode, xmit_policy) that takes both parameters
explicitly, avoiding the need to read them from the bond struct.
bond_xdp_check() becomes a thin wrapper around it.
bond_option_xmit_hash_policy_set() then uses __bond_xdp_check_mode()
directly, passing the candidate xmit_policy before it is committed,
mirroring exactly what bond_option_mode_set() already does for mode
changes.

Patch 1 adds the kernel fix.
Patch 2 adds a selftest that reproduces the WARNING by attaching native
XDP to a bond in 802.3ad mode, then attempting to change xmit_hash_policy
to vlan+srcmac -- verifying the change is rejected with the fix applied.

Jiayuan Chen (2):
  bpf/bonding: reject vlan+srcmac xmit_hash_policy change when XDP is
    loaded
  selftests/bpf: add test for xdp_bonding xmit_hash_policy compat

 drivers/net/bonding/bond_main.c               |  9 ++-
 drivers/net/bonding/bond_options.c            |  2 +
 include/net/bonding.h                         |  1 +
 .../selftests/bpf/prog_tests/xdp_bonding.c    | 58 +++++++++++++++++++
 4 files changed, 68 insertions(+), 2 deletions(-)

-- 
2.43.0