[PATCH net v6 0/2] net,bpf: fix null-ptr-deref in xdp_master_redirect() for bonding and add selftest

[Jiayuan Chen <jiayuan.chen@linux.dev>]

From: Jiayuan Chen <jiayuan.chen@shopee.com>

This series has gone through several rounds of discussion and the
maintainers hold different views on where the fix should live (in the
generic xdp_master_redirect() path vs. inside bonding). I respect all
of the suggestions, but I would like to get the crash fixed first, so
this version takes the approach of checking whether the master device
is up in xdp_master_redirect(), as suggested by Daniel Borkmann. If a
different shape is preferred later it can be done as a follow-up, but
the null-ptr-deref should not linger.

syzkaller reported a kernel panic, full decoded trace here:
https://syzkaller.appspot.com/bug?extid=80e046b8da2820b6ba73

Problem Description

bond_rr_gen_slave_id() dereferences bond->rr_tx_counter without a NULL
check. rr_tx_counter is a per-CPU counter that bonding only allocates
in bond_open() when the mode is round-robin. If the bond device was
never brought up, rr_tx_counter stays NULL.

The XDP redirect path can still reach that code on a bond that was
never opened: bpf_master_redirect_enabled_key is a global static key,
so as soon as any bond device has native XDP attached, the
XDP_TX -> xdp_master_redirect() interception is enabled for every
slave system-wide. The path xdp_master_redirect() ->
bond_xdp_get_xmit_slave() -> bond_xdp_xmit_roundrobin_slave_get() ->
bond_rr_gen_slave_id() then runs against a bond that has no
rr_tx_counter and crashes.

Solution

Patch 1: Fix this in the generic xdp_master_redirect() by skipping
master interception when the master device is not running. Returning
XDP_TX keeps the original XDP_TX behaviour on the receiving slave, and
avoids calling into any master ->ndo_xdp_get_xmit_slave() on a device
that has not fully initialized its XDP state. This is not specific to
bonding: any current or future master that defers XDP state allocation
to ->ndo_open() is protected.
Patch 2: Add a selftest that reproduces the above scenario.

Changes since v5:
https://lore.kernel.org/netdev/20260309030659.xxxxx-1-jiayuan.chen@linux.dev/
- Moved the fix back into xdp_master_redirect() and use netif_running()
  on the master device to decide whether to intercept
  (Suggested by Daniel Borkmann, seconded by Paolo Abeni and Eric Dumazet)

Changes since v4:
https://lore.kernel.org/netdev/20260304074301.35482-1-jiayuan.chen@linux.dev/
- Reverted unconditional alloc in bond_init(); instead add a NULL check
  with unlikely()/READ_ONCE() in bond_rr_gen_slave_id() and WRITE_ONCE()
  in bond_open(), avoiding memory waste for non-RR modes
  (Suggested by Nikolay Aleksandrov, patch by Jay Vosburgh)

Changes since v3:
https://lore.kernel.org/netdev/20260228021918.141002-1-jiayuan.chen@linux.dev/T/#t
- Added code comment and commit log explaining why rr_tx_counter is
  allocated unconditionally for all modes (Suggested by Jay Vosburgh)

Changes since v2:
https://lore.kernel.org/netdev/20260227092254.272603-1-jiayuan.chen@linux.dev/T/#t
- Moved allocation from bond_create_init() helper into bond_init()
  (ndo_init), which is the natural single point covering both creation
  paths and also handles post-creation mode changes to round-robin

Changes since v1:
https://lore.kernel.org/netdev/20260224112545.37888-1-jiayuan.chen@linux.dev/T/#t
- Moved the guard for NULL rr_tx_counter from xdp_master_redirect()
  into the bonding subsystem itself
  (Suggested by Sebastian Andrzej Siewior <bigeasy@linutronix.de>)

[1] https://syzkaller.appspot.com/bug?extid=80e046b8da2820b6ba73

Jiayuan Chen (2):
  net, bpf: fix null-ptr-deref in xdp_master_redirect() for down master
  selftests/bpf: add test for xdp_master_redirect with bond not up

 net/core/filter.c                             |   2 +
 .../selftests/bpf/prog_tests/xdp_bonding.c    | 101 +++++++++++++++++-
 2 files changed, 101 insertions(+), 2 deletions(-)

-- 
2.43.0