On PREEMPT_RT kernels, local_bh_disable() only calls migrate_disable()
(when PREEMPT_RT_NEEDS_BH_LOCK is not set) and does not disable
preemption. This means CFS scheduling can preempt a task inside the
per-CPU bulk queue (bq) operations in cpumap and devmap, allowing
another task on the same CPU to concurrently access the same bq,
leading to use-after-free, list corruption, and kernel panics.

Patch 1 fixes the cpumap race in bq_flush_to_queue(), originally
reported by syzbot [1].

Patch 2 fixes the same class of race in devmap's bq_xmit_all(),
identified by code inspection after Sebastian Andrzej Siewior pointed
out that devmap has the same per-CPU bulk queue pattern [2].

Both patches use local_lock_nested_bh() to serialize access to the
per-CPU bq. On non-RT this is a pure lockdep annotation with no
overhead; on PREEMPT_RT it provides a per-CPU sleeping lock.

To reproduce the devmap race, insert an mdelay(100) in bq_xmit_all()
after "cnt = bq->count" and before the actual transmit loop. Then pin
two threads to the same CPU, each running BPF_PROG_TEST_RUN with an XDP
program that redirects to a DEVMAP entry (e.g. a veth pair). CFS
timeslicing during the mdelay window causes interleaving. Without the
fix, KASAN reports null-ptr-deref due to operating on freed frames:

  BUG: KASAN: null-ptr-deref in __build_skb_around+0x22d/0x340
  Write of size 32 at addr 0000000000000d50 by task devmap_race_rep/449

  CPU: 0 UID: 0 PID: 449 Comm: devmap_race_rep Not tainted 6.19.0+ #31 PREEMPT_RT
  Call Trace:
   <TASK>
   __build_skb_around+0x22d/0x340
   build_skb_around+0x25/0x260
   __xdp_build_skb_from_frame+0x103/0x860
   veth_xdp_rcv_bulk_skb.isra.0+0x162/0x320
   veth_xdp_rcv.constprop.0+0x61e/0xbb0
   veth_poll+0x280/0xb50
   __napi_poll.constprop.0+0xa5/0x590
   net_rx_action+0x4b0/0xea0
   handle_softirqs.isra.0+0x1b3/0x780
   __local_bh_enable_ip+0x12a/0x240
   xdp_test_run_batch.constprop.0+0xedd/0x1f60
   bpf_test_run_xdp_live+0x304/0x640
   bpf_prog_test_run_xdp+0xd24/0x1b70
   __sys_bpf+0x61c/0x3e00
   </TASK>

  Kernel panic - not syncing: Fatal exception in interrupt

[1] https://lore.kernel.org/all/69369331.a70a0220.38f243.009d.GAE@google.com/T/
[2] https://lore.kernel.org/bpf/20260212023634.366343-1-jiayuan.chen@linux.dev/

---
v3 -> v4: https://lore.kernel.org/all/20260213034018.284146-1-jiayuan.chen@linux.dev/
- Move panic trace to cover letter. (Sebastian Andrzej Siewior)
- Add Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> to both patches
  from cover letter.

v2 -> v3: https://lore.kernel.org/bpf/20260212023634.366343-1-jiayuan.chen@linux.dev/
- Fix commit message: remove incorrect "spin_lock() becomes rt_mutex"
claim, the per-CPU bq has no spin_lock at all. (Sebastian Andrzej Siewior)
- Fix commit message: accurately describe local_lock_nested_bh()
behavior instead of referencing local_lock(). (Sebastian Andrzej Siewior)
- Remove incomplete discussion of snapshot alternative.
(Sebastian Andrzej Siewior)
- Remove panic trace from commit message. (Sebastian Andrzej Siewior)
- Add patch 2/2 for devmap, same race pattern. (Sebastian Andrzej Siewior)

v1 -> v2: https://lore.kernel.org/bpf/20260211064417.196401-1-jiayuan.chen@linux.dev/
- Use local_lock_nested_bh()/local_unlock_nested_bh() instead of
local_lock()/local_unlock(), since these paths already run under
local_bh_disable(). (Sebastian Andrzej Siewior)
- Replace "Caller must hold bq->bq_lock" comment with
lockdep_assert_held() in bq_flush_to_queue(). (Sebastian Andrzej Siewior)
- Fix Fixes tag to 3253cb49cbad ("softirq: Allow to drop the
softirq-BKL lock on PREEMPT_RT") which is the actual commit that
makes the race possible. (Sebastian Andrzej Siewior)

Jiayuan Chen (2):
  bpf: cpumap: fix race in bq_flush_to_queue on PREEMPT_RT
  bpf: devmap: fix race in bq_xmit_all on PREEMPT_RT

 kernel/bpf/cpumap.c | 17 +++++++++++++++--
 kernel/bpf/devmap.c | 25 +++++++++++++++++++++----
 2 files changed, 36 insertions(+), 6 deletions(-)

-- 
2.43.0