Subject: [PATCH bpf v1] bpf: cpumap: fix race in bq_flush_to_queue on PREEMPT_RT

From: Jiayuan Chen <[email-masked]>
Date: Wed, 11 Feb 2026 14:44:16 +0800

From: Jiayuan Chen <[email-masked]>

On PREEMPT_RT kernels, the per-CPU xdp_bulk_queue (bq) can be accessed
concurrently by multiple preemptible tasks on the same CPU.

The original code assumes bq_enqueue() and __cpu_map_flush() run
atomically with respect to each other on the same CPU, relying on
local_bh_disable() to prevent preemption. However, on PREEMPT_RT,
local_bh_disable() only calls migrate_disable() and does not disable
preemption. spin_lock() also becomes a sleeping rt_mutex. Together,
this allows CFS scheduling to preempt a task during bq_flush_to_queue(),
enabling another task on the same CPU to enter bq_enqueue() and operate
on the same per-CPU bq concurrently.

This leads to several races:

1. Double __list_del_clearprev(): after spin_unlock() in
 bq_flush_to_queue(), a preempting task can call bq_enqueue() ->
 bq_flush_to_queue() on the same bq when bq->count reaches
 CPU_MAP_BULK_SIZE. Both tasks then call __list_del_clearprev()
 on the same bq->flush_node, the second call dereferences the
 prev pointer that was already set to NULL by the first.

2. bq->count and bq->q[] races: concurrent bq_enqueue() can corrupt
 the packet queue while bq_flush_to_queue() is processing it.

The race between task A (__cpu_map_flush -> bq_flush_to_queue) and
task B (bq_enqueue -> bq_flush_to_queue) on the same CPU:

Task A (xdp_do_flush)          Task B (cpu_map_enqueue)
----------------------         ------------------------
bq_flush_to_queue(bq)
  spin_lock(&q->producer_lock)
  /* flush bq->q[] to ptr_ring */
  bq->count = 0
  spin_unlock(&q->producer_lock)
                                 bq_enqueue(rcpu, xdpf)
  <-- CFS preempts Task A -->      bq->q[bq->count++] = xdpf
                                   /* ... more enqueues until full ... */
                                   bq_flush_to_queue(bq)
                                     spin_lock(&q->producer_lock)
                                     /* flush to ptr_ring */
                                     spin_unlock(&q->producer_lock)
                                     __list_del_clearprev(flush_node)
                                       /* sets flush_node.prev = NULL */
  <-- Task A resumes -->
  __list_del_clearprev(flush_node)
    flush_node.prev->next = ...
    /* prev is NULL -> kernel oops */

Fix this by adding a local_lock_t to xdp_bulk_queue and acquiring it
in bq_enqueue() and __cpu_map_flush(). On non-RT kernels, local_lock
maps to preempt_disable/enable with zero additional overhead. On
PREEMPT_RT, it provides a per-CPU sleeping lock that serializes
access to the bq.

An alternative approach of snapshotting bq->count and bq->q[] before
releasing the producer_lock was considered, but it requires copying
the entire bq->q[] array on every flush, adding unnecessary overhead.

To reproduce, insert an mdelay(100) between spin_unlock() and
__list_del_clearprev() in bq_flush_to_queue(), then run reproducer
provided by syzkaller.

Panic:
===
BUG: kernel NULL pointer dereference, address: 0000000000000000
PF: supervisor write access in kernel mode
PF: error_code(0x0002) - not-present page
PGD 0 P4D 0
Oops: Oops: 0002 [#1] SMP PTI
CPU: 0 UID: 0 PID: 377 Comm: a.out Not tainted 6.19.0+ #21 PREEMPT_RT
RIP: 0010:bq_flush_to_queue+0x145/0x200
RSP: 0018:ffffc90000eb78e8 EFLAGS: 00010246
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: ffffc90000eb7a10 RSI: 0000000000000000 RDI: 0000000000000000
RBP: ffffc90000eb7928 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000001
R13: ffffe8ffffc255c0 R14: 0000000000000000 R15: ffff88800c01b800
FS:  00007fae0762d6c0(0000) GS:ffff8880f9c75000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 000000000d698005 CR4: 0000000000770ef0
PKRU: 55555554
Call Trace:
 <TASK>
 __cpu_map_flush+0x2c/0x70
 xdp_do_flush+0x64/0x1b0
 xdp_test_run_batch.constprop.0+0x4d4/0x6d0
 bpf_test_run_xdp_live+0x24b/0x3e0
 ? get_page_from_freelist+0x5ea/0x1cf0
 ? __pfx_xdp_test_run_init_page+0x10/0x10
 ? __check_object_size+0x25e/0x320
 ? bpf_dispatcher_change_prog+0x20d/0x4e0
 bpf_prog_test_run_xdp+0x4a1/0x6e0
 ? rt_spin_unlock+0x61/0xb0
 __sys_bpf+0x44a/0x2760
 ? set_ptes.isra.0+0x3b/0x90
 ? rt_spin_unlock+0x61/0xb0
 ? do_anonymous_page+0x106/0x430
 __x64_sys_bpf+0x1a/0x30
 x64_sys_call+0x146c/0x26e0
 do_syscall_64+0xd5/0x5a0
 ? do_user_addr_fault+0x1d0/0x870
 ? irqentry_exit+0x3e/0x670
 ? clear_bhb_loop+0x30/0x80
 entry_SYSCALL_64_after_hwframe+0x76/0x7e
RIP: 0033:0x7fae0777928d
RSP: 002b:00007fae0762ce68 EFLAGS: 00000206 ORIG_RAX: 0000000000000141
RAX: ffffffffffffffda RBX: 00007fae0762dcdc RCX: 00007fae0777928d
RDX: 0000000000000050 RSI: 0000200000000000 RDI: 000000000000000a
RBP: 00007fae0762ce90 R08: 00007fae0762d6c0 R09: 203a6362696c6720
R10: 0000000000000000 R11: 0000000000000206 R12: 00007fae0762d6c0
R13: ffffffffffffff88 R14: 000000000000006e R15: 00007ffdd6e54110
 </TASK>
Modules linked in:
CR2: 0000000000000000
---[ end trace 0000000000000000 ]---
RIP: 0010:bq_flush_to_queue+0x145/0x200
RSP: 0018:ffffc90000eb78e8 EFLAGS: 00010246
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: ffffc90000eb7a10 RSI: 0000000000000000 RDI: 0000000000000000
RBP: ffffc90000eb7928 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000001
R13: ffffe8ffffc255c0 R14: 0000000000000000 R15: ffff88800c01b800
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 000000000d698005 CR4: 0000000000770ef0
PKRU: 55555554
Kernel panic - not syncing: Fatal exception in interrupt
Kernel Offset: disabled
---[ end Kernel panic - not syncing: Fatal exception in interrupt ]---

Fixes: d2d6422f8bd1 ("x86: Allow to enable PREEMPT_RT.")
Reported-by: [email-masked]
Closes: https://lore.kernel.org/all/[email-masked]/T/
Signed-off-by: Jiayuan Chen <[email-masked]>
Signed-off-by: Jiayuan Chen <[email-masked]>
---
 kernel/bpf/cpumap.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/cpumap.c b/kernel/bpf/cpumap.c
index 04171fbc39cb..7fda8421ec40 100644
--- a/kernel/bpf/cpumap.c
+++ b/kernel/bpf/cpumap.c
@@ -29,6 +29,7 @@
 #include <linux/sched.h>
 #include <linux/workqueue.h>
 #include <linux/kthread.h>
+#include <linux/local_lock.h>
 #include <linux/completion.h>
 #include <trace/events/xdp.h>
 #include <linux/btf_ids.h>
@@ -52,6 +53,7 @@ struct xdp_bulk_queue {
 	struct list_head flush_node;
 	struct bpf_cpu_map_entry *obj;
 	unsigned int count;
+	local_lock_t bq_lock;
 };
 
 /* Struct for every remote "destination" CPU in map */
@@ -451,6 +453,7 @@ __cpu_map_entry_alloc(struct bpf_map *map, struct bpf_cpumap_val *value,
 	for_each_possible_cpu(i) {
 		bq = per_cpu_ptr(rcpu->bulkq, i);
 		bq->obj = rcpu;
+		local_lock_init(&bq->bq_lock);
 	}
 
 	/* Alloc queue */
@@ -714,6 +717,7 @@ const struct bpf_map_ops cpu_map_ops = {
 	.map_redirect		= cpu_map_redirect,
 };
 
+/* Caller must hold bq->bq_lock */
 static void bq_flush_to_queue(struct xdp_bulk_queue *bq)
 {
 	struct bpf_cpu_map_entry *rcpu = bq->obj;
@@ -750,10 +754,16 @@ static void bq_flush_to_queue(struct xdp_bulk_queue *bq)
 
 /* Runs under RCU-read-side, plus in softirq under NAPI protection.
  * Thus, safe percpu variable access.
+ *
+ * On PREEMPT_RT, local_bh_disable() does not disable preemption,
+ * so we use local_lock to serialize access to the per-CPU bq.
  */
 static void bq_enqueue(struct bpf_cpu_map_entry *rcpu, struct xdp_frame *xdpf)
 {
-	struct xdp_bulk_queue *bq = this_cpu_ptr(rcpu->bulkq);
+	struct xdp_bulk_queue *bq;
+
+	local_lock(&rcpu->bulkq->bq_lock);
+	bq = this_cpu_ptr(rcpu->bulkq);
 
 	if (unlikely(bq->count == CPU_MAP_BULK_SIZE))
 		bq_flush_to_queue(bq);
@@ -774,6 +784,8 @@ static void bq_enqueue(struct bpf_cpu_map_entry *rcpu, struct xdp_frame *xdpf)
 
 		list_add(&bq->flush_node, flush_list);
 	}
+
+	local_unlock(&rcpu->bulkq->bq_lock);
 }
 
 int cpu_map_enqueue(struct bpf_cpu_map_entry *rcpu, struct xdp_frame *xdpf,
@@ -810,7 +822,9 @@ void __cpu_map_flush(struct list_head *flush_list)
 	struct xdp_bulk_queue *bq, *tmp;
 
 	list_for_each_entry_safe(bq, tmp, flush_list, flush_node) {
+		local_lock(&bq->obj->bulkq->bq_lock);
 		bq_flush_to_queue(bq);
+		local_unlock(&bq->obj->bulkq->bq_lock);
 
 		/* If already running, costs spin_lock_irqsave + smb_mb */
 		wake_up_process(bq->obj->kthread);
-- 
2.43.0