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

[Jiayuan Chen]

February 11, 2026 at 19:44, "Sebastian Andrzej Siewior" <bigeasy@xxxxxxxxxxxxx mailto:bigeasy@xxxxxxxxxxxxx?to=%22Sebastian%20Andrzej%20Siewior%22%20%3Cbigeasy%40linutronix.de%3E > wrote:


> 
> On 2026-02-11 14:44:16 [+0800], Jiayuan Chen wrote:
> 
> > 
> > From: Jiayuan Chen <jiayuan.chen@xxxxxxxxxx>
> >  
> >  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.
> > 
> …
> 
> > 
> > Fixes: d2d6422f8bd1 ("x86: Allow to enable PREEMPT_RT.")
> > 
> Can you reproduce this? It should not trigger with the commit above.
> It should trigger starting with
>  3253cb49cbad4 ("softirq: Allow to drop the softirq-BKL lock on PREEMPT_RT")


Thanks for the review, Sebastian.

You are right. The race only becomes possible after the softirq BKL is dropped.

> > 
> > Reported-by: syzbot+2b3391f44313b3983e91@xxxxxxxxxxxxxxxxxxxxxxxxx
> >  Closes: https://lore.kernel.org/all/69369331.a70a0220.38f243.009d.GAE@xxxxxxxxxx/T/
> >  Signed-off-by: Jiayuan Chen <jiayuan.chen@xxxxxxxxxx>
> >  Signed-off-by: Jiayuan Chen <jiayuan.chen@xxxxxxxxx>
> >  ---
> >  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
> >  @@ -714,6 +717,7 @@ const struct bpf_map_ops cpu_map_ops = {
> >  .map_redirect = cpu_map_redirect,
> >  };
> >  
> >  +/* Caller must hold bq->bq_lock */
> > 
> If this information is important please use lockdep_assert_held() in the
> function below. This can be used by lockdep and is understood by humans
> while the comment is only visible to humans.


Will add lockdep_assert_held() in bq_flush_to_queue() and drop the
comment.

> > 
> > 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.
> > 
> + PREEMPT_RT relies on local_lock_nested_bh().
> 
> > 
> > + *
> >  + * 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);
> > 
> local_lock_nested_bh() & the matching unlock here and in the other
> places, please.


Makes sense. Since these paths already run under local_bh_disable(),
local_lock_nested_bh() is the correct primitive.

> Sebastian
>