linux-kernel - Re: [PATCH sched_ext/for-6.18] sched_ext: Fix NULL dereference in scx_bpf_cpu

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Message-ID: <aLnWztwCj4GyJFyS@gpd4>
Date: Thu, 4 Sep 2025 20:13:34 +0200
From: Andrea Righi <arighi@...dia.com>
To: Tejun Heo <tj@...nel.org>
Cc: David Vernet <void@...ifault.com>, Changwoo Min <changwoo@...lia.com>,
	Christian Loehle <christian.loehle@....com>,
	Jake Hillion <jakehillion@...a.com>, sched-ext@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH sched_ext/for-6.18] sched_ext: Fix NULL dereference in
 scx_bpf_cpu_rq() warning

On Thu, Sep 04, 2025 at 07:39:34AM -1000, Tejun Heo wrote:
> On Thu, Sep 04, 2025 at 02:55:22PM +0200, Andrea Righi wrote:
> > When printing the deprecation warning for scx_bpf_cpu_rq(), we may hit a
> > NULL pointer dereference if the kfunc is called before a BPF scheduler
> > is fully attached, for example, when invoked from a BPF timer or during
> > ops.init():
> > 
> >  [   50.752775] BUG: kernel NULL pointer dereference, address: 0000000000000331
> >  ...
> >  [   50.764205] RIP: 0010:scx_bpf_cpu_rq+0x30/0xa0
> >  ...
> >  [   50.787661] Call Trace:
> >  [   50.788398]  <TASK>
> >  [   50.789061]  bpf_prog_08f7fd2dcb187aaf_wakeup_timerfn+0x75/0x1a8
> >  [   50.792477]  bpf_timer_cb+0x7e/0x140
> >  [   50.796003]  hrtimer_run_softirq+0x91/0xe0
> >  [   50.796952]  handle_softirqs+0xce/0x3c0
> >  [   50.799087]  run_ksoftirqd+0x3e/0x70
> >  [   50.800197]  smpboot_thread_fn+0x133/0x290
> >  [   50.802320]  kthread+0x115/0x220
> >  [   50.804984]  ret_from_fork+0x17a/0x1d0
> >  [   50.806920]  ret_from_fork_asm+0x1a/0x30
> >  [   50.807799]  </TASK>
> > 
> > Fix this by only printing the warning once the scheduler is fully
> > registered.
> > 
> > Fixes: 5c48d88fe0049 ("sched_ext: deprecation warn for scx_bpf_cpu_rq()")
> > Cc: Christian Loehle <christian.loehle@....com>
> > Signed-off-by: Andrea Righi <arighi@...dia.com>
> > ---
> >  kernel/sched/ext.c | 7 +++++--
> >  1 file changed, 5 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> > index 489462290142a..04fefd34db238 100644
> > --- a/kernel/sched/ext.c
> > +++ b/kernel/sched/ext.c
> > @@ -6362,17 +6362,20 @@ __bpf_kfunc s32 scx_bpf_task_cpu(const struct task_struct *p)
> >   */
> >  __bpf_kfunc struct rq *scx_bpf_cpu_rq(s32 cpu)
> >  {
> > -	struct scx_sched *sch = scx_root;
> > +	struct scx_sched *sch;
> >  
> >  	if (!kf_cpu_valid(cpu, NULL))
> >  		return NULL;
> >  
> > -	if (!sch->warned_deprecated_rq) {
> > +	rcu_read_lock();
> > +	sch = rcu_dereference(scx_root);
> > +	if (sch && sch->warned_deprecated_rq) {
> 
> This drops ! from the condition.
> 
> Thanks.

Oops, thanks for noticing it Tejun. I'll send a v2.

-Andrea