Re: [PATCH sched_ext/for-6.16] sched_ext: Avoid NULL scx_root deref in __scx_exit()

[Tejun Heo]

On Wed, Apr 30, 2025 at 10:40:21AM +0200, Andrea Righi wrote:
> A sched_ext scheduler may trigger __scx_exit() from a BPF timer
> callback, where scx_root may not be safely dereferenced.
> 
> This can lead to a NULL pointer dereference as shown below (triggered by
> scx_tickless):
> 
>  BUG: kernel NULL pointer dereference, address: 0000000000000330
> ...
>  CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted 6.14.0-virtme #1 PREEMPT(full)
>  RIP: 0010:__scx_exit+0x2b/0x190
> ...
>  Call Trace:
>   <IRQ>
>   scx_bpf_get_idle_smtmask+0x59/0x80
>   bpf_prog_8320d4217989178c_dispatch_all_cpus+0x35/0x1b6
> ...
>   bpf_prog_97f847d871513f95_sched_timerfn+0x4c/0x264
>   bpf_timer_cb+0x7a/0x140
>   __hrtimer_run_queues+0x1f9/0x3a0
>   hrtimer_run_softirq+0x8c/0xd0
>   handle_softirqs+0xd3/0x3d0
>   __irq_exit_rcu+0x9a/0xc0
>   irq_exit_rcu+0xe/0x20
> 
> Fix this by checking for a valid scx_root and adding proper RCU
> protection.
> 
> Fixes: 48e1267773866 ("sched_ext: Introduce scx_sched")
> Signed-off-by: Andrea Righi <arighi@xxxxxxxxxx>

Applied to sched_ext/for-6.16.

Thanks.

-- 
tejun