Re: [PATCH] sched_ext: Fix scx_enable() crash on helper kthread creation failure

[Andrea Righi <arighi@nvidia.com>]

On Wed, Nov 19, 2025 at 04:07:22PM +0530, Saket Kumar Bhaskar wrote:
> A crash was observed when the sched_ext selftests runner was
> terminated with Ctrl+\ while test 15 was running:
> 
> NIP [c00000000028fa58] scx_enable.constprop.0+0x358/0x12b0
> LR [c00000000028fa2c] scx_enable.constprop.0+0x32c/0x12b0
> Call Trace:
> scx_enable.constprop.0+0x32c/0x12b0 (unreliable)
> bpf_struct_ops_link_create+0x18c/0x22c
> __sys_bpf+0x23f8/0x3044
> sys_bpf+0x2c/0x6c
> system_call_exception+0x124/0x320
> system_call_vectored_common+0x15c/0x2ec
> 
> kthread_run_worker() returns an ERR_PTR() on failure rather than NULL,
> but the current code in scx_alloc_and_add_sched() only checks for a NULL
> helper. Incase of failure on SIGQUIT, the error is not handled in
> scx_alloc_and_add_sched() and scx_enable() ends up dereferencing an
> error pointer.
> 
> Error handling is fixed in scx_alloc_and_add_sched() to propagate
> PTR_ERR() into ret, so that scx_enable() jumps to the existing error
> path, avoiding random dereference on failure.
> 
> Fixes: bff3b5aec1b7 ("sched_ext: Move disable machinery into scx_sched")
> Reported-by: Samir Mulani <samir@linux.ibm.com>
> Signed-off-by: Saket Kumar Bhaskar <skb99@linux.ibm.com>

Good catch, makes sense to me.

Reviewed-by: Andrea Righi <arighi@nvidia.com>

Thanks,
-Andrea

> ---
>  kernel/sched/ext.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index 2b0e88206d07..7fc0cce68a1b 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -4392,8 +4392,11 @@ static struct scx_sched *scx_alloc_and_add_sched(struct sched_ext_ops *ops)
>  		goto err_free_gdsqs;
>  
>  	sch->helper = kthread_run_worker(0, "sched_ext_helper");
> -	if (!sch->helper)
> +	if (IS_ERR(sch->helper)) {
> +		ret = PTR_ERR(sch->helper);
>  		goto err_free_pcpu;
> +	}
> +
>  	sched_set_fifo(sch->helper->task);
>  
>  	atomic_set(&sch->exit_kind, SCX_EXIT_NONE);
> -- 
> 2.51.0
>