Re: [PATCH 3/3] sched_ext: Guard the cid kfuncs against unallocated cid tables

Hi Tejun,

On Sun, Jul 19, 2026 at 10:26:05PM -1000, Tejun Heo wrote:
> The cid tables are allocated by the first enable's scx_cid_init(), which
> runs after scx_alloc_and_add_sched() has published ops->priv. A TRACING or
> SYSCALL program associated with the enabling struct_ops map gets a non-NULL
> sched from scx_prog_sched() as soon as ops->priv is set, so a cid kfunc
> called in that window dereferences the still-NULL table pointer. Only the
> first enable since boot is exposed as the tables are never freed.
> 
> scx_bpf_this_cid() and scx_bpf_task_cid() already handle the window by
> testing the table pointer. Do the same in scx_cid_to_cpu(),
> scx_cpu_to_cid() and scx_bpf_cid_topo(), returning -EINVAL / all-(-1) topo
> as before any scheduler is enabled. __scx_cid_to_cpu() and
> __scx_cpu_to_cid() stay unchecked for callers with the tables guaranteed
> allocated - ops invocations on a live scheduler and the enable path itself.
> 
> Fixes: e9b55af47edf ("sched_ext: Add topological CPU IDs (cids)")
> Signed-off-by: Tejun Heo <[email protected]>
> ---
>  kernel/sched/ext/cid.c |  5 +++--
>  kernel/sched/ext/cid.h | 23 +++++++++++++++--------
>  2 files changed, 18 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/sched/ext/cid.c b/kernel/sched/ext/cid.c
> index 9dfd242be34f..699cb8db228d 100644
> --- a/kernel/sched/ext/cid.c
> +++ b/kernel/sched/ext/cid.c
> @@ -875,17 +875,18 @@ bool scx_cmask_empty(const struct scx_cmask *m)
>  __bpf_kfunc void scx_bpf_cid_topo(s32 cid, struct scx_cid_topo *out__uninit,
>  				  const struct bpf_prog_aux *aux)
>  {
> +	struct scx_cid_topo *topo = READ_ONCE(scx_cid_topo);

I think a non-NULL pointer here doesn't necessarily mean that the table is
initialized. IIUC scx_cid_arrays_alloc() publishes these pointers before
scx_cid_init() populates them, so a racing program can still observe zero-filled
uninitialized cid_topo entries.

This also applies to the other enables, where the tables renamin non-NULL while
being rewritten. Should we add a defer publication until initialization is
complete or a readiness flag?

Thanks,
-Andrea

>  	struct scx_sched *sch;
>  
>  	guard(rcu)();
>  
>  	sch = scx_prog_sched(aux);
> -	if (unlikely(!sch) || !cid_valid(sch, cid)) {
> +	if (unlikely(!sch) || !cid_valid(sch, cid) || unlikely(!topo)) {
>  		*out__uninit = SCX_CID_TOPO_NEG;
>  		return;
>  	}
>  
> -	*out__uninit = READ_ONCE(scx_cid_topo)[cid];
> +	*out__uninit = topo[cid];
>  }
>  
>  __bpf_kfunc_end_defs();
> diff --git a/kernel/sched/ext/cid.h b/kernel/sched/ext/cid.h
> index b36a1a28eac8..1498efb81549 100644
> --- a/kernel/sched/ext/cid.h
> +++ b/kernel/sched/ext/cid.h
> @@ -90,9 +90,10 @@ static inline bool cid_valid(struct scx_sched *sch, s32 cid)
>   * __scx_cid_to_cpu - Unchecked cid->cpu table lookup
>   * @cid: cid to look up. Must be in [0, num_possible_cpus()).
>   *
> - * Intended for callsites that have already validated @cid and that hold a
> - * non-NULL @sch from scx_prog_sched() - a live sched implies the table has
> - * been allocated, so no NULL check is needed here.
> + * Intended for callsites that have already validated @cid and where the
> + * tables are guaranteed allocated - ops invocations on a live scheduler or
> + * the enable path itself. Prog-facing kfuncs, which can run while the first
> + * enable is still allocating the tables, use the checked wrappers instead.
>   */
>  static inline s32 __scx_cid_to_cpu(s32 cid)
>  {
> @@ -119,13 +120,17 @@ static inline s32 __scx_cpu_to_cid(s32 cpu)
>   * Return the cpu for @cid or a negative errno on failure. Invalid cid triggers
>   * scx_error() on @sch. The cid arrays are allocated on first scheduler enable
>   * and never freed, so the returned cpu is stable for the lifetime of the loaded
> - * scheduler.
> + * scheduler. Return -EINVAL without triggering scx_error() if the tables are
> + * not allocated yet, which a prog-facing kfunc can observe while racing the
> + * first enable.
>   */
>  static inline s32 scx_cid_to_cpu(struct scx_sched *sch, s32 cid)
>  {
> -	if (!cid_valid(sch, cid))
> +	s16 *tbl = READ_ONCE(scx_cid_to_cpu_tbl);
> +
> +	if (!cid_valid(sch, cid) || unlikely(!tbl))
>  		return -EINVAL;
> -	return __scx_cid_to_cpu(cid);
> +	return tbl[cid];
>  }
>  
>  /**
> @@ -138,9 +143,11 @@ static inline s32 scx_cid_to_cpu(struct scx_sched *sch, s32 cid)
>   */
>  static inline s32 scx_cpu_to_cid(struct scx_sched *sch, s32 cpu)
>  {
> -	if (!scx_cpu_valid(sch, cpu, NULL))
> +	s16 *tbl = READ_ONCE(scx_cpu_to_cid_tbl);
> +
> +	if (!scx_cpu_valid(sch, cpu, NULL) || unlikely(!tbl))
>  		return -EINVAL;
> -	return __scx_cpu_to_cid(cpu);
> +	return tbl[cpu];
>  }
>  
>  /**
> -- 
> 2.55.0
>