Re: [PATCH] sched_ext: Reject NMI calls to lock-taking kfuncs

[Andrea Righi]

Hi Wanwu,

On Tue, Sep 01, 2026 at 05:56:52PM +0800, Wanwu Li wrote:
> commit e06ece82d7b0 ("sched_ext: Report NMI kicks with scx_error()") made
> scx_bpf_kick_cpu() reject NMI calls, and its cover letter describes the
> reachability: sched_ext kfuncs in the "any" category "are callable from
> tracing progs that can attach to functions running in NMI", and an unlucky
> call from there "could deadlock the machine". The fix in that series made
> the error/exit path lock-free so scx_error() is safe to call from NMI. That
> closes the *error* path of every kfunc, but not a kfunc's own
> business-logic lock acquisition on its success path.
> 
> The remaining lock-taking kfuncs that scx_kfunc_context_filter() exposes to
> BPF_PROG_TYPE_TRACING have the same hazard: if an NMI lands on a CPU whose
> interrupted context already holds the lock, the kfunc's raw spinlock
> acquisition spins forever and hard-locks the CPU:
> 
>  - scx_bpf_destroy_dsq()                    -> dsq->lock
>  - scx_bpf_dsq_reenq()                      -> rq's deferred_reenq_lock
>  - scx_bpf_cpuperf_set() / scx_bpf_cidperf_set() -> rq->lock
>  - scx_bpf_sub_grant() / scx_bpf_sub_revoke()     -> pshard lock
>    (via the shared sub_cap_preamble())
>  - bpf_iter_scx_dsq_next() / bpf_iter_scx_dsq_destroy() -> dsq->lock
>    (bpf_iter_scx_dsq_new() is lockless and needs no guard)
> 
> As things stand, there is no scenario for reenqueueing, iterating a DSQ,
> setting a performance target or granting sub-caps from NMI. The guards
> defend against a buggy or malicious BPF program turning an "any"-category
> kfunc into a machine-wide hard-lockup through the door that
> scx_kfunc_context_filter() already opens. This matches the intent of
> scx_bpf_kick_cpu()'s NMI check, which the commit cited above added not to
> enable an NMI use case but to surface such a bug as a clean abort.

The deadlock scenario makes sense to me, but I wonder whether we should prevent
these kfuncs from being called by tracing programs altogether instead of adding
runtime checks to the scheduler paths.

AFAICS, the lock-taking and state-changing kfuncs do not have a meaningful use
from BPF_PROG_TYPE_TRACING. We could move them out of scx_kfunc_ids_any into a
separate set registered only for BPF_PROG_TYPE_STRUCT_OPS. The read-only kfuncs
could remain available to tracing programs.

This should include:
 - scx_bpf_kick_cpu() / scx_bpf_kick_cid()
 - scx_bpf_destroy_dsq()
 - scx_bpf_dsq_reenq() / scx_bpf_reenqueue_local___v2()
 - bpf_iter_scx_dsq_{new,next,destroy}()
 - scx_bpf_cpuperf_set() / scx_bpf_cidperf_set()
 - scx_bpf_sub_grant() / scx_bpf_sub_revoke()
 (... maybe others that I'm missing ...)

That would reject invalid programs at verification time, avoid the runtime
overhead and make the API boundary explicit: tracing programs can observe
sched_ext state, while only sched_ext schedulers can modify it.

If BPF_PROG_TYPE_SYSCALL registration is needed for test_run or selftests, we
can retain that separately since it cannot execute from NMI.

What do you think?

Thanks,
-Andrea

> 
> Route all of them through a new scx_kfunc_nmi_safe() helper and reuse
> scx_bpf_kick_cpu()'s existing in_nmi() check - now shared with its cid
> equivalent scx_bpf_kick_cid() through scx_kick_cpu() - so the rule is
> stated once and the coverage is auditable from one place. scx_error() is
> already NMI-safe (commit f883dbb64ca5 ("sched_ext: Make exit claiming
> lock-free")), so the reject-abort cannot deadlock the lock acquisition.
> 
> Read-only members of the reachable sets (dsq_peek, dsq_nr_queued,
> cpuperf_cur/cap, sub_caps, the idle cpumask helpers and the cid lookups)
> take no scheduler lock on the path a tracing program reaches them, and were
> audited to that effect; they are correctly left unguarded. The select_cpu
> kfuncs do take pi_lock, but scx_kfunc_context_filter() only exposes the
> any/idle/cid sets to BPF_PROG_TYPE_TRACING, and struct_ops run in task
> context, so no lock-taking path here is reachable from NMI.
> 
> Signed-off-by: Wanwu Li <liwanwu@xxxxxxxxxx>
> ---
>  kernel/sched/ext/ext.c      | 27 ++++++++++++++++++++-------
>  kernel/sched/ext/internal.h | 22 ++++++++++++++++++++++
>  kernel/sched/ext/sub.c      |  3 +++
>  3 files changed, 45 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
> index 10af28a9f2c0..a0b886975e1d 100644
> --- a/kernel/sched/ext/ext.c
> +++ b/kernel/sched/ext/ext.c
> @@ -5108,6 +5108,9 @@ static void destroy_dsq(struct scx_sched *sch, u64 dsq_id)
>  	struct scx_dispatch_q *dsq;
>  	unsigned long flags;
>  
> +	if (!scx_kfunc_nmi_safe("scx_bpf_destroy_dsq()", sch))
> +		return;
> +
>  	rcu_read_lock();
>  
>  	dsq = find_user_dsq(sch, dsq_id);
> @@ -9518,14 +9521,8 @@ void scx_kick_cpu(struct scx_sched *sch, s32 cpu, u64 flags)
>  	struct rq *this_rq;
>  	unsigned long irq_flags;
>  
> -	/*
> -	 * The per-cpu kick list is guarded only by local_irq_save(), which does
> -	 * not mask NMIs, so kicking from NMI could corrupt it and is unsupported.
> -	 */
> -	if (unlikely(in_nmi())) {
> -		scx_error(sch, "scx_bpf_kick_cpu() called from NMI");
> +	if (!scx_kfunc_nmi_safe("scx_bpf_kick_cpu()", sch))
>  		return;
> -	}
>  
>  	local_irq_save(irq_flags);
>  
> @@ -9756,6 +9753,9 @@ __bpf_kfunc struct task_struct *bpf_iter_scx_dsq_next(struct bpf_iter_scx_dsq *i
>  	if (!kit->dsq)
>  		return NULL;
>  
> +	if (!scx_kfunc_nmi_safe(__func__, kit->dsq->sched))
> +		return NULL;
> +
>  	guard(raw_spinlock_irqsave)(&kit->dsq->lock);
>  
>  	return nldsq_cursor_next_task(&kit->cursor, kit->dsq);
> @@ -9777,6 +9777,9 @@ __bpf_kfunc void bpf_iter_scx_dsq_destroy(struct bpf_iter_scx_dsq *it)
>  	if (!list_empty(&kit->cursor.node)) {
>  		unsigned long flags;
>  
> +		if (!scx_kfunc_nmi_safe(__func__, kit->dsq->sched))
> +			return;
> +
>  		raw_spin_lock_irqsave(&kit->dsq->lock, flags);
>  		list_del_init(&kit->cursor.node);
>  		raw_spin_unlock_irqrestore(&kit->dsq->lock, flags);
> @@ -9857,6 +9860,9 @@ __bpf_kfunc void scx_bpf_dsq_reenq(u64 dsq_id, u64 reenq_flags,
>  		return;
>  	}
>  
> +	if (!scx_kfunc_nmi_safe(__func__, sch))
> +		return;
> +
>  	/* not specifying any filter bits is the same as %SCX_REENQ_ANY */
>  	if (!(reenq_flags & __SCX_REENQ_FILTER_MASK))
>  		reenq_flags |= SCX_REENQ_ANY;
> @@ -10244,6 +10250,9 @@ __bpf_kfunc void scx_bpf_cpuperf_set(s32 cpu, u32 perf, const struct bpf_prog_au
>  	if (unlikely(!sch))
>  		return;
>  
> +	if (!scx_kfunc_nmi_safe(__func__, sch))
> +		return;
> +
>  	scx_cpuperf_set(sch, cpu, perf);
>  }
>  
> @@ -10269,6 +10278,10 @@ __bpf_kfunc s32 scx_bpf_cidperf_set(s32 cid, u32 perf,
>  	sch = scx_prog_sched(aux);
>  	if (unlikely(!sch))
>  		return -ENODEV;
> +
> +	if (!scx_kfunc_nmi_safe(__func__, sch))
> +		return -EBUSY;
> +
>  	cpu = scx_cid_to_cpu(sch, cid);
>  	if (cpu < 0)
>  		return cpu;
> diff --git a/kernel/sched/ext/internal.h b/kernel/sched/ext/internal.h
> index 27bbf5e04d90..49b165effd67 100644
> --- a/kernel/sched/ext/internal.h
> +++ b/kernel/sched/ext/internal.h
> @@ -2091,6 +2091,28 @@ extern struct scx_sched *scx_enabling_sub_sched;
>  #define scx_error(sch, fmt, args...)						\
>  	scx_exit((sch), SCX_EXIT_ERROR, 0, fmt, ##args)
>  
> +/*
> + * sched_ext kfuncs that take scheduler locks are not NMI-safe: a
> + * BPF_PROG_TYPE_TRACING program can be attached to a function that runs in
> + * NMI, and scx_kfunc_context_filter() lets such a program call every kfunc in
> + * the any/cid/idle sets. Acquiring the rq, dsq or pshard raw spinlocks - or
> + * touching the irq-masking-only kick list - from NMI while the interrupted
> + * context on the same CPU already holds them deadlocks (or corrupts) it.
> + * scx_bpf_kick_cpu() was the first guard; route all of them through here.
> + *
> + * Returns true when the caller may proceed, false when running from NMI and
> + * the kfunc must bail without touching locks. scx_error() is NMI-safe (see the
> + * lock-free ->aborting claim).
> + */
> +static inline bool scx_kfunc_nmi_safe(const char *who, struct scx_sched *sch)
> +{
> +	if (unlikely(in_nmi())) {
> +		scx_error(sch, "%s called from NMI", who);
> +		return false;
> +	}
> +	return true;
> +}
> +
>  /**
>   * scx_root_protected_live - Root sched for paths that only run while live
>   *
> diff --git a/kernel/sched/ext/sub.c b/kernel/sched/ext/sub.c
> index 0554448835bd..b5125a871562 100644
> --- a/kernel/sched/ext/sub.c
> +++ b/kernel/sched/ext/sub.c
> @@ -2265,6 +2265,9 @@ static s32 sub_cap_preamble(u64 cgroup_id, u64 caps, const struct bpf_prog_aux *
>  	if (unlikely(!parent))
>  		return -ENODEV;
>  
> +	if (!scx_kfunc_nmi_safe("sub-cap kfuncs", parent))
> +		return -EBUSY;
> +
>  	if (!scx_is_cid_type()) {
>  		scx_error(parent, "sub-cap kfuncs require a cid-form scheduler");
>  		return -EOPNOTSUPP;
> -- 
> 2.25.1
>