From: Tejun Heo Date: Fri, 10 Apr 2026 17:54:06 +0000 (-1000) Subject: sched_ext: Add verifier-time kfunc context filter X-Git-Tag: v7.1-rc1~162^2~7 X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=d1d3c1c6ae3691869be9d94730d6e5325aaae8c6;p=thirdparty%2Fkernel%2Flinux.git sched_ext: Add verifier-time kfunc context filter Move enforcement of SCX context-sensitive kfunc restrictions from per-task runtime kf_mask checks to BPF verifier-time filtering, using the BPF core's struct_ops context information. A shared .filter callback is attached to each context-sensitive BTF set and consults a per-op allow table (scx_kf_allow_flags[]) indexed by SCX ops member offset. Disallowed calls are now rejected at program load time instead of at runtime. The old model split reachability across two places: each SCX_CALL_OP*() set bits naming its op context, and each kfunc's scx_kf_allowed() check OR'd together the bits it accepted. A kfunc was callable when those two masks overlapped. The new model transposes the result to the caller side - each op's allow flags directly list the kfunc groups it may call. The old bit assignments were: Call-site bits: ops.select_cpu = ENQUEUE | SELECT_CPU ops.enqueue = ENQUEUE ops.dispatch = DISPATCH ops.cpu_release = CPU_RELEASE Kfunc-group accepted bits: enqueue group = ENQUEUE | DISPATCH select_cpu group = SELECT_CPU | ENQUEUE dispatch group = DISPATCH cpu_release group = CPU_RELEASE Intersecting them yields the reachability now expressed directly by scx_kf_allow_flags[]: ops.select_cpu -> SELECT_CPU | ENQUEUE ops.enqueue -> SELECT_CPU | ENQUEUE ops.dispatch -> ENQUEUE | DISPATCH ops.cpu_release -> CPU_RELEASE Unlocked ops carried no kf_mask bits and reached only unlocked kfuncs; that maps directly to UNLOCKED in the new table. Equivalence was checked by walking every (op, kfunc-group) combination across SCX ops, SYSCALL, and non-SCX struct_ops callers against the old scx_kf_allowed() runtime checks. With two intended exceptions (see below), all combinations reach the same verdict; disallowed calls are now caught at load time instead of firing scx_error() at runtime. scx_bpf_dsq_move_set_slice() and scx_bpf_dsq_move_set_vtime() are exceptions: they have no runtime check at all, but the new filter rejects them from ops outside dispatch/unlocked. The affected cases are nonsensical - the values these setters store are only read by scx_bpf_dsq_move{,_vtime}(), which is itself restricted to dispatch/unlocked, so a setter call from anywhere else was already dead code. Runtime scx_kf_mask enforcement is left in place by this patch and removed in a follow-up. Original-patch-by: Juntong Deng Original-patch-by: Cheng-Yang Chou Signed-off-by: Tejun Heo Reviewed-by: Andrea Righi --- diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c index be5b6755f1d1e..edf51d91bab2a 100644 --- a/kernel/sched/ext.c +++ b/kernel/sched/ext.c @@ -8133,6 +8133,7 @@ BTF_KFUNCS_END(scx_kfunc_ids_enqueue_dispatch) static const struct btf_kfunc_id_set scx_kfunc_set_enqueue_dispatch = { .owner = THIS_MODULE, .set = &scx_kfunc_ids_enqueue_dispatch, + .filter = scx_kfunc_context_filter, }; static bool scx_dsq_move(struct bpf_iter_scx_dsq_kern *kit, @@ -8511,6 +8512,7 @@ BTF_KFUNCS_END(scx_kfunc_ids_dispatch) static const struct btf_kfunc_id_set scx_kfunc_set_dispatch = { .owner = THIS_MODULE, .set = &scx_kfunc_ids_dispatch, + .filter = scx_kfunc_context_filter, }; __bpf_kfunc_start_defs(); @@ -8551,6 +8553,7 @@ BTF_KFUNCS_END(scx_kfunc_ids_cpu_release) static const struct btf_kfunc_id_set scx_kfunc_set_cpu_release = { .owner = THIS_MODULE, .set = &scx_kfunc_ids_cpu_release, + .filter = scx_kfunc_context_filter, }; __bpf_kfunc_start_defs(); @@ -8628,6 +8631,7 @@ BTF_KFUNCS_END(scx_kfunc_ids_unlocked) static const struct btf_kfunc_id_set scx_kfunc_set_unlocked = { .owner = THIS_MODULE, .set = &scx_kfunc_ids_unlocked, + .filter = scx_kfunc_context_filter, }; __bpf_kfunc_start_defs(); @@ -9603,6 +9607,115 @@ static const struct btf_kfunc_id_set scx_kfunc_set_any = { .set = &scx_kfunc_ids_any, }; +/* + * Per-op kfunc allow flags. Each bit corresponds to a context-sensitive kfunc + * group; an op may permit zero or more groups, with the union expressed in + * scx_kf_allow_flags[]. The verifier-time filter (scx_kfunc_context_filter()) + * consults this table to decide whether a context-sensitive kfunc is callable + * from a given SCX op. + */ +enum scx_kf_allow_flags { + SCX_KF_ALLOW_UNLOCKED = 1 << 0, + SCX_KF_ALLOW_CPU_RELEASE = 1 << 1, + SCX_KF_ALLOW_DISPATCH = 1 << 2, + SCX_KF_ALLOW_ENQUEUE = 1 << 3, + SCX_KF_ALLOW_SELECT_CPU = 1 << 4, +}; + +/* + * Map each SCX op to the union of kfunc groups it permits, indexed by + * SCX_OP_IDX(op). Ops not listed only permit kfuncs that are not + * context-sensitive. + */ +static const u32 scx_kf_allow_flags[] = { + [SCX_OP_IDX(select_cpu)] = SCX_KF_ALLOW_SELECT_CPU | SCX_KF_ALLOW_ENQUEUE, + [SCX_OP_IDX(enqueue)] = SCX_KF_ALLOW_SELECT_CPU | SCX_KF_ALLOW_ENQUEUE, + [SCX_OP_IDX(dispatch)] = SCX_KF_ALLOW_ENQUEUE | SCX_KF_ALLOW_DISPATCH, + [SCX_OP_IDX(cpu_release)] = SCX_KF_ALLOW_CPU_RELEASE, + [SCX_OP_IDX(init_task)] = SCX_KF_ALLOW_UNLOCKED, + [SCX_OP_IDX(dump)] = SCX_KF_ALLOW_UNLOCKED, +#ifdef CONFIG_EXT_GROUP_SCHED + [SCX_OP_IDX(cgroup_init)] = SCX_KF_ALLOW_UNLOCKED, + [SCX_OP_IDX(cgroup_exit)] = SCX_KF_ALLOW_UNLOCKED, + [SCX_OP_IDX(cgroup_prep_move)] = SCX_KF_ALLOW_UNLOCKED, + [SCX_OP_IDX(cgroup_cancel_move)] = SCX_KF_ALLOW_UNLOCKED, + [SCX_OP_IDX(cgroup_set_weight)] = SCX_KF_ALLOW_UNLOCKED, + [SCX_OP_IDX(cgroup_set_bandwidth)] = SCX_KF_ALLOW_UNLOCKED, + [SCX_OP_IDX(cgroup_set_idle)] = SCX_KF_ALLOW_UNLOCKED, +#endif /* CONFIG_EXT_GROUP_SCHED */ + [SCX_OP_IDX(sub_attach)] = SCX_KF_ALLOW_UNLOCKED, + [SCX_OP_IDX(sub_detach)] = SCX_KF_ALLOW_UNLOCKED, + [SCX_OP_IDX(cpu_online)] = SCX_KF_ALLOW_UNLOCKED, + [SCX_OP_IDX(cpu_offline)] = SCX_KF_ALLOW_UNLOCKED, + [SCX_OP_IDX(init)] = SCX_KF_ALLOW_UNLOCKED, + [SCX_OP_IDX(exit)] = SCX_KF_ALLOW_UNLOCKED, +}; + +/* + * Verifier-time filter for context-sensitive SCX kfuncs. Registered via the + * .filter field on each per-group btf_kfunc_id_set. The BPF core invokes this + * for every kfunc call in the registered hook (BPF_PROG_TYPE_STRUCT_OPS or + * BPF_PROG_TYPE_SYSCALL), regardless of which set originally introduced the + * kfunc - so the filter must short-circuit on kfuncs it doesn't govern (e.g. + * scx_kfunc_ids_any) by falling through to "allow" when none of the + * context-sensitive sets contain the kfunc. + */ +int scx_kfunc_context_filter(const struct bpf_prog *prog, u32 kfunc_id) +{ + bool in_unlocked = btf_id_set8_contains(&scx_kfunc_ids_unlocked, kfunc_id); + bool in_select_cpu = btf_id_set8_contains(&scx_kfunc_ids_select_cpu, kfunc_id); + bool in_enqueue = btf_id_set8_contains(&scx_kfunc_ids_enqueue_dispatch, kfunc_id); + bool in_dispatch = btf_id_set8_contains(&scx_kfunc_ids_dispatch, kfunc_id); + bool in_cpu_release = btf_id_set8_contains(&scx_kfunc_ids_cpu_release, kfunc_id); + u32 moff, flags; + + /* Not a context-sensitive kfunc (e.g. from scx_kfunc_ids_any) - allow. */ + if (!(in_unlocked || in_select_cpu || in_enqueue || in_dispatch || in_cpu_release)) + return 0; + + /* SYSCALL progs (e.g. BPF test_run()) may call unlocked and select_cpu kfuncs. */ + if (prog->type == BPF_PROG_TYPE_SYSCALL) + return (in_unlocked || in_select_cpu) ? 0 : -EACCES; + + if (prog->type != BPF_PROG_TYPE_STRUCT_OPS) + return -EACCES; + + /* + * add_subprog_and_kfunc() collects all kfunc calls, including dead code + * guarded by bpf_ksym_exists(), before check_attach_btf_id() sets + * prog->aux->st_ops. Allow all kfuncs when st_ops is not yet set; + * do_check_main() re-runs the filter with st_ops set and enforces the + * actual restrictions. + */ + if (!prog->aux->st_ops) + return 0; + + /* + * Non-SCX struct_ops: only unlocked kfuncs are safe. The other + * context-sensitive kfuncs assume the rq lock is held by the SCX + * dispatch path, which doesn't apply to other struct_ops users. + */ + if (prog->aux->st_ops != &bpf_sched_ext_ops) + return in_unlocked ? 0 : -EACCES; + + /* SCX struct_ops: check the per-op allow list. */ + moff = prog->aux->attach_st_ops_member_off; + flags = scx_kf_allow_flags[SCX_MOFF_IDX(moff)]; + + if ((flags & SCX_KF_ALLOW_UNLOCKED) && in_unlocked) + return 0; + if ((flags & SCX_KF_ALLOW_CPU_RELEASE) && in_cpu_release) + return 0; + if ((flags & SCX_KF_ALLOW_DISPATCH) && in_dispatch) + return 0; + if ((flags & SCX_KF_ALLOW_ENQUEUE) && in_enqueue) + return 0; + if ((flags & SCX_KF_ALLOW_SELECT_CPU) && in_select_cpu) + return 0; + + return -EACCES; +} + static int __init scx_init(void) { int ret; @@ -9612,11 +9725,12 @@ static int __init scx_init(void) * register_btf_kfunc_id_set() needs most of the system to be up. * * Some kfuncs are context-sensitive and can only be called from - * specific SCX ops. They are grouped into BTF sets accordingly. - * Unfortunately, BPF currently doesn't have a way of enforcing such - * restrictions. Eventually, the verifier should be able to enforce - * them. For now, register them the same and make each kfunc explicitly - * check using scx_kf_allowed(). + * specific SCX ops. They are grouped into per-context BTF sets, each + * registered with scx_kfunc_context_filter as its .filter callback. The + * BPF core dedups identical filter pointers per hook + * (btf_populate_kfunc_set()), so the filter is invoked exactly once per + * kfunc lookup; it consults scx_kf_allow_flags[] to enforce per-op + * restrictions at verify time. */ if ((ret = register_btf_kfunc_id_set(BPF_PROG_TYPE_STRUCT_OPS, &scx_kfunc_set_enqueue_dispatch)) || diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c index f99ceeba2e56c..ec49e0c9892e6 100644 --- a/kernel/sched/ext_idle.c +++ b/kernel/sched/ext_idle.c @@ -1491,6 +1491,7 @@ BTF_KFUNCS_END(scx_kfunc_ids_select_cpu) static const struct btf_kfunc_id_set scx_kfunc_set_select_cpu = { .owner = THIS_MODULE, .set = &scx_kfunc_ids_select_cpu, + .filter = scx_kfunc_context_filter, }; int scx_idle_init(void) diff --git a/kernel/sched/ext_idle.h b/kernel/sched/ext_idle.h index fa583f141f35d..dc35f850481e1 100644 --- a/kernel/sched/ext_idle.h +++ b/kernel/sched/ext_idle.h @@ -12,6 +12,8 @@ struct sched_ext_ops; +extern struct btf_id_set8 scx_kfunc_ids_select_cpu; + void scx_idle_update_selcpu_topology(struct sched_ext_ops *ops); void scx_idle_init_masks(void); diff --git a/kernel/sched/ext_internal.h b/kernel/sched/ext_internal.h index 54da08a223b7c..62ce4eaf6a3f5 100644 --- a/kernel/sched/ext_internal.h +++ b/kernel/sched/ext_internal.h @@ -6,6 +6,7 @@ * Copyright (c) 2025 Tejun Heo */ #define SCX_OP_IDX(op) (offsetof(struct sched_ext_ops, op) / sizeof(void (*)(void))) +#define SCX_MOFF_IDX(moff) ((moff) / sizeof(void (*)(void))) enum scx_consts { SCX_DSP_DFL_MAX_BATCH = 32, @@ -1363,6 +1364,8 @@ enum scx_ops_state { extern struct scx_sched __rcu *scx_root; DECLARE_PER_CPU(struct rq *, scx_locked_rq_state); +int scx_kfunc_context_filter(const struct bpf_prog *prog, u32 kfunc_id); + /* * Return the rq currently locked from an scx callback, or NULL if no rq is * locked.