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sched_ext: Deny SCX kfuncs to non-SCX struct_ops programs

scx_kfunc_context_filter() currently allows non-SCX struct_ops programs (e.g. tcp_congestion_ops) to call SCX unlocked kfuncs. This is wrong for two reasons: - It is semantically incorrect: a TCP congestion control program has no business calling SCX kfuncs such as scx_bpf_kick_cpu(). - With CONFIG_EXT_SUB_SCHED=y, kfuncs like scx_bpf_kick_cpu() call scx_prog_sched(aux), which invokes bpf_prog_get_assoc_struct_ops(aux) and casts the result to struct sched_ext_ops * before reading ops->priv. For a non-SCX struct_ops program the returned pointer is the kdata of that struct_ops type, which is far smaller than sched_ext_ops, making the read an out-of-bounds access (confirmed with KASAN). Extend the filter to cover scx_kfunc_set_any and scx_kfunc_set_idle as well, and deny all SCX kfuncs for any struct_ops program that is not the SCX struct_ops. This addresses both issues: the semantic contract is enforced at the verifier level, and the runtime out-of-bounds access becomes unreachable.

Cheng-Yang Chou committed 6 months ago

Commit 2d2b0261 parent 87019cb3 files · +20 −14Browse files

Unified · Split

3 files
  1. kernel/sched/ext.c +18 −14
  2. kernel/sched/ext_idle.c +1 −0
  3. kernel/sched/ext_idle.h +1 −0
kernel/sched/ext.c+18−14
@@ -9480,6 +9480,7 @@ BTF_KFUNCS_END(scx_kfunc_ids_any) · 9479 unmodified lines
94809480 static const struct btf_kfunc_id_set scx_kfunc_set_any = {
94819481 .owner = THIS_MODULE,
94829482 .set = &scx_kfunc_ids_any,
9483+ .filter = scx_kfunc_context_filter,
94839484 };
94849485
94859486 /*
@@ -9527,13 +9528,12 @@ static const u32 scx_kf_allow_flags[] = { · 41 unmodified lines
95279528 };
95289529
95299530 /*
9530− * Verifier-time filter for context-sensitive SCX kfuncs. Registered via the
9531− * .filter field on each per-group btf_kfunc_id_set. The BPF core invokes this
9532− * for every kfunc call in the registered hook (BPF_PROG_TYPE_STRUCT_OPS or
9531+ * Verifier-time filter for SCX kfuncs. Registered via the .filter field on
9532+ * each per-group btf_kfunc_id_set. The BPF core invokes this for every kfunc
9533+ * call in the registered hook (BPF_PROG_TYPE_STRUCT_OPS or
95339534 * BPF_PROG_TYPE_SYSCALL), regardless of which set originally introduced the
9534− * kfunc - so the filter must short-circuit on kfuncs it doesn't govern (e.g.
9535− * scx_kfunc_ids_any) by falling through to "allow" when none of the
9536− * context-sensitive sets contain the kfunc.
9535+ * kfunc - so the filter must short-circuit on kfuncs it doesn't govern by
9536+ * falling through to "allow" when none of the SCX sets contain the kfunc.
95379537 */
95389538 int scx_kfunc_context_filter(const struct bpf_prog *prog, u32 kfunc_id)
95399539 {
@@ -9542,18 +9542,21 @@ int scx_kfunc_context_filter(const struct bpf_prog *prog, u32 kfunc_id) · 2 unmodified lines
95429542 bool in_enqueue = btf_id_set8_contains(&scx_kfunc_ids_enqueue_dispatch, kfunc_id);
95439543 bool in_dispatch = btf_id_set8_contains(&scx_kfunc_ids_dispatch, kfunc_id);
95449544 bool in_cpu_release = btf_id_set8_contains(&scx_kfunc_ids_cpu_release, kfunc_id);
9545+ bool in_idle = btf_id_set8_contains(&scx_kfunc_ids_idle, kfunc_id);
9546+ bool in_any = btf_id_set8_contains(&scx_kfunc_ids_any, kfunc_id);
95459547 u32 moff, flags;
95469548
9547− /* Not a context-sensitive kfunc (e.g. from scx_kfunc_ids_any) - allow. */
9548− if (!(in_unlocked || in_select_cpu || in_enqueue || in_dispatch || in_cpu_release))
9549+ /* Not an SCX kfunc - allow. */
9550+ if (!(in_unlocked || in_select_cpu || in_enqueue || in_dispatch ||
9551+ in_cpu_release || in_idle || in_any))
95499552 return 0;
95509553
95519554 /* SYSCALL progs (e.g. BPF test_run()) may call unlocked and select_cpu kfuncs. */
95529555 if (prog->type == BPF_PROG_TYPE_SYSCALL)
9553− return (in_unlocked || in_select_cpu) ? 0 : -EACCES;
9556+ return (in_unlocked || in_select_cpu || in_idle || in_any) ? 0 : -EACCES;
95549557
95559558 if (prog->type != BPF_PROG_TYPE_STRUCT_OPS)
9556− return -EACCES;
9559+ return (in_any || in_idle) ? 0 : -EACCES;
95579560
95589561 /*
95599562 * add_subprog_and_kfunc() collects all kfunc calls, including dead code
@@ -9566,14 +9569,15 @@ int scx_kfunc_context_filter(const struct bpf_prog *prog, u32 kfunc_id) · 6 unmodified lines
95669569 return 0;
95679570
95689571 /*
9569− * Non-SCX struct_ops: only unlocked kfuncs are safe. The other
9570− * context-sensitive kfuncs assume the rq lock is held by the SCX
9571− * dispatch path, which doesn't apply to other struct_ops users.
9572+ * Non-SCX struct_ops: SCX kfuncs are not permitted.
95729573 */
95739574 if (prog->aux->st_ops != &bpf_sched_ext_ops)
9574− return in_unlocked ? 0 : -EACCES;
9575+ return -EACCES;
95759576
95769577 /* SCX struct_ops: check the per-op allow list. */
9578+ if (in_any || in_idle)
9579+ return 0;
9580+
95779581 moff = prog->aux->attach_st_ops_member_off;
95789582 flags = scx_kf_allow_flags[SCX_MOFF_IDX(moff)];
95799583
kernel/sched/ext_idle.c+1−0
@@ -1467,6 +1467,7 @@ BTF_KFUNCS_END(scx_kfunc_ids_idle) · 1466 unmodified lines
14671467 static const struct btf_kfunc_id_set scx_kfunc_set_idle = {
14681468 .owner = THIS_MODULE,
14691469 .set = &scx_kfunc_ids_idle,
1470+ .filter = scx_kfunc_context_filter,
14701471 };
14711472
14721473 /*
kernel/sched/ext_idle.h+1−0
@@ -12,6 +12,7 @@ · 11 unmodified lines
1212
1313 struct sched_ext_ops;
1414
15+extern struct btf_id_set8 scx_kfunc_ids_idle;
1516 extern struct btf_id_set8 scx_kfunc_ids_select_cpu;
1617
1718 void scx_idle_update_selcpu_topology(struct sched_ext_ops *ops);