btf_kfunc_id_set_contains() is called by fetch_kfunc_meta() in the BPF verifier to get the kfunc flags stored in the .BTF_ids ELF section. If it returns NULL instead of a valid pointer, it's interpreted as an illegal kfunc usage failing the verification. There are two potential reasons for btf_kfunc_id_set_contains() to return NULL: 1. Provided kfunc BTF id is not present in relevant kfunc id sets. 2. The kfunc is not allowed, as determined by the program type specific filter [1]. The filter functions accept a pointer to `struct bpf_prog`, so they might implicitly depend on earlier stages of verification, when bpf_prog members are set. For example, bpf_qdisc_kfunc_filter() in linux/net/sched/bpf_qdisc.c inspects prog->aux->st_ops [2], which is initialized in: check_attach_btf_id() -> check_struct_ops_btf_id() So far this hasn't been an issue, because fetch_kfunc_meta() is the only caller of btf_kfunc_id_set_contains(). However in subsequent patches of this series it is necessary to inspect kfunc flags earlier in BPF verifier, in the add_kfunc_call(). To resolve this, refactor btf_kfunc_id_set_contains() into two interface functions: * btf_kfunc_flags() that simply returns pointer to kfunc_flags without applying the filters * btf_kfunc_is_allowed() that both checks for kfunc_flags existence (which is a requirement for a kfunc to be allowed) and applies the prog filters See [3] for the previous version of this patch. [1] https://lore.kernel.org/all/20230519225157.760788-7-aditi.ghag@isovalent.com/ [2] https://lore.kernel.org/all/20250409214606.2000194-4-ameryhung@gmail.com/ [3] https://lore.kernel.org/bpf/20251029190113.3323406-3-ihor.solodrai@linux.dev/ Signed-off-by: Ihor Solodrai --- include/linux/btf.h | 4 +-- kernel/bpf/btf.c | 70 ++++++++++++++++++++++++++++++++----------- kernel/bpf/verifier.c | 6 ++-- 3 files changed, 58 insertions(+), 22 deletions(-) diff --git a/include/linux/btf.h b/include/linux/btf.h index 691f09784933..bd261015c4bc 100644 --- a/include/linux/btf.h +++ b/include/linux/btf.h @@ -574,8 +574,8 @@ const char *btf_name_by_offset(const struct btf *btf, u32 offset); const char *btf_str_by_offset(const struct btf *btf, u32 offset); struct btf *btf_parse_vmlinux(void); struct btf *bpf_prog_get_target_btf(const struct bpf_prog *prog); -u32 *btf_kfunc_id_set_contains(const struct btf *btf, u32 kfunc_btf_id, - const struct bpf_prog *prog); +u32 *btf_kfunc_flags(const struct btf *btf, u32 kfunc_btf_id, const struct bpf_prog *prog); +bool btf_kfunc_is_allowed(const struct btf *btf, u32 kfunc_btf_id, const struct bpf_prog *prog); u32 *btf_kfunc_is_modify_return(const struct btf *btf, u32 kfunc_btf_id, const struct bpf_prog *prog); int register_btf_kfunc_id_set(enum bpf_prog_type prog_type, diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c index 539c9fdea41d..32974c11664d 100644 --- a/kernel/bpf/btf.c +++ b/kernel/bpf/btf.c @@ -8640,24 +8640,17 @@ static int btf_populate_kfunc_set(struct btf *btf, enum btf_kfunc_hook hook, return ret; } -static u32 *__btf_kfunc_id_set_contains(const struct btf *btf, - enum btf_kfunc_hook hook, - u32 kfunc_btf_id, - const struct bpf_prog *prog) +static u32 *btf_kfunc_id_set_contains(const struct btf *btf, + enum btf_kfunc_hook hook, + u32 kfunc_btf_id) { - struct btf_kfunc_hook_filter *hook_filter; struct btf_id_set8 *set; - u32 *id, i; + u32 *id; if (hook >= BTF_KFUNC_HOOK_MAX) return NULL; if (!btf->kfunc_set_tab) return NULL; - hook_filter = &btf->kfunc_set_tab->hook_filters[hook]; - for (i = 0; i < hook_filter->nr_filters; i++) { - if (hook_filter->filters[i](prog, kfunc_btf_id)) - return NULL; - } set = btf->kfunc_set_tab->sets[hook]; if (!set) return NULL; @@ -8668,6 +8661,28 @@ static u32 *__btf_kfunc_id_set_contains(const struct btf *btf, return id + 1; } +static bool __btf_kfunc_is_allowed(const struct btf *btf, + enum btf_kfunc_hook hook, + u32 kfunc_btf_id, + const struct bpf_prog *prog) +{ + struct btf_kfunc_hook_filter *hook_filter; + int i; + + if (hook >= BTF_KFUNC_HOOK_MAX) + return false; + if (!btf->kfunc_set_tab) + return false; + + hook_filter = &btf->kfunc_set_tab->hook_filters[hook]; + for (i = 0; i < hook_filter->nr_filters; i++) { + if (hook_filter->filters[i](prog, kfunc_btf_id)) + return false; + } + + return true; +} + static int bpf_prog_type_to_kfunc_hook(enum bpf_prog_type prog_type) { switch (prog_type) { @@ -8715,6 +8730,26 @@ static int bpf_prog_type_to_kfunc_hook(enum bpf_prog_type prog_type) } } +bool btf_kfunc_is_allowed(const struct btf *btf, + u32 kfunc_btf_id, + const struct bpf_prog *prog) +{ + enum bpf_prog_type prog_type = resolve_prog_type(prog); + enum btf_kfunc_hook hook; + u32 *kfunc_flags; + + kfunc_flags = btf_kfunc_id_set_contains(btf, BTF_KFUNC_HOOK_COMMON, kfunc_btf_id); + if (kfunc_flags && __btf_kfunc_is_allowed(btf, BTF_KFUNC_HOOK_COMMON, kfunc_btf_id, prog)) + return true; + + hook = bpf_prog_type_to_kfunc_hook(prog_type); + kfunc_flags = btf_kfunc_id_set_contains(btf, hook, kfunc_btf_id); + if (kfunc_flags && __btf_kfunc_is_allowed(btf, hook, kfunc_btf_id, prog)) + return true; + + return false; +} + /* Caution: * Reference to the module (obtained using btf_try_get_module) corresponding to * the struct btf *MUST* be held when calling this function from verifier @@ -8722,26 +8757,27 @@ static int bpf_prog_type_to_kfunc_hook(enum bpf_prog_type prog_type) * keeping the reference for the duration of the call provides the necessary * protection for looking up a well-formed btf->kfunc_set_tab. */ -u32 *btf_kfunc_id_set_contains(const struct btf *btf, - u32 kfunc_btf_id, - const struct bpf_prog *prog) +u32 *btf_kfunc_flags(const struct btf *btf, u32 kfunc_btf_id, const struct bpf_prog *prog) { enum bpf_prog_type prog_type = resolve_prog_type(prog); enum btf_kfunc_hook hook; u32 *kfunc_flags; - kfunc_flags = __btf_kfunc_id_set_contains(btf, BTF_KFUNC_HOOK_COMMON, kfunc_btf_id, prog); + kfunc_flags = btf_kfunc_id_set_contains(btf, BTF_KFUNC_HOOK_COMMON, kfunc_btf_id); if (kfunc_flags) return kfunc_flags; hook = bpf_prog_type_to_kfunc_hook(prog_type); - return __btf_kfunc_id_set_contains(btf, hook, kfunc_btf_id, prog); + return btf_kfunc_id_set_contains(btf, hook, kfunc_btf_id); } u32 *btf_kfunc_is_modify_return(const struct btf *btf, u32 kfunc_btf_id, const struct bpf_prog *prog) { - return __btf_kfunc_id_set_contains(btf, BTF_KFUNC_HOOK_FMODRET, kfunc_btf_id, prog); + if (!__btf_kfunc_is_allowed(btf, BTF_KFUNC_HOOK_FMODRET, kfunc_btf_id, prog)) + return NULL; + + return btf_kfunc_id_set_contains(btf, BTF_KFUNC_HOOK_FMODRET, kfunc_btf_id); } static int __register_btf_kfunc_id_set(enum btf_kfunc_hook hook, diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 53635ea2e41b..bee49c9cde21 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -13722,10 +13722,10 @@ static int fetch_kfunc_meta(struct bpf_verifier_env *env, *kfunc_name = func_name; func_proto = btf_type_by_id(desc_btf, func->type); - kfunc_flags = btf_kfunc_id_set_contains(desc_btf, func_id, env->prog); - if (!kfunc_flags) { + if (!btf_kfunc_is_allowed(desc_btf, func_id, env->prog)) return -EACCES; - } + + kfunc_flags = btf_kfunc_flags(desc_btf, func_id, env->prog); memset(meta, 0, sizeof(*meta)); meta->btf = desc_btf; -- 2.52.0