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([2600:1700:6cf8:1240:d785:ba42:22fc:942e]) by smtp.gmail.com with ESMTPSA id 3f1490d57ef6-e0b29f4f90asm865078276.16.2024.07.26.11.22.14 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 26 Jul 2024 11:22:15 -0700 (PDT) Message-ID: <71fa8e2c-953d-447d-905a-e2c596839cea@gmail.com> Date: Fri, 26 Jul 2024 11:22:13 -0700 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v9 03/11] bpf: Allow struct_ops prog to return referenced kptr To: Amery Hung Cc: netdev@vger.kernel.org, bpf@vger.kernel.org, yangpeihao@sjtu.edu.cn, daniel@iogearbox.net, andrii@kernel.org, alexei.starovoitov@gmail.com, martin.lau@kernel.org, toke@redhat.com, jhs@mojatatu.com, jiri@resnulli.us, sdf@google.com, xiyou.wangcong@gmail.com, yepeilin.cs@gmail.com References: <20240714175130.4051012-1-amery.hung@bytedance.com> <20240714175130.4051012-4-amery.hung@bytedance.com> <5b527381-ef28-470e-954d-45ce27e8d9d9@gmail.com> Content-Language: en-US From: Kui-Feng Lee In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 7/24/24 13:44, Amery Hung wrote: > On Tue, Jul 23, 2024 at 10:36 PM Kui-Feng Lee wrote: >> >> >> >> On 7/14/24 10:51, Amery Hung wrote: >>> Allow a struct_ops program to return a referenced kptr if the struct_ops >>> operator has pointer to struct as the return type. To make sure the >>> returned pointer continues to be valid in the kernel, several >>> constraints are required: >>> >>> 1) The type of the pointer must matches the return type >>> 2) The pointer originally comes from the kernel (not locally allocated) >>> 3) The pointer is in its unmodified form >>> >>> In addition, since the first user, Qdisc_ops::dequeue, allows a NULL >>> pointer to be returned when there is no skb to be dequeued, we will allow >>> a scalar value with value equals to NULL to be returned. >>> >>> In the future when there is a struct_ops user that always expects a valid >>> pointer to be returned from an operator, we may extend tagging to the >>> return value. We can tell the verifier to only allow NULL pointer return >>> if the return value is tagged with MAY_BE_NULL. >>> >>> The check is split into two parts since check_reference_leak() happens >>> before check_return_code(). We first allow a reference object to leak >>> through return if it is in the return register and the type matches the >>> return type. Then, we check whether the pointer to-be-returned is valid in >>> check_return_code(). >>> >>> Signed-off-by: Amery Hung >>> --- >>> kernel/bpf/verifier.c | 50 +++++++++++++++++++++++++++++++++++++++---- >>> 1 file changed, 46 insertions(+), 4 deletions(-) >>> >>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c >>> index f614ab283c37..e7f356098902 100644 >>> --- a/kernel/bpf/verifier.c >>> +++ b/kernel/bpf/verifier.c >>> @@ -10188,16 +10188,36 @@ record_func_key(struct bpf_verifier_env *env, struct bpf_call_arg_meta *meta, >>> >>> static int check_reference_leak(struct bpf_verifier_env *env, bool exception_exit) >>> { >>> + enum bpf_prog_type type = resolve_prog_type(env->prog); >>> + u32 regno = exception_exit ? BPF_REG_1 : BPF_REG_0; >>> + struct bpf_reg_state *reg = reg_state(env, regno); >>> struct bpf_func_state *state = cur_func(env); >>> + const struct bpf_prog *prog = env->prog; >>> + const struct btf_type *ret_type = NULL; >>> bool refs_lingering = false; >>> + struct btf *btf; >>> int i; >>> >>> if (!exception_exit && state->frameno && !state->in_callback_fn) >>> return 0; >>> >>> + if (type == BPF_PROG_TYPE_STRUCT_OPS && >>> + reg->type & PTR_TO_BTF_ID && reg->ref_obj_id) { >>> + btf = bpf_prog_get_target_btf(prog); >>> + ret_type = btf_type_by_id(btf, prog->aux->attach_func_proto->type); >>> + if (reg->btf_id != ret_type->type) { >>> + verbose(env, "Return kptr type, struct %s, doesn't match function prototype, struct %s\n", >>> + btf_type_name(reg->btf, reg->btf_id), >>> + btf_type_name(btf, ret_type->type)); >>> + return -EINVAL; >>> + } >>> + } >>> + >>> for (i = 0; i < state->acquired_refs; i++) { >>> if (!exception_exit && state->in_callback_fn && state->refs[i].callback_ref != state->frameno) >>> continue; >>> + if (ret_type && reg->ref_obj_id == state->refs[i].id) >>> + continue; >> >> Is it possible having two kptrs that both are in the returned type >> passing into a function? >> > > Just to make sure I understand the question correctly: Are you asking > what would happen here if a struct_ops operator has the following > signature? > > struct *foo xxx_ops__dummy_op(struct foo *foo_a__ref, struct foo *foo_b__ref) Right! What would happen to this case? Could one of them leak without being detected? > >> >>> verbose(env, "Unreleased reference id=%d alloc_insn=%d\n", >>> state->refs[i].id, state->refs[i].insn_idx); >>> refs_lingering = true; >>> @@ -15677,12 +15697,15 @@ static int check_return_code(struct bpf_verifier_env *env, int regno, const char >>> const char *exit_ctx = "At program exit"; >>> struct tnum enforce_attach_type_range = tnum_unknown; >>> const struct bpf_prog *prog = env->prog; >>> - struct bpf_reg_state *reg; >>> + struct bpf_reg_state *reg = reg_state(env, regno); >>> struct bpf_retval_range range = retval_range(0, 1); >>> enum bpf_prog_type prog_type = resolve_prog_type(env->prog); >>> int err; >>> struct bpf_func_state *frame = env->cur_state->frame[0]; >>> const bool is_subprog = frame->subprogno; >>> + struct btf *btf = bpf_prog_get_target_btf(prog); >>> + bool st_ops_ret_is_kptr = false; >>> + const struct btf_type *t; >>> >>> /* LSM and struct_ops func-ptr's return type could be "void" */ >>> if (!is_subprog || frame->in_exception_callback_fn) { >>> @@ -15691,10 +15714,26 @@ static int check_return_code(struct bpf_verifier_env *env, int regno, const char >>> if (prog->expected_attach_type == BPF_LSM_CGROUP) >>> /* See below, can be 0 or 0-1 depending on hook. */ >>> break; >>> - fallthrough; >>> + if (!prog->aux->attach_func_proto->type) >>> + return 0; >>> + break; >>> case BPF_PROG_TYPE_STRUCT_OPS: >>> if (!prog->aux->attach_func_proto->type) >>> return 0; >>> + >>> + t = btf_type_by_id(btf, prog->aux->attach_func_proto->type); >>> + if (btf_type_is_ptr(t)) { >>> + /* Allow struct_ops programs to return kptr or null if >>> + * the return type is a pointer type. >>> + * check_reference_leak has ensured the returning kptr >>> + * matches the type of the function prototype and is >>> + * the only leaking reference. Thus, we can safely return >>> + * if the pointer is in its unmodified form >>> + */ >>> + if (reg->type & PTR_TO_BTF_ID) >>> + return __check_ptr_off_reg(env, reg, regno, false); >>> + st_ops_ret_is_kptr = true; >>> + } >>> break; >>> default: >>> break; >>> @@ -15716,8 +15755,6 @@ static int check_return_code(struct bpf_verifier_env *env, int regno, const char >>> return -EACCES; >>> } >>> >>> - reg = cur_regs(env) + regno; >>> - >>> if (frame->in_async_callback_fn) { >>> /* enforce return zero from async callbacks like timer */ >>> exit_ctx = "At async callback return"; >>> @@ -15804,6 +15841,11 @@ static int check_return_code(struct bpf_verifier_env *env, int regno, const char >>> case BPF_PROG_TYPE_NETFILTER: >>> range = retval_range(NF_DROP, NF_ACCEPT); >>> break; >>> + case BPF_PROG_TYPE_STRUCT_OPS: >>> + if (!st_ops_ret_is_kptr) >>> + return 0; >>> + range = retval_range(0, 0); >>> + break; >>> case BPF_PROG_TYPE_EXT: >>> /* freplace program can return anything as its return value >>> * depends on the to-be-replaced kernel func or bpf program.