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Message-ID: <5b527381-ef28-470e-954d-45ce27e8d9d9@gmail.com>
Date: Tue, 23 Jul 2024 22:36:50 -0700
From: Kui-Feng Lee <sinquersw@...il.com>
To: Amery Hung <ameryhung@...il.com>, netdev@...r.kernel.org
Cc: bpf@...r.kernel.org, yangpeihao@...u.edu.cn, daniel@...earbox.net,
andrii@...nel.org, alexei.starovoitov@...il.com, martin.lau@...nel.org,
toke@...hat.com, jhs@...atatu.com, jiri@...nulli.us, sdf@...gle.com,
xiyou.wangcong@...il.com, yepeilin.cs@...il.com
Subject: Re: [RFC PATCH v9 03/11] bpf: Allow struct_ops prog to return
referenced kptr
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 <amery.hung@...edance.com>
> ---
> 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?
> 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.
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