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Message-ID: <CAMB2axOR-9_h2wCiibRD0bwoCntZi7h_g79E1naRLFOurWscTg@mail.gmail.com>
Date: Wed, 24 Jul 2024 13:44:23 -0700
From: Amery Hung <ameryhung@...il.com>
To: Kui-Feng Lee <sinquersw@...il.com>
Cc: netdev@...r.kernel.org, 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 Tue, Jul 23, 2024 at 10:36 PM Kui-Feng Lee <sinquersw@...il.com> 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 <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?
>
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)
>
> > 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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