Subject: Re: [RFC PATCH v9 03/11] bpf: Allow struct_ops prog to return referenced kptr

From: Kui-Feng Lee <[email-masked]>
Date: Fri, 26 Jul 2024 11:22:13 -0700

On 7/24/24 13:44, Amery Hung wrote:
> On Tue, Jul 23, 2024 at 10:36 PM Kui-Feng Lee <[email-masked]> 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 <[email-masked]>
>>> ---
>>>    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.