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References: <20220712145850.599666-1-benjamin.tissoires@redhat.com> <20220712145850.599666-6-benjamin.tissoires@redhat.com>
In-Reply-To: <20220712145850.599666-6-benjamin.tissoires@redhat.com>
From:   Kumar Kartikeya Dwivedi <memxor@gmail.com>
Date:   Sat, 16 Jul 2022 22:32:54 +0200
Message-ID: <CAP01T77nCee6R9DL_gHJCrkVgcoJH9n52McKA87KqE3Ud8qwTg@mail.gmail.com>
Subject: Re: [PATCH bpf-next v6 05/23] bpf/verifier: allow kfunc to return an
 allocated mem
To:     Benjamin Tissoires <benjamin.tissoires@redhat.com>
Cc:     Greg KH <gregkh@linuxfoundation.org>,
        Jiri Kosina <jikos@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>, Shuah Khan <shuah@kernel.org>,
        Dave Marchevsky <davemarchevsky@fb.com>,
        Joe Stringer <joe@cilium.io>, Jonathan Corbet <corbet@lwn.net>,
        Tero Kristo <tero.kristo@linux.intel.com>,
        linux-kernel@vger.kernel.org, linux-input@vger.kernel.org,
        netdev@vger.kernel.org, bpf@vger.kernel.org,
        linux-kselftest@vger.kernel.org, linux-doc@vger.kernel.org
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Precedence: bulk

On Tue, 12 Jul 2022 at 17:03, Benjamin Tissoires
<benjamin.tissoires@redhat.com> wrote:
>
> When a kfunc is not returning a pointer to a struct but to a plain type,
> we can consider it is a valid allocated memory assuming that:
> - one of the arguments is either called rdonly_buf_size or
>   rdwr_buf_size
> - and this argument is a const from the caller point of view
>
> We can then use this parameter as the size of the allocated memory.
>
> The memory is either read-only or read-write based on the name
> of the size parameter.
>
> Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
>
> ---
>
> changes in v6:
> - code review from Kartikeya:
>   - remove comment change that had no reasons to be
>   - remove handling of PTR_TO_MEM with kfunc releases
>   - introduce struct bpf_kfunc_arg_meta
>   - do rdonly/rdwr_buf_size check in btf_check_kfunc_arg_match
>   - reverted most of the changes in verifier.c
>   - make sure kfunc acquire is using a struct pointer, not just a plain
>     pointer
>   - also forward ref_obj_id to PTR_TO_MEM in kfunc to not use after free
>     the allocated memory
>
> changes in v5:
> - updated PTR_TO_MEM comment in btf.c to match upstream
> - make it read-only or read-write based on the name of size
>
> new in v4
> ---
>  include/linux/bpf.h   | 10 ++++++-
>  include/linux/btf.h   | 12 ++++++++
>  kernel/bpf/btf.c      | 67 ++++++++++++++++++++++++++++++++++++++++---
>  kernel/bpf/verifier.c | 49 +++++++++++++++++++++++--------
>  4 files changed, 121 insertions(+), 17 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 2b21f2a3452f..5b8eadb6e7bc 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1916,12 +1916,20 @@ int btf_distill_func_proto(struct bpf_verifier_log *log,
>                            const char *func_name,
>                            struct btf_func_model *m);
>
> +struct bpf_kfunc_arg_meta {
> +       u64 r0_size;
> +       bool r0_rdonly;
> +       int ref_obj_id;
> +       bool multiple_ref_obj_id;
> +};
> +
>  struct bpf_reg_state;
>  int btf_check_subprog_arg_match(struct bpf_verifier_env *env, int subprog,
>                                 struct bpf_reg_state *regs);
>  int btf_check_kfunc_arg_match(struct bpf_verifier_env *env,
>                               const struct btf *btf, u32 func_id,
> -                             struct bpf_reg_state *regs);
> +                             struct bpf_reg_state *regs,
> +                             struct bpf_kfunc_arg_meta *meta);
>  int btf_prepare_func_args(struct bpf_verifier_env *env, int subprog,
>                           struct bpf_reg_state *reg);
>  int btf_check_type_match(struct bpf_verifier_log *log, const struct bpf_prog *prog,
> diff --git a/include/linux/btf.h b/include/linux/btf.h
> index 1bfed7fa0428..31da4273c2ec 100644
> --- a/include/linux/btf.h
> +++ b/include/linux/btf.h
> @@ -420,4 +420,16 @@ static inline int register_btf_id_dtor_kfuncs(const struct btf_id_dtor_kfunc *dt
>  }
>  #endif
>
> +static inline bool btf_type_is_struct_ptr(struct btf *btf, const struct btf_type *t)
> +{
> +       /* t comes in already as a pointer */
> +       t = btf_type_by_id(btf, t->type);
> +
> +       /* allow const */
> +       if (BTF_INFO_KIND(t->info) == BTF_KIND_CONST)
> +               t = btf_type_by_id(btf, t->type);

Any specific reason to not allow any other modifiers apart from const?
volatile, restrict, typedef..?
If not, just use btf_type_skip_modifiers instead.

> +
> +       return btf_type_is_struct(t);
> +}
> +
>  #endif
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 4423045b8ff3..552d7bc05a0c 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -6168,10 +6168,36 @@ static bool is_kfunc_arg_mem_size(const struct btf *btf,
>         return true;
>  }
>
> +static bool btf_is_kfunc_arg_mem_size(const struct btf *btf,
> +                                     const struct btf_param *arg,
> +                                     const struct bpf_reg_state *reg,
> +                                     const char *name)

It would be nicer if we could reuse some code from
is_kfunc_arg_mem_size, the only difference is matching suffix vs full
string. But don't feel too strongly about it.

> +{
> +       int len, target_len = strlen(name);
> +       const struct btf_type *t;
> +       const char *param_name;
> +
> +       t = btf_type_skip_modifiers(btf, arg->type, NULL);
> +       if (!btf_type_is_scalar(t) || reg->type != SCALAR_VALUE)
> +               return false;
> +
> +       param_name = btf_name_by_offset(btf, arg->name_off);
> +       if (str_is_empty(param_name))
> +               return false;
> +       len = strlen(param_name);
> +       if (len != target_len)
> +               return false;
> +       if (strncmp(param_name, name, target_len))
> +               return false;
> +
> +       return true;
> +}
> +
>  static int btf_check_func_arg_match(struct bpf_verifier_env *env,
>                                     const struct btf *btf, u32 func_id,
>                                     struct bpf_reg_state *regs,
> -                                   bool ptr_to_mem_ok)
> +                                   bool ptr_to_mem_ok,
> +                                   struct bpf_kfunc_arg_meta *kfunc_meta)
>  {
>         enum bpf_prog_type prog_type = resolve_prog_type(env->prog);
>         struct bpf_verifier_log *log = &env->log;
> @@ -6225,6 +6251,30 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
>
>                 t = btf_type_skip_modifiers(btf, args[i].type, NULL);
>                 if (btf_type_is_scalar(t)) {
> +                       if (is_kfunc && kfunc_meta) {
> +                               bool is_buf_size = false;
> +
> +                               /* check for any const scalar parameter of name "rdonly_buf_size"
> +                                * or "rdwr_buf_size"
> +                                */
> +                               if (btf_is_kfunc_arg_mem_size(btf, &args[i], reg,
> +                                                             "rdonly_buf_size")) {
> +                                       kfunc_meta->r0_rdonly = true;
> +                                       is_buf_size = true;
> +                               } else if (btf_is_kfunc_arg_mem_size(btf, &args[i], reg,
> +                                                                    "rdwr_buf_size"))
> +                                       is_buf_size = true;
> +
> +                               if (is_buf_size) {
> +                                       if (kfunc_meta->r0_size) {
> +                                               bpf_log(log, "2 or more rdonly/rdwr_buf_size parameters for kfunc");
> +                                               return -EINVAL;
> +                                       }
> +
> +                                       kfunc_meta->r0_size = reg->var_off.value;

As Yonghong pointed out, you need to ensure the register holds a
constant value, by using tnum_is_const(reg->var_off), and giving an
error otherwise, because we need a constant size to be set for R0.

> +                               }
> +                       }
> +
>                         if (reg->type == SCALAR_VALUE)
>                                 continue;
>                         bpf_log(log, "R%d is not a scalar\n", regno);
> @@ -6246,6 +6296,14 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
>                 if (ret < 0)
>                         return ret;
>
> +               /* kptr_get is only valid for kfunc */

Invalid comment

> +               if (kfunc_meta && reg->ref_obj_id) {
> +                       /* check for any one ref_obj_id to keep track of memory */
> +                       if (kfunc_meta->ref_obj_id)
> +                               kfunc_meta->multiple_ref_obj_id = true;

Why not just return the error here itself? And then no need to keep
the multiple_ref_obj_id member.
When you return the error here, you can move a similar check in the if
(reg->type == PTR_TO_BTF_ID) block to this place so that we don't do
it twice.

> +                       kfunc_meta->ref_obj_id = reg->ref_obj_id;
> +               }
> +
>                 /* kptr_get is only true for kfunc */
>                 if (i == 0 && kptr_get) {
>                         struct bpf_map_value_off_desc *off_desc;
> @@ -6441,7 +6499,7 @@ int btf_check_subprog_arg_match(struct bpf_verifier_env *env, int subprog,
>                 return -EINVAL;
>
>         is_global = prog->aux->func_info_aux[subprog].linkage == BTF_FUNC_GLOBAL;
> -       err = btf_check_func_arg_match(env, btf, btf_id, regs, is_global);
> +       err = btf_check_func_arg_match(env, btf, btf_id, regs, is_global, NULL);
>
>         /* Compiler optimizations can remove arguments from static functions
>          * or mismatched type can be passed into a global function.
> @@ -6454,9 +6512,10 @@ int btf_check_subprog_arg_match(struct bpf_verifier_env *env, int subprog,
>
>  int btf_check_kfunc_arg_match(struct bpf_verifier_env *env,
>                               const struct btf *btf, u32 func_id,
> -                             struct bpf_reg_state *regs)
> +                             struct bpf_reg_state *regs,
> +                             struct bpf_kfunc_arg_meta *meta)
>  {
> -       return btf_check_func_arg_match(env, btf, func_id, regs, true);
> +       return btf_check_func_arg_match(env, btf, func_id, regs, true, meta);
>  }
>
>  /* Convert BTF of a function into bpf_reg_state if possible
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 3adcc0d123af..77556132db15 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -7561,6 +7561,7 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
>  {
>         const struct btf_type *t, *func, *func_proto, *ptr_type;
>         struct bpf_reg_state *regs = cur_regs(env);
> +       struct bpf_kfunc_arg_meta meta = { 0 };
>         const char *func_name, *ptr_type_name;
>         u32 i, nargs, func_id, ptr_type_id;
>         int err, insn_idx = *insn_idx_p;
> @@ -7592,7 +7593,7 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
>                                         BTF_KFUNC_TYPE_ACQUIRE, func_id);
>
>         /* Check the arguments */
> -       err = btf_check_kfunc_arg_match(env, desc_btf, func_id, regs);
> +       err = btf_check_kfunc_arg_match(env, desc_btf, func_id, regs, &meta);
>         if (err < 0)
>                 return err;
>         /* In case of release function, we get register number of refcounted
> @@ -7613,7 +7614,7 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
>         /* Check return type */
>         t = btf_type_skip_modifiers(desc_btf, func_proto->type, NULL);
>
> -       if (acq && !btf_type_is_ptr(t)) {
> +       if (acq && !btf_type_is_struct_ptr(desc_btf, t)) {
>                 verbose(env, "acquire kernel function does not return PTR_TO_BTF_ID\n");
>                 return -EINVAL;
>         }
> @@ -7625,17 +7626,41 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
>                 ptr_type = btf_type_skip_modifiers(desc_btf, t->type,
>                                                    &ptr_type_id);
>                 if (!btf_type_is_struct(ptr_type)) {
> -                       ptr_type_name = btf_name_by_offset(desc_btf,
> -                                                          ptr_type->name_off);
> -                       verbose(env, "kernel function %s returns pointer type %s %s is not supported\n",
> -                               func_name, btf_type_str(ptr_type),
> -                               ptr_type_name);
> -                       return -EINVAL;
> +                       if (!meta.r0_size) {
> +                               ptr_type_name = btf_name_by_offset(desc_btf,
> +                                                                  ptr_type->name_off);
> +                               verbose(env,
> +                                       "kernel function %s returns pointer type %s %s is not supported\n",
> +                                       func_name,
> +                                       btf_type_str(ptr_type),
> +                                       ptr_type_name);
> +                               return -EINVAL;
> +                       }
> +
> +                       if (meta.multiple_ref_obj_id) {
> +                               verbose(env,
> +                                       "kernel function %s has multiple memory tracked objects\n",
> +                                       func_name);
> +                               return -EINVAL;
> +                       }
> +
> +                       mark_reg_known_zero(env, regs, BPF_REG_0);
> +                       regs[BPF_REG_0].type = PTR_TO_MEM;
> +                       regs[BPF_REG_0].mem_size = meta.r0_size;
> +
> +                       if (meta.r0_rdonly)
> +                               regs[BPF_REG_0].type |= MEM_RDONLY;
> +
> +                       /* Ensures we don't access the memory after a release_reference() */
> +                       if (meta.ref_obj_id)
> +                               regs[BPF_REG_0].ref_obj_id = meta.ref_obj_id;
> +               } else {
> +                       mark_reg_known_zero(env, regs, BPF_REG_0);
> +                       regs[BPF_REG_0].btf = desc_btf;
> +                       regs[BPF_REG_0].type = PTR_TO_BTF_ID;
> +                       regs[BPF_REG_0].btf_id = ptr_type_id;
>                 }
> -               mark_reg_known_zero(env, regs, BPF_REG_0);
> -               regs[BPF_REG_0].btf = desc_btf;
> -               regs[BPF_REG_0].type = PTR_TO_BTF_ID;
> -               regs[BPF_REG_0].btf_id = ptr_type_id;
> +
>                 if (btf_kfunc_id_set_contains(desc_btf, resolve_prog_type(env->prog),
>                                               BTF_KFUNC_TYPE_RET_NULL, func_id)) {
>                         regs[BPF_REG_0].type |= PTR_MAYBE_NULL;
> --
> 2.36.1
>