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SubjectRe: [PATCH bpf-next v6 05/23] bpf/verifier: allow kfunc to return an allocated mem
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
>

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