On Mon, Jun 20, 2022 at 6:29 PM KP Singh <kpsingh@xxxxxxxxxx> wrote:
>
> kfuncs can handle pointers to memory when the next argument is
> the size of the memory that can be read and verify these as
> ARG_CONST_SIZE_OR_ZERO
>
> Similarly add support for string constants (const char *) and
> verify it similar to ARG_PTR_TO_CONST_STR.
>
> Signed-off-by: KP Singh <kpsingh@xxxxxxxxxx>
> ---
> include/linux/bpf_verifier.h | 2 +
> kernel/bpf/btf.c | 29 ++++++++++++
> kernel/bpf/verifier.c | 85 ++++++++++++++++++++----------------
> 3 files changed, 79 insertions(+), 37 deletions(-)
>
[...]
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 668ecf61649b..02d7951591ae 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -6162,6 +6162,26 @@ static bool is_kfunc_arg_mem_size(const struct btf *btf,
> return true;
> }
>
> +static bool btf_param_is_const_str_ptr(const struct btf *btf,
> + const struct btf_param *param)
> +{
> + const struct btf_type *t;
> +
> + t = btf_type_by_id(btf, param->type);
> + if (!btf_type_is_ptr(t))
> + return false;
> +
> + t = btf_type_by_id(btf, t->type);
> + if (!(BTF_INFO_KIND(t->info) == BTF_KIND_CONST))
"if (BTF_INFO_KIND(t->info) != BTF_KIND_CONST)" looks clearer to me
> + return false;
> +
> + t = btf_type_skip_modifiers(btf, t->type, NULL);
> + if (!strcmp(btf_name_by_offset(btf, t->name_off), "char"))
"return !strcmp(btf_name_by_offset(btf, t->name_off), "char")" looks
clearer to me here too
> + return true;
> +
> + return false;
> +}
> +
> static int btf_check_func_arg_match(struct bpf_verifier_env *env,
> const struct btf *btf, u32 func_id,
> struct bpf_reg_state *regs,
> @@ -6344,6 +6364,7 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
> } else if (ptr_to_mem_ok) {
> const struct btf_type *resolve_ret;
> u32 type_size;
> + int err;
>
> if (is_kfunc) {
> bool arg_mem_size = i + 1 < nargs && is_kfunc_arg_mem_size(btf, &args[i + 1], ®s[regno + 1]);
> @@ -6354,6 +6375,14 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
> * When arg_mem_size is true, the pointer can be
> * void *.
> */
> + if (btf_param_is_const_str_ptr(btf, &args[i])) {
> + err = check_const_str(env, reg, regno);
> + if (err < 0)
> + return err;
> + i++;
> + continue;
If I'm understanding it correctly, this patch is intended to allow
helper functions to take in a kfunc as an arg as long as the next arg
is the size of the memory. Do we need to check the memory size access
here (eg like a call to check_mem_size_reg() in the verifier) to
ensure that memory accesses of that size are safe?
> + }
> +
> if (!btf_type_is_scalar(ref_t) &&
> !__btf_type_is_scalar_struct(log, btf, ref_t, 0) &&
> (arg_mem_size ? !btf_type_is_void(ref_t) : 1)) {
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 2859901ffbe3..14a434792d7b 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -5840,6 +5840,52 @@ static u32 stack_slot_get_id(struct bpf_verifier_env *env, struct bpf_reg_state
> return state->stack[spi].spilled_ptr.id;
> }
[...]
> +
> static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
> struct bpf_call_arg_meta *meta,
> const struct bpf_func_proto *fn)
> @@ -6074,44 +6120,9 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
> return err;
> err = check_ptr_alignment(env, reg, 0, size, true);
> } else if (arg_type == ARG_PTR_TO_CONST_STR) {
> - struct bpf_map *map = reg->map_ptr;
> - int map_off;
> - u64 map_addr;
> - char *str_ptr;
> -
> - if (!bpf_map_is_rdonly(map)) {
> - verbose(env, "R%d does not point to a readonly map'\n", regno);
> - return -EACCES;
> - }
> -
> - if (!tnum_is_const(reg->var_off)) {
> - verbose(env, "R%d is not a constant address'\n", regno);
> - return -EACCES;
> - }
> -
> - if (!map->ops->map_direct_value_addr) {
> - verbose(env, "no direct value access support for this map type\n");
> - return -EACCES;
> - }
> -
> - err = check_map_access(env, regno, reg->off,
> - map->value_size - reg->off, false,
> - ACCESS_HELPER);
> - if (err)
> - return err;
> -
> - map_off = reg->off + reg->var_off.value;
> - err = map->ops->map_direct_value_addr(map, &map_addr, map_off);
> - if (err) {
> - verbose(env, "direct value access on string failed\n");
> + err = check_const_str(env, reg, regno);
> + if (err < 0)
> return err;
nit: I don't think you need the if check here since thsi function will
return err automatically in the next line
> - }
> -
> - str_ptr = (char *)(long)(map_addr);
> - if (!strnchr(str_ptr + map_off, map->value_size - map_off, 0)) {
> - verbose(env, "string is not zero-terminated\n");
> - return -EINVAL;
> - }
> } else if (arg_type == ARG_PTR_TO_KPTR) {
> if (process_kptr_func(env, regno, meta))
> return -EACCES;
> --
> 2.37.0.rc0.104.g0611611a94-goog
>
