On Mon, Apr 19, 2021 at 05:52:39PM +0200, Florent Revest wrote:
> This type provides the guarantee that an argument is going to be a const
> pointer to somewhere in a read-only map value. It also checks that this
> pointer is followed by a zero character before the end of the map value.
>
> Signed-off-by: Florent Revest <revest@xxxxxxxxxxxx>
> Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> ---
> include/linux/bpf.h | 1 +
> kernel/bpf/verifier.c | 41 +++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 42 insertions(+)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 77d1d8c65b81..c160526fc8bf 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -309,6 +309,7 @@ enum bpf_arg_type {
> ARG_PTR_TO_PERCPU_BTF_ID, /* pointer to in-kernel percpu type */
> ARG_PTR_TO_FUNC, /* pointer to a bpf program function */
> ARG_PTR_TO_STACK_OR_NULL, /* pointer to stack or NULL */
> + ARG_PTR_TO_CONST_STR, /* pointer to a null terminated read-only string */
> __BPF_ARG_TYPE_MAX,
> };
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 852541a435ef..5f46dd6f3383 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -4787,6 +4787,7 @@ static const struct bpf_reg_types spin_lock_types = { .types = { PTR_TO_MAP_VALU
> static const struct bpf_reg_types percpu_btf_ptr_types = { .types = { PTR_TO_PERCPU_BTF_ID } };
> static const struct bpf_reg_types func_ptr_types = { .types = { PTR_TO_FUNC } };
> static const struct bpf_reg_types stack_ptr_types = { .types = { PTR_TO_STACK } };
> +static const struct bpf_reg_types const_str_ptr_types = { .types = { PTR_TO_MAP_VALUE } };
>
> static const struct bpf_reg_types *compatible_reg_types[__BPF_ARG_TYPE_MAX] = {
> [ARG_PTR_TO_MAP_KEY] = &map_key_value_types,
> @@ -4817,6 +4818,7 @@ static const struct bpf_reg_types *compatible_reg_types[__BPF_ARG_TYPE_MAX] = {
> [ARG_PTR_TO_PERCPU_BTF_ID] = &percpu_btf_ptr_types,
> [ARG_PTR_TO_FUNC] = &func_ptr_types,
> [ARG_PTR_TO_STACK_OR_NULL] = &stack_ptr_types,
> + [ARG_PTR_TO_CONST_STR] = &const_str_ptr_types,
> };
>
> static int check_reg_type(struct bpf_verifier_env *env, u32 regno,
> @@ -5067,6 +5069,45 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
> if (err)
> 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 (reg->type != PTR_TO_MAP_VALUE || !map ||
I think the 'type' check is redundant,
since check_reg_type() did it via compatible_reg_types.
If so it's probably better to remove it here ?
'!map' looks unnecessary. Can it ever happen? If yes, it's a verifier bug.
For example in check_mem_access() we just deref reg->map_ptr without checking
which, I think, is correct.
> + !bpf_map_is_rdonly(map)) {
This check is needed, of course.
> + 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);
> + 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) {
since the code checks it here the same check in check_bpf_snprintf_call() should
probably do:
if (err) {
verbose("verifier bug\n");
return -EFAULT;
}
instead of just "return err;"
?
> + verbose(env, "direct value access on string failed\n");
I think the message doesn't tell users much, but they probably should never
see it unless they try to do lookup from readonly array with
more than one element.
So I guess it's fine to keep this one as-is. Just flagging.
Anyway the whole set looks great, so I've applied to bpf-next.
Thanks!
