Re: [PATCH bpf-next v5 2/6] bpf: Add a ARG_PTR_TO_CONST_STR argument type

[Alexei Starovoitov]

On Tue, Apr 20, 2021 at 5:35 AM Florent Revest <revest@xxxxxxxxxxxx> wrote:
>
> On Tue, Apr 20, 2021 at 12:54 AM Alexei Starovoitov
> <alexei.starovoitov@xxxxxxxxx> wrote:
> >
> > 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.
>
> I agree with all of the above. I only thought it's better to be safe
> than sorry but if you'd like I could follow up with a patch that
> removes some checks?
...
> Sure, does not hurt. I can also follow up with a patch unless if you
> prefer doing it yourself.

Please send a follow up patch.
I consider this kind of "safe than sorry" to be defensive programming that
promotes less-thinking-is-fine-because-its-faster-to-code style.
I'm sure you've seen my rants against defensive programming in the past :)