linux-kernel - Re: [PATCH bpf-next 1/5] bpf: Add a ARG_PTR_TO_CONST

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Date:   Wed, 17 Mar 2021 00:58:30 +0100
From:   Florent Revest <revest@...omium.org>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Yonghong Song <yhs@...com>, KP Singh <kpsingh@...nel.org>,
        Brendan Jackman <jackmanb@...omium.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next 1/5] bpf: Add a ARG_PTR_TO_CONST_STR argument type

On Tue, Mar 16, 2021 at 2:03 AM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
> On Wed, Mar 10, 2021 at 2:02 PM Florent Revest <revest@...omium.org> wrote:
> > +       } else if (arg_type == ARG_PTR_TO_CONST_STR) {
> > +               struct bpf_map *map = reg->map_ptr;
> > +               int map_off, i;
> > +               u64 map_addr;
> > +               char *map_ptr;
> > +
> > +               if (!map || !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_helper_mem_access(env, regno,
> > +                                             map->value_size - reg->off,
> > +                                             false, meta);
>
> you expect reg to be PTR_TO_MAP_VALUE, so probably better to directly
> use check_map_access(). And double-check that register is of expected
> type. just the presence of ref->map_ptr might not be sufficient?

Sorry, just making sure I understand your comment correctly, are you
suggesting that we:
1- skip the check_map_access_type() currently done by
check_helper_mem_access()? or did you implicitly mean that we should
call it as well next to check_map_access() ?
2- enforce (reg->type == PTR_TO_MAP_VALUE) even if currently
guaranteed by compatible_reg_types, just to stay on the safe side ?