linux-kernel - Re: [PATCH bpf-next 1/3] bpf: consider CONST_PTR_TO_MAP as trusted pointer to struct bpf

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Date:   Tue, 18 Jul 2023 17:54:27 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Anton Protopopov <aspsk@...valent.com>
Cc:     Martin KaFai Lau <martin.lau@...ux.dev>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>,
        Hou Tao <houtao1@...wei.com>, Joe Stringer <joe@...valent.com>,
        bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH bpf-next 1/3] bpf: consider CONST_PTR_TO_MAP as trusted
 pointer to struct bpf_map

On Sun, Jul 16, 2023 at 12:49 AM Anton Protopopov <aspsk@...valent.com> wrote:
>
> On Fri, Jul 14, 2023 at 10:56:00AM -0700, Alexei Starovoitov wrote:
> > On Fri, Jul 14, 2023 at 7:20 AM Anton Protopopov <aspsk@...valent.com> wrote:
> > >
> > > Patch verifier to regard values of type CONST_PTR_TO_MAP as trusted
> > > pointers to struct bpf_map. This allows kfuncs to work with `struct
> > > bpf_map *` arguments.
> > >
> > > Save some bytes by defining btf_bpf_map_id as BTF_ID_LIST_GLOBAL_SINGLE
> > > (which is u32[1]), not as BTF_ID_LIST (which is u32[64]).
> > >
> > > Signed-off-by: Anton Protopopov <aspsk@...valent.com>
> > > ---
> > >  include/linux/btf_ids.h | 1 +
> > >  kernel/bpf/map_iter.c   | 3 +--
> > >  kernel/bpf/verifier.c   | 5 ++++-
> > >  3 files changed, 6 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/include/linux/btf_ids.h b/include/linux/btf_ids.h
> > > index 00950cc03bff..a3462a9b8e18 100644
> > > --- a/include/linux/btf_ids.h
> > > +++ b/include/linux/btf_ids.h
> > > @@ -267,5 +267,6 @@ MAX_BTF_TRACING_TYPE,
> > >  extern u32 btf_tracing_ids[];
> > >  extern u32 bpf_cgroup_btf_id[];
> > >  extern u32 bpf_local_storage_map_btf_id[];
> > > +extern u32 btf_bpf_map_id[];
> > >
> > >  #endif
> > > diff --git a/kernel/bpf/map_iter.c b/kernel/bpf/map_iter.c
> > > index d06d3b7150e5..b67996147895 100644
> > > --- a/kernel/bpf/map_iter.c
> > > +++ b/kernel/bpf/map_iter.c
> > > @@ -78,8 +78,7 @@ static const struct seq_operations bpf_map_seq_ops = {
> > >         .show   = bpf_map_seq_show,
> > >  };
> > >
> > > -BTF_ID_LIST(btf_bpf_map_id)
> > > -BTF_ID(struct, bpf_map)
> > > +BTF_ID_LIST_GLOBAL_SINGLE(btf_bpf_map_id, struct, bpf_map)
> > >
> > >  static const struct bpf_iter_seq_info bpf_map_seq_info = {
> > >         .seq_ops                = &bpf_map_seq_ops,
> > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > > index 0b9da95331d7..5663f97ef292 100644
> > > --- a/kernel/bpf/verifier.c
> > > +++ b/kernel/bpf/verifier.c
> > > @@ -5419,6 +5419,9 @@ static bool is_trusted_reg(const struct bpf_reg_state *reg)
> > >         if (reg->ref_obj_id)
> > >                 return true;
> > >
> > > +       if (reg->type == CONST_PTR_TO_MAP)
> > > +               return true;
> > > +
> >
> > Overall it looks great.
> > Instead of above, how about the following instead:
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 0b9da95331d7..cd08167dc347 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -10775,7 +10775,7 @@ static int check_kfunc_args(struct
> > bpf_verifier_env *env, struct bpf_kfunc_call_
> >                         if (!is_kfunc_trusted_args(meta) && !is_kfunc_rcu(meta))
> >                                 break;
> >
> > -                       if (!is_trusted_reg(reg)) {
> > +                       if (!is_trusted_reg(reg) &&
> > !reg2btf_ids[base_type(reg->type)]) {
> >
> >
> > This way we won't need to list every convertible type in is_trusted_reg.
> >
> > I'm a bit hesitant to put reg2btf_ids[] check directly into is_trusted_reg().
> > Maybe it's ok, but it needs more analysis.
>
> I am not sure I see a difference in adding a check you proposed above and
> adding the reg2btf_ids[] check directly into the is_trusted_reg() function.
> Basically, we say "if type is in reg2btf_ids[], then consider it trusted" in
> both cases. AFAIS, currently the reg2btf_ids[] contains only trusted types,
> however, could it happen that we add a non-trusted type there?
>
> So, I would leave the patch as is (which also makes sense because the
> const-ptr-to-map is a special case), or add the "reg2btf_ids[] check"
> directly into the is_trusted_reg() function.

Fair enough. Let's add reg2btf_ids[] to is_trusted_reg() directly.