linux-kernel - Re: [PATCH bpf-next 4/6] bpf: override the meaning of ARG_PTR_TO_MAP

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Date:   Thu, 20 Aug 2020 13:33:57 +0100
From:   Lorenz Bauer <lmb@...udflare.com>
To:     Yonghong Song <yhs@...com>
Cc:     Jakub Sitnicki <jakub@...udflare.com>,
        John Fastabend <john.fastabend@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        kernel-team <kernel-team@...udflare.com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next 4/6] bpf: override the meaning of
 ARG_PTR_TO_MAP_VALUE for sockmap and sockhash

On Wed, 19 Aug 2020 at 21:13, Yonghong Song <yhs@...com> wrote:
>
>
>
> On 8/19/20 2:24 AM, Lorenz Bauer wrote:
> > The verifier assumes that map values are simple blobs of memory, and
> > therefore treats ARG_PTR_TO_MAP_VALUE, etc. as such. However, there are
> > map types where this isn't true. For example, sockmap and sockhash store
> > sockets. In general this isn't a big problem: we can just
> > write helpers that explicitly requests PTR_TO_SOCKET instead of
> > ARG_PTR_TO_MAP_VALUE.
> >
> > The one exception are the standard map helpers like map_update_elem,
> > map_lookup_elem, etc. Here it would be nice we could overload the
> > function prototype for different kinds of maps. Unfortunately, this
> > isn't entirely straight forward:
> > We only know the type of the map once we have resolved meta->map_ptr
> > in check_func_arg. This means we can't swap out the prototype
> > in check_helper_call until we're half way through the function.
> >
> > Instead, modify check_func_arg to treat ARG_PTR_TO_MAP_VALUE* to
> > mean "the native type for the map" instead of "pointer to memory"
> > for sockmap and sockhash. This means we don't have to modify the
> > function prototype at all
> >
> > Signed-off-by: Lorenz Bauer <lmb@...udflare.com>
> > ---
> >   kernel/bpf/verifier.c | 40 ++++++++++++++++++++++++++++++++++++++++
> >   1 file changed, 40 insertions(+)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index b6ccfce3bf4c..47f9b94bb9d4 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -3872,6 +3872,38 @@ static int int_ptr_type_to_size(enum bpf_arg_type type)
> >       return -EINVAL;
> >   }
> >
> > +static int override_map_arg_type(struct bpf_verifier_env *env,
> > +                              const struct bpf_call_arg_meta *meta,
> > +                              enum bpf_arg_type *arg_type)
> > +{
> > +     if (!meta->map_ptr) {
> > +             /* kernel subsystem misconfigured verifier */
> > +             verbose(env, "invalid map_ptr to access map->type\n");
> > +             return -EACCES;
> > +     }
> > +
> > +     switch (meta->map_ptr->map_type) {
> > +     case BPF_MAP_TYPE_SOCKMAP:
> > +     case BPF_MAP_TYPE_SOCKHASH:
> > +             switch (*arg_type) {
> > +             case ARG_PTR_TO_MAP_VALUE:
> > +                     *arg_type = ARG_PTR_TO_SOCKET;
> > +                     break;
> > +             case ARG_PTR_TO_MAP_VALUE_OR_NULL:
> > +                     *arg_type = ARG_PTR_TO_SOCKET_OR_NULL;
> > +                     break;
> > +             default:
> > +                     verbose(env, "invalid arg_type for sockmap/sockhash\n");
> > +                     return -EINVAL;
> > +             }
> > +             break;
> > +
> > +     default:
> > +             break;
> > +     }
> > +     return 0;
> > +}
> > +
> >   static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
> >                         struct bpf_call_arg_meta *meta,
> >                         const struct bpf_func_proto *fn)
> > @@ -3904,6 +3936,14 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
> >               return -EACCES;
> >       }
> >
> > +     if (arg_type == ARG_PTR_TO_MAP_VALUE ||
> > +         arg_type == ARG_PTR_TO_UNINIT_MAP_VALUE ||
> > +         arg_type == ARG_PTR_TO_MAP_VALUE_OR_NULL) {
>
> We probably do not need ARG_PTR_TO_UNINIT_MAP_VALUE here.
>
> Do we need ARG_PTR_TO_MAP_VALUE_OR_NULL? bpf_map_update_elem arg type
> is ARG_PTR_TO_MAP_VALUE.

I did this to be consistent: in a single function definition you
either get the map specific
types or the regular semantics. You don't get to mix and match them.
For the same
reason I included ARG_PTR_TO_UNINIT_MAP_VALUE: the semantics don't make
sense for sockmap, so a function using this doesn't make sense either.

>
> > +             err = override_map_arg_type(env, meta, &arg_type);
> > +             if (err)
> > +                     return err;
> > +     }
> > +
> >       if (arg_type == ARG_PTR_TO_MAP_KEY ||
> >           arg_type == ARG_PTR_TO_MAP_VALUE ||
> >           arg_type == ARG_PTR_TO_UNINIT_MAP_VALUE ||
> >



-- 
Lorenz Bauer  |  Systems Engineer
6th Floor, County Hall/The Riverside Building, SE1 7PB, UK

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