Re: [PATCH bpf-next 1/2] bpf: change uapi for bpf iterator map elements

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Sat, Aug 1, 2020 at 9:22 PM Yonghong Song <yhs@fb.com> wrote:
>
> Commit a5cbe05a6673 ("bpf: Implement bpf iterator for
> map elements") added bpf iterator support for
> map elements. The map element bpf iterator requires
> info to identify a particular map. In the above
> commit, the attr->link_create.target_fd is used
> to carry map_fd and an enum bpf_iter_link_info
> is added to uapi to specify the target_fd actually
> representing a map_fd:
>     enum bpf_iter_link_info {
>         BPF_ITER_LINK_UNSPEC = 0,
>         BPF_ITER_LINK_MAP_FD = 1,
>
>         MAX_BPF_ITER_LINK_INFO,
>     };
>
> This is an extensible approach as we can grow
> enumerator for pid, cgroup_id, etc. and we can
> unionize target_fd for pid, cgroup_id, etc.
> But in the future, there are chances that
> more complex customization may happen, e.g.,
> for tasks, it could be filtered based on
> both cgroup_id and user_id.
>
> This patch changed the uapi to have fields
>         __aligned_u64   iter_info;
>         __u32           iter_info_len;
> for additional iter_info for link_create.
> The iter_info is defined as
>         union bpf_iter_link_info {
>                 struct {
>                         __u32   map_fd;
>                 } map;
>         };
>
> So future extension for additional customization
> will be easier. The bpf_iter_link_info will be
> passed to target callback to validate and generic
> bpf_iter framework does not need to deal it any
> more.
>
> Signed-off-by: Yonghong Song <yhs@fb.com>
> ---
>  include/linux/bpf.h            | 10 ++++---
>  include/uapi/linux/bpf.h       | 15 +++++-----
>  kernel/bpf/bpf_iter.c          | 52 +++++++++++++++-------------------
>  kernel/bpf/map_iter.c          | 37 ++++++++++++++++++------
>  kernel/bpf/syscall.c           |  2 +-
>  net/core/bpf_sk_storage.c      | 37 ++++++++++++++++++------
>  tools/include/uapi/linux/bpf.h | 15 +++++-----
>  7 files changed, 104 insertions(+), 64 deletions(-)
>

[...]

>  int bpf_iter_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
>  {
> +       union bpf_iter_link_info __user *ulinfo;
>         struct bpf_link_primer link_primer;
>         struct bpf_iter_target_info *tinfo;
> -       struct bpf_iter_aux_info aux = {};
> +       union bpf_iter_link_info linfo;
>         struct bpf_iter_link *link;
> -       u32 prog_btf_id, target_fd;
> +       u32 prog_btf_id, linfo_len;
>         bool existed = false;
> -       struct bpf_map *map;
>         int err;
>
> +       memset(&linfo, 0, sizeof(union bpf_iter_link_info));
> +
> +       ulinfo = u64_to_user_ptr(attr->link_create.iter_info);
> +       linfo_len = attr->link_create.iter_info_len;
> +       if (ulinfo && linfo_len) {

We probably want to be more strict here: if either pointer or len is
non-zero, both should be present and valid. Otherwise we can have
garbage in iter_info, as long as iter_info_len is zero.

> +               err = bpf_check_uarg_tail_zero(ulinfo, sizeof(linfo),
> +                                              linfo_len);
> +               if (err)
> +                       return err;
> +               linfo_len = min_t(u32, linfo_len, sizeof(linfo));
> +               if (copy_from_user(&linfo, ulinfo, linfo_len))
> +                       return -EFAULT;
> +       }
> +
>         prog_btf_id = prog->aux->attach_btf_id;
>         mutex_lock(&targets_mutex);
>         list_for_each_entry(tinfo, &targets, list) {
> @@ -411,13 +425,6 @@ int bpf_iter_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
>         if (!existed)
>                 return -ENOENT;
>
> -       /* Make sure user supplied flags are target expected. */
> -       target_fd = attr->link_create.target_fd;
> -       if (attr->link_create.flags != tinfo->reg_info->req_linfo)
> -               return -EINVAL;
> -       if (!attr->link_create.flags && target_fd)
> -               return -EINVAL;
> -

Please still ensure that no flags are specified.


>         link = kzalloc(sizeof(*link), GFP_USER | __GFP_NOWARN);
>         if (!link)
>                 return -ENOMEM;
> @@ -431,28 +438,15 @@ int bpf_iter_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
>                 return err;
>         }
>

[...]

> -static int bpf_iter_check_map(struct bpf_prog *prog,
> -                             struct bpf_iter_aux_info *aux)
> +static int bpf_iter_attach_map(struct bpf_prog *prog,
> +                              union bpf_iter_link_info *linfo,
> +                              struct bpf_iter_aux_info *aux)
>  {
> -       struct bpf_map *map = aux->map;
> +       struct bpf_map *map;
> +       int err = -EINVAL;
>
> -       if (map->map_type != BPF_MAP_TYPE_SK_STORAGE)
> +       if (!linfo->map.map_fd)
>                 return -EINVAL;

This could be -EBADF?

>
> -       if (prog->aux->max_rdonly_access > map->value_size)
> -               return -EACCES;
> +       map = bpf_map_get_with_uref(linfo->map.map_fd);
> +       if (IS_ERR(map))
> +               return PTR_ERR(map);
> +
> +       if (map->map_type != BPF_MAP_TYPE_SK_STORAGE)
> +               goto put_map;
> +
> +       if (prog->aux->max_rdonly_access > map->value_size) {
> +               err = -EACCES;
> +               goto put_map;
> +       }

[...]