Re: [PATCH bpf-next v2 05/10] bpf: lsm: BTF API for LSM hooks

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

On Wed, Jan 15, 2020 at 9:14 AM KP Singh <kpsingh@chromium.org> wrote:
>
> From: KP Singh <kpsingh@google.com>
>
> The BTF API provides information required by the BPF verifier to
> attach eBPF programs to the LSM hooks by using the BTF information of
> two types:
>
> - struct security_hook_heads: This type provides the offset which
>   a new dynamically allocated security hook must be attached to.
> - union security_list_options: This provides the information about the
>   function prototype required by the hook.
>
> When the program is loaded:
>
> - The verifier receives the index of a member in struct
>   security_hook_heads to which a program must be attached as
>   prog->aux->lsm_hook_index. The index is one-based for better
>   verification.
> - bpf_lsm_type_by_index is used to determine the func_proto of
>   the LSM hook and updates prog->aux->attach_func_proto
> - bpf_lsm_head_by_index is used to determine the hlist_head to which
>   the BPF program must be attached.
>
> Signed-off-by: KP Singh <kpsingh@google.com>
> ---
>  include/linux/bpf_lsm.h |  12 +++++
>  security/bpf/Kconfig    |   1 +
>  security/bpf/hooks.c    | 104 ++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 117 insertions(+)
>
> diff --git a/include/linux/bpf_lsm.h b/include/linux/bpf_lsm.h
> index 9883cf25241c..a9b4f7b41c65 100644
> --- a/include/linux/bpf_lsm.h
> +++ b/include/linux/bpf_lsm.h
> @@ -19,6 +19,8 @@ extern struct security_hook_heads bpf_lsm_hook_heads;
>
>  int bpf_lsm_srcu_read_lock(void);
>  void bpf_lsm_srcu_read_unlock(int idx);
> +const struct btf_type *bpf_lsm_type_by_index(struct btf *btf, u32 offset);
> +const struct btf_member *bpf_lsm_head_by_index(struct btf *btf, u32 id);
>
>  #define CALL_BPF_LSM_VOID_HOOKS(FUNC, ...)                     \
>         do {                                                    \
> @@ -65,6 +67,16 @@ static inline int bpf_lsm_srcu_read_lock(void)
>         return 0;
>  }
>  static inline void bpf_lsm_srcu_read_unlock(int idx) {}
> +static inline const struct btf_type *bpf_lsm_type_by_index(
> +       struct btf *btf, u32 index)
> +{
> +       return ERR_PTR(-EOPNOTSUPP);
> +}
> +static inline const struct btf_member *bpf_lsm_head_by_index(
> +       struct btf *btf, u32 id)
> +{
> +       return ERR_PTR(-EOPNOTSUPP);
> +}
>
>  #endif /* CONFIG_SECURITY_BPF */
>
> diff --git a/security/bpf/Kconfig b/security/bpf/Kconfig
> index 595e4ad597ae..9438d899b618 100644
> --- a/security/bpf/Kconfig
> +++ b/security/bpf/Kconfig
> @@ -7,6 +7,7 @@ config SECURITY_BPF
>         depends on SECURITY
>         depends on BPF_SYSCALL
>         depends on SRCU
> +       depends on DEBUG_INFO_BTF
>         help
>           This enables instrumentation of the security hooks with
>           eBPF programs.
> diff --git a/security/bpf/hooks.c b/security/bpf/hooks.c
> index b123d9cb4cd4..82725611693d 100644
> --- a/security/bpf/hooks.c
> +++ b/security/bpf/hooks.c
> @@ -5,6 +5,8 @@
>   */
>
>  #include <linux/bpf_lsm.h>
> +#include <linux/bpf.h>
> +#include <linux/btf.h>
>  #include <linux/srcu.h>
>
>  DEFINE_STATIC_SRCU(security_hook_srcu);
> @@ -18,3 +20,105 @@ void bpf_lsm_srcu_read_unlock(int idx)
>  {
>         return srcu_read_unlock(&security_hook_srcu, idx);
>  }
> +
> +static inline int validate_hlist_head(struct btf *btf, u32 type_id)
> +{
> +       s32 hlist_id;
> +
> +       hlist_id = btf_find_by_name_kind(btf, "hlist_head", BTF_KIND_STRUCT);
> +       if (hlist_id < 0 || hlist_id != type_id)
> +               return -EINVAL;

This feels backwards and expensive. You already have type_id you want
to check. Do a quick look up, check type and other attributes, if you
want. There is no need to do linear search for struct named
"hlist_head".

But in reality, you should trust kernel BTF, you already know that you
found correct "security_hook_heads" struct, so its member has to be
hlist_head, no?

> +
> +       return 0;
> +}
> +
> +/* Find the BTF representation of the security_hook_heads member for a member
> + * with a given index in struct security_hook_heads.
> + */
> +const struct btf_member *bpf_lsm_head_by_index(struct btf *btf, u32 index)
> +{
> +       const struct btf_member *member;
> +       const struct btf_type *t;
> +       u32 off, i;
> +       int ret;
> +
> +       t = btf_type_by_name_kind(btf, "security_hook_heads", BTF_KIND_STRUCT);
> +       if (WARN_ON_ONCE(IS_ERR(t)))
> +               return ERR_CAST(t);
> +
> +       for_each_member(i, t, member) {
> +               /* We've found the id requested and need to check the
> +                * the following:
> +                *
> +                * - Is it at a valid alignment for struct hlist_head?
> +                *
> +                * - Is it a valid hlist_head struct?
> +                */
> +               if (index == i) {

Also not efficient. Check index to be < vlen(t), then member =
btf_type_member(t) + index;


> +                       off = btf_member_bit_offset(t, member);
> +                       if (off % 8)
> +                               /* valid c code cannot generate such btf */
> +                               return ERR_PTR(-EINVAL);
> +                       off /= 8;
> +
> +                       if (off % __alignof__(struct hlist_head))
> +                               return ERR_PTR(-EINVAL);
> +
> +                       ret = validate_hlist_head(btf, member->type);
> +                       if (ret < 0)
> +                               return ERR_PTR(ret);
> +
> +                       return member;

This feels a bit over-cautious to double-check this. If
security_hook_heads definition is controlled by kernel sources, then
we could just trust vmlinux BTF?

> +               }
> +       }
> +
> +       return ERR_PTR(-ENOENT);
> +}
> +
> +/* Given an index of a member in security_hook_heads return the
> + * corresponding type for the LSM hook. The members of the union
> + * security_list_options have the same name as the security_hook_heads which
> + * is ensured by the LSM_HOOK_INIT macro defined in include/linux/lsm_hooks.h
> + */
> +const struct btf_type *bpf_lsm_type_by_index(struct btf *btf, u32 index)
> +{
> +       const struct btf_member *member, *hook_head = NULL;
> +       const struct btf_type *t, *hook_type = NULL;
> +       u32 i;
> +
> +       hook_head = bpf_lsm_head_by_index(btf, index);
> +       if (IS_ERR(hook_head))
> +               return ERR_PTR(PTR_ERR(hook_head));
> +
> +       t = btf_type_by_name_kind(btf, "security_list_options", BTF_KIND_UNION);
> +       if (WARN_ON_ONCE(IS_ERR(t)))
> +               return ERR_CAST(t);

btf_type_by_name_kind() is a linear search (at least right now), so it
might be a good idea to cache found type_id's of security_list_options
and security_hook_heads?

> +
> +       for_each_member(i, t, member) {
> +               if (hook_head->name_off == member->name_off) {
> +                       /* There should be only one member with the same name
> +                        * as the LSM hook. This should never really happen
> +                        * and either indicates malformed BTF or someone trying
> +                        * trick the LSM.
> +                        */
> +                       if (WARN_ON(hook_type))
> +                               return ERR_PTR(-EINVAL);
> +
> +                       hook_type = btf_type_by_id(btf, member->type);
> +                       if (unlikely(!hook_type))
> +                               return ERR_PTR(-EINVAL);
> +
> +                       if (!btf_type_is_ptr(hook_type))
> +                               return ERR_PTR(-EINVAL);
> +               }
> +       }
> +
> +       if (!hook_type)
> +               return ERR_PTR(-ENOENT);
> +
> +       t = btf_type_by_id(btf, hook_type->type);
> +       if (unlikely(!t))
> +               return ERR_PTR(-EINVAL);

why not do this inside the loop when you find correct member and not
continue processing all the fields?

> +
> +       return t;
> +}
> --
> 2.20.1
>