Re: [PATCH bpf-next 4/7] bpf: Attachment verification for BPF_MODIFY_RETURN

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

On Tue, Mar 3, 2020 at 6:12 AM KP Singh <kpsingh@chromium.org> wrote:
>
> From: KP Singh <kpsingh@google.com>
>
> - Functions that are whitlisted by for error injection i.e.
>   within_error_injection_list.
> - Security hooks, this is expected to be cleaned up after the KRSI
>   patches introduce the LSM_HOOK macro:
>
>     https://lore.kernel.org/bpf/20200220175250.10795-1-kpsingh@chromium.org/

Commit message can use a bit more work for sure. Why (and even what)
of the changes is not really explained well.

>
> - The attachment is currently limited to functions that return an int.
>   This can be extended later other types (e.g. PTR).
>
> Signed-off-by: KP Singh <kpsingh@google.com>
> ---
>  kernel/bpf/btf.c      | 28 ++++++++++++++++++++--------
>  kernel/bpf/verifier.c | 31 +++++++++++++++++++++++++++++++
>  2 files changed, 51 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 30841fb8b3c0..50080add2ab9 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -3710,14 +3710,26 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
>                 nr_args--;
>         }
>
> -       if ((prog->expected_attach_type == BPF_TRACE_FEXIT ||
> -            prog->expected_attach_type == BPF_MODIFY_RETURN) &&
> -           arg == nr_args) {
> -               if (!t)
> -                       /* Default prog with 5 args. 6th arg is retval. */
> -                       return true;
> -               /* function return type */
> -               t = btf_type_by_id(btf, t->type);
> +       if (arg == nr_args) {
> +               if (prog->expected_attach_type == BPF_TRACE_FEXIT) {
> +                       if (!t)
> +                               return true;
> +                       t = btf_type_by_id(btf, t->type);
> +               } else if (prog->expected_attach_type == BPF_MODIFY_RETURN) {
> +                       /* For now the BPF_MODIFY_RETURN can only be attached to
> +                        * functions that return an int.
> +                        */
> +                       if (!t)
> +                               return false;
> +
> +                       t = btf_type_skip_modifiers(btf, t->type, NULL);
> +                       if (!btf_type_is_int(t)) {

Should the size of int be verified here? E.g., if some function
returns u8, is that ok for BPF program to return, say, (1<<30) ?

> +                               bpf_log(log,
> +                                       "ret type %s not allowed for fmod_ret\n",
> +                                       btf_kind_str[BTF_INFO_KIND(t->info)]);
> +                               return false;
> +                       }
> +               }
>         } else if (arg >= nr_args) {
>                 bpf_log(log, "func '%s' doesn't have %d-th argument\n",
>                         tname, arg + 1);
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 2460c8e6b5be..ae32517d4ccd 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -19,6 +19,7 @@
>  #include <linux/sort.h>
>  #include <linux/perf_event.h>
>  #include <linux/ctype.h>
> +#include <linux/error-injection.h>
>
>  #include "disasm.h"
>
> @@ -9800,6 +9801,33 @@ static int check_struct_ops_btf_id(struct bpf_verifier_env *env)
>
>         return 0;
>  }
> +#define SECURITY_PREFIX "security_"
> +
> +static int check_attach_modify_return(struct bpf_verifier_env *env)
> +{
> +       struct bpf_prog *prog = env->prog;
> +       unsigned long addr = (unsigned long) prog->aux->trampoline->func.addr;
> +
> +       if (within_error_injection_list(addr))
> +               return 0;
> +
> +       /* This is expected to be cleaned up in the future with the KRSI effort
> +        * introducing the LSM_HOOK macro for cleaning up lsm_hooks.h.
> +        */
> +       if (!strncmp(SECURITY_PREFIX, prog->aux->attach_func_name,
> +                    sizeof(SECURITY_PREFIX) - 1)) {
> +
> +               if (!capable(CAP_MAC_ADMIN))
> +                       return -EPERM;
> +
> +               return 0;
> +       }
> +
> +       verbose(env, "fmod_ret attach_btf_id %u (%s) is not modifiable\n",
> +               prog->aux->attach_btf_id, prog->aux->attach_func_name);
> +
> +       return -EINVAL;
> +}
>
>  static int check_attach_btf_id(struct bpf_verifier_env *env)
>  {
> @@ -10000,6 +10028,9 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
>                 }
>                 tr->func.addr = (void *)addr;
>                 prog->aux->trampoline = tr;
> +
> +               if (prog->expected_attach_type == BPF_MODIFY_RETURN)
> +                       ret = check_attach_modify_return(env);
>  out:
>                 mutex_unlock(&tr->mutex);
>                 if (ret)
> --
> 2.20.1
>