Re: [PATCH bpf-next v5 4/7] bpf: lsm: Implement attach, detach and execution

[Andrii Nakryiko]

On Mon, Mar 23, 2020 at 9:45 AM KP Singh <kpsingh@xxxxxxxxxxxx> wrote:
>
> From: KP Singh <kpsingh@xxxxxxxxxx>
>
> JITed BPF programs are dynamically attached to the LSM hooks
> using BPF trampolines. The trampoline prologue generates code to handle
> conversion of the signature of the hook to the appropriate BPF context.
>
> The allocated trampoline programs are attached to the nop functions
> initialized as LSM hooks.
>
> BPF_PROG_TYPE_LSM programs must have a GPL compatible license and
> and need CAP_SYS_ADMIN (required for loading eBPF programs).
>
> Upon attachment:
>
> * A BPF fexit trampoline is used for LSM hooks with a void return type.
> * A BPF fmod_ret trampoline is used for LSM hooks which return an
>   int. The attached programs can override the return value of the
>   bpf LSM hook to indicate a MAC Policy decision.
>
> Signed-off-by: KP Singh <kpsingh@xxxxxxxxxx>
> Reviewed-by: Brendan Jackman <jackmanb@xxxxxxxxxx>
> Reviewed-by: Florent Revest <revest@xxxxxxxxxx>
> ---
>  include/linux/bpf.h     |  4 ++++
>  include/linux/bpf_lsm.h | 11 +++++++++++
>  kernel/bpf/bpf_lsm.c    | 29 +++++++++++++++++++++++++++++
>  kernel/bpf/btf.c        |  9 ++++++++-
>  kernel/bpf/syscall.c    | 26 ++++++++++++++++++++++----
>  kernel/bpf/trampoline.c | 17 +++++++++++++----
>  kernel/bpf/verifier.c   | 19 +++++++++++++++----
>  7 files changed, 102 insertions(+), 13 deletions(-)
>

[...]

>
> +#define BPF_LSM_SYM_PREFX  "bpf_lsm_"
> +
> +int bpf_lsm_verify_prog(struct bpf_verifier_log *vlog,
> +                       const struct bpf_prog *prog)
> +{
> +       /* Only CAP_MAC_ADMIN users are allowed to make changes to LSM hooks
> +        */
> +       if (!capable(CAP_MAC_ADMIN))
> +               return -EPERM;
> +
> +       if (!prog->gpl_compatible) {
> +               bpf_log(vlog,
> +                       "LSM programs must have a GPL compatible license\n");
> +               return -EINVAL;
> +       }
> +
> +       if (strncmp(BPF_LSM_SYM_PREFX, prog->aux->attach_func_name,
> +                   strlen(BPF_LSM_SYM_PREFX))) {

sizeof(BPF_LSM_SYM_PREFIX) - 1?

> +               bpf_log(vlog, "attach_btf_id %u points to wrong type name %s\n",
> +                       prog->aux->attach_btf_id, prog->aux->attach_func_name);
> +               return -EINVAL;
> +       }
> +
> +       return 0;
> +}
> +

[...]

> @@ -2367,10 +2369,24 @@ static int bpf_tracing_prog_attach(struct bpf_prog *prog)
>         struct file *link_file;
>         int link_fd, err;
>
> -       if (prog->expected_attach_type != BPF_TRACE_FENTRY &&
> -           prog->expected_attach_type != BPF_TRACE_FEXIT &&
> -           prog->expected_attach_type != BPF_MODIFY_RETURN &&
> -           prog->type != BPF_PROG_TYPE_EXT) {
> +       switch (prog->type) {
> +       case BPF_PROG_TYPE_TRACING:
> +               if (prog->expected_attach_type != BPF_TRACE_FENTRY &&
> +                   prog->expected_attach_type != BPF_TRACE_FEXIT &&
> +                   prog->expected_attach_type != BPF_MODIFY_RETURN) {
> +                       err = -EINVAL;
> +                       goto out_put_prog;
> +               }
> +               break;
> +       case BPF_PROG_TYPE_EXT:

It looks like an omission that we don't enforce expected_attach_type
to be 0 here. Should we fix it until it's too late?

> +               break;
> +       case BPF_PROG_TYPE_LSM:
> +               if (prog->expected_attach_type != BPF_LSM_MAC) {
> +                       err = -EINVAL;
> +                       goto out_put_prog;
> +               }
> +               break;
> +       default:
>                 err = -EINVAL;
>                 goto out_put_prog;
>         }
> @@ -2452,12 +2468,14 @@ static int bpf_raw_tracepoint_open(const union bpf_attr *attr)
>         if (prog->type != BPF_PROG_TYPE_RAW_TRACEPOINT &&
>             prog->type != BPF_PROG_TYPE_TRACING &&
>             prog->type != BPF_PROG_TYPE_EXT &&
> +           prog->type != BPF_PROG_TYPE_LSM &&
>             prog->type != BPF_PROG_TYPE_RAW_TRACEPOINT_WRITABLE) {
>                 err = -EINVAL;
>                 goto out_put_prog;
>         }
>
>         if (prog->type == BPF_PROG_TYPE_TRACING ||
> +           prog->type == BPF_PROG_TYPE_LSM ||
>             prog->type == BPF_PROG_TYPE_EXT) {


can you please refactor this into a nicer explicit switch instead of
combination of if/elses?

>                 if (attr->raw_tracepoint.name) {
>                         /* The attach point for this category of programs
> diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
> index f30bca2a4d01..9be85aa4ec5f 100644
> --- a/kernel/bpf/trampoline.c
> +++ b/kernel/bpf/trampoline.c
> @@ -6,6 +6,7 @@
>  #include <linux/ftrace.h>
>  #include <linux/rbtree_latch.h>
>  #include <linux/perf_event.h>
> +#include <linux/btf.h>
>

[...]