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

Andrii Nakryiko
2020-03-23T20:19:08Z

On Mon, Mar 23, 2020 at 9:45 AM KP Singh <kpsingh@chromium.org> wrote:
>
> From: KP Singh <kpsingh@google.com>
>
> 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@google.com>
> Reviewed-by: Brendan Jackman <jackmanb@google.com>
> Reviewed-by: Florent Revest <revest@google.com>
> ---
> 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>
> …