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

Stephen Smalley
2020-03-24T14:34:10Z

On Mon, Mar 23, 2020 at 12:46 PM 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>
> ---

> diff --git a/kernel/bpf/bpf_lsm.c b/kernel/bpf/bpf_lsm.c
> index 530d137f7a84..2a8131b640b8 100644
> --- a/kernel/bpf/bpf_lsm.c
> +++ b/kernel/bpf/bpf_lsm.c
> @@ -9,6 +9,9 @@
> #include <linux/btf.h>
> #include <linux/lsm_hooks.h>
> #include <linux/bpf_lsm.h>
> +#include <linux/jump_label.h>
> +#include <linux/kallsyms.h>
> +#include <linux/bpf_verifier.h>
>
> /* For every LSM hook that allows attachment of BPF programs, declare a NOP
> * function where a BPF program can be attached as an fexit trampoline.
> @@ -27,6 +30,32 @@ noinline __weak void bpf_lsm_##NAME(__VA_ARGS__) {}
> #include <linux/lsm_hook_names.h>
> #undef LSM_HOOK
>
> +#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;

I had asked before, and will ask again: please provide an explicit LSM
hook for mediating whether one can make changes to the LSM hooks.
Neither CAP_MAC_ADMIN nor CAP_SYS_ADMIN suffices to check this for SELinux.