BPF LSM and fexit [was: [PATCH bpf-next v3 04/10] bpf: lsm: Add mutable hooks list for the BPF LSM]

[Casey Schaufler]

On 2/11/2020 6:45 PM, Alexei Starovoitov wrote:
> On Wed, Feb 12, 2020 at 01:09:07AM +0100, Daniel Borkmann wrote:
>> Another approach could be to have a special nop inside call_int_hook()
>> macro which would then get patched to avoid these situations. Somewhat
>> similar like static keys where it could be defined anywhere in text but
>> with updating of call_int_hook()'s RC for the verdict.

Tell me again why you can't register your BPF hooks like all the
other security modules do? You keep reintroducing BPF as a special
case, and I don't see why.

> Sounds nice in theory. I couldn't quite picture how that would look
> in the code, so I hacked:
> diff --git a/security/security.c b/security/security.c
> index 565bc9b67276..ce4bc1e5e26c 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -28,6 +28,7 @@
>  #include <linux/string.h>
>  #include <linux/msg.h>
>  #include <net/flow.h>
> +#include <linux/jump_label.h>
>
>  #define MAX_LSM_EVM_XATTR      2
>
> @@ -678,12 +679,26 @@ static void __init lsm_early_task(struct task_struct *task)
>   *     This is a hook that returns a value.
>   */
>
> +#define LSM_HOOK_NAME(FUNC) \
> +       DEFINE_STATIC_KEY_FALSE(bpf_lsm_key_##FUNC);
> +#include <linux/lsm_hook_names.h>
> +#undef LSM_HOOK_NAME
> +__diag_push();
> +__diag_ignore(GCC, 8, "-Wstrict-prototypes", "");
> +#define LSM_HOOK_NAME(FUNC) \
> +       int bpf_lsm_call_##FUNC() {return 0;}
> +#include <linux/lsm_hook_names.h>
> +#undef LSM_HOOK_NAME
> +__diag_pop();
> +
>  #define call_void_hook(FUNC, ...)                              \
>         do {                                                    \
>                 struct security_hook_list *P;                   \
>                                                                 \
>                 hlist_for_each_entry(P, &security_hook_heads.FUNC, list) \
>                         P->hook.FUNC(__VA_ARGS__);              \
> +               if (static_branch_unlikely(&bpf_lsm_key_##FUNC)) \
> +                      (void)bpf_lsm_call_##FUNC(__VA_ARGS__); \
>         } while (0)
>
>  #define call_int_hook(FUNC, IRC, ...) ({                       \
> @@ -696,6 +711,8 @@ static void __init lsm_early_task(struct task_struct *task)
>                         if (RC != 0)                            \
>                                 break;                          \
>                 }                                               \
> +               if (RC == IRC && static_branch_unlikely(&bpf_lsm_key_##FUNC)) \
> +                      RC = bpf_lsm_call_##FUNC(__VA_ARGS__); \
>         } while (0);                                            \
>         RC;                                                     \
>  })
>
> The assembly looks good from correctness and performance points.
> union security_list_options can be split into lsm_hook_names.h too
> to avoid __diag_ignore. Is that what you have in mind?
> I don't see how one can improve call_int_hook() macro without
> full refactoring of linux/lsm_hooks.h
> imo static_key doesn't have to be there in the first set. We can add this
> optimization later.