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

[Alexei Starovoitov]

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.

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.