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

[Daniel Borkmann]

On 2/12/20 3:45 AM, 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.
> 
> 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__); \

Nit: the `RC == IRC` test could be moved behind the static_branch_unlikely() so
that it would be bypassed when not enabled.

>          } 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.

Yes, like the above diff looks good, and then we'd dynamically attach the program
at bpf_lsm_call_##FUNC()'s fexit hook for a direct jump, so all the security_blah()
internals could stay as-is which then might also address Jann's concerns wrt
concrete annotation as well as potential locking changes inside security_blah().
Agree that patching out via static key could be optional but since you were talking
about avoiding indirect jumps..

Thanks,
Daniel