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

[KP Singh]

On 12-Feb 14:27, Daniel Borkmann wrote:
> 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..

I like this approach as well. Will give it a go and update the
patches. Thanks a lot for your inputs!

- KP

> 
> Thanks,
> Daniel