[PATCH RESEND bpf-next 3/4] security: Replace indirect LSM hook calls with static calls

[Song Liu]

On Mon, Feb 6, 2023 at 8:29 AM Casey Schaufler <casey at schaufler-ca.com> wrote:
>
> On 2/6/2023 5:04 AM, KP Singh wrote:
> > On Fri, Jan 20, 2023 at 5:36 AM Kees Cook <keescook at chromium.org> wrote:
> >> On Fri, Jan 20, 2023 at 01:08:17AM +0100, KP Singh wrote:
> >>> The indirect calls are not really needed as one knows the addresses of
> > [...]
> >
> >>> +/*
> >>> + * Define static calls and static keys for each LSM hook.
> >>> + */
> >>> +
> >>> +#define DEFINE_LSM_STATIC_CALL(NUM, NAME, RET, ...)                  \
> >>> +     DEFINE_STATIC_CALL_NULL(LSM_STATIC_CALL(NAME, NUM),             \
> >>> +                             *((RET(*)(__VA_ARGS__))NULL));          \
> >>> +     DEFINE_STATIC_KEY_FALSE(SECURITY_HOOK_ENABLED_KEY(NAME, NUM));
> >> Hm, another place where we would benefit from having separated logic for
> >> "is it built?" and "is it enabled by default?" and we could use
> >> DEFINE_STATIC_KEY_MAYBE(). But, since we don't, I think we need to use
> >> DEFINE_STATIC_KEY_TRUE() here or else won't all the calls be
> >> out-of-line? (i.e. the default compiled state will be NOPs?) If we're
> >> trying to optimize for having LSMs, I think we should default to inline
> >> calls. (The machine code in the commit log seems to indicate that they
> >> are out of line -- it uses jumps.)
> >>
> > I should have added it in the commit description, actually we are
> > optimizing for "hot paths are less likely to have LSM hooks enabled"
> > (eg. socket_sendmsg).
>
> How did you come to that conclusion? Where is there a correlation between
> "hot path" and "less likely to be enabled"?

I could echo KP's reasoning here. AFAICT, the correlation is that LSMs on
hot path will give more performance overhead. In our use cases (Meta),
we are very careful with "small" performance hits. 0.25% is significant
overhead; 1% overhead will not fly without very good reasons (Do we
have to do this? Are there any other alternatives?). If it is possible to
achieve similar security on a different hook, we will not enable the hook on
the hot path. For example, we may not enable socket_sendmsg, but try
to disallow opening such sockets instead.

>
> >  But I do see that there are LSMs that have these
> > enabled. Maybe we can put this behind a config option, possibly
> > depending on CONFIG_EXPERT?
>
> Help me, as the maintainer of one of those LSMs, understand why that would
> be a good idea.

IIUC, this is also from performance concerns. We would like to manage
the complexity at compile time for performance benefits.

Thanks,
Song