[PATCH bpf-next v4 3/8] bpf: lsm: provide attachment points for BPF LSM programs

[Kees Cook]

On Thu, Feb 20, 2020 at 03:49:05PM -0800, Casey Schaufler wrote:
> On 2/20/2020 9:52 AM, KP Singh wrote:
> > From: KP Singh <kpsingh at google.com>
> 
> Sorry about the heavy list pruning - the original set
> blows thunderbird up.

(I've added some people back; I had to dig this thread back out of lkml
since I didn't get a direct copy...)

> > The BPF LSM programs are implemented as fexit trampolines to avoid the
> > overhead of retpolines. These programs cannot be attached to security_*
> > wrappers as there are quite a few security_* functions that do more than
> > just calling the LSM callbacks.
> >
> > This was discussed on the lists in:
> >
> >   https://lore.kernel.org/bpf/20200123152440.28956-1-kpsingh@chromium.org/T/#m068becce588a0cdf01913f368a97aea4c62d8266
> >
> > Adding a NOP callback after all the static LSM callbacks are called has
> > the following benefits:
> >
> > - The BPF programs run at the right stage of the security_* wrappers.
> > - They run after all the static LSM hooks allowed the operation,
> >   therefore cannot allow an action that was already denied.
> 
> I still say that the special call-out to BPF is unnecessary.
> I remain unconvinced by the arguments. You aren't doing anything
> so special that the general mechanism won't work.

If I'm understanding this correctly, there are two issues:

1- BPF needs to be run last due to fexit trampolines (?)

2- BPF hooks don't know what may be attached at any given time, so
   ALL LSM hooks need to be universally hooked. THIS turns out to create
   a measurable performance problem in that the cost of the indirect call
   on the (mostly/usually) empty BPF policy is too high.

"1" can be solved a lot of ways, and doesn't seem to be a debated part
of this series.

"2" is interesting -- it creates a performance problem for EVERYONE that
builds in this kernel feature, regardless of them using it. Excepting
SELinux, "traditional" LSMs tends to be relatively sparse in their hooking:

$ grep '^      struct hlist_head' include/linux/lsm_hooks.h | wc -l
230
$ for i in apparmor loadpin lockdown safesetid selinux smack tomoyo yama ; \
  do echo -n "$i " && (cd $i && git grep LSM_HOOK_INIT | wc -l) ; done
apparmor   68
loadpin     3
lockdown    1
safesetid   2
selinux   202
smack     108
tomoyo     28
yama        4

So, trying to avoid the indirect calls is, as you say, an optimization,
but it might be a needed one due to the other limitations.

To me, some questions present themselves:

a) What, exactly, are the performance characteristics of:
	"before"
	"with indirect calls"
	"with static keys optimization"

b) Would there actually be a global benefit to using the static keys
   optimization for other LSMs? (Especially given that they're already
   sparsely populated and policy likely determines utility -- all the
   LSMs would just turn ON all their static keys or turn off ALL their
   static keys depending on having policy loaded.)

If static keys are justified for KRSI (by "a") then it seems the approach
here should stand. If "b" is also true, then we need an additional
series to apply this optimization for the other LSMs (but that seems
distinctly separate from THIS series).

-- 
Kees Cook