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

[KP Singh]

Hi Kees,

Thanks for the feedback!

On 21-Feb 20:22, Kees Cook wrote:
> 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"

Good suggestion!

I will do some analysis and come back with the numbers.

> 
> 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.)

As Alexei mentioned, we can use the patches for static calls after
they are merged:

https://lore.kernel.org/lkml/8bc857824f82462a296a8a3c4913a11a7f801e74.1547073843.git.jpoimboe@redhat.com/

to make the framework better (as a separate series) especially given
that we are unsure how they work with BPF.

- KP

> 
> 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