Re: [PATCH bpf-next v4 0/8] MAC and Audit policy using eBPF (KRSI)

[Kees Cook]

On Fri, Feb 21, 2020 at 02:31:18PM -0800, Casey Schaufler wrote:
> On 2/21/2020 11:41 AM, KP Singh wrote:
> > On 21-Feb 11:19, Casey Schaufler wrote:
> >> On 2/20/2020 9:52 AM, KP Singh wrote:
> >>> From: KP Singh <kpsingh@xxxxxxxxxx>
> >>> # v3 -> v4
> >>>
> >>>   https://lkml.org/lkml/2020/1/23/515
> >>>
> >>> * Moved away from allocating a separate security_hook_heads and adding a
> >>>   new special case for arch_prepare_bpf_trampoline to using BPF fexit
> >>>   trampolines called from the right place in the LSM hook and toggled by
> >>>   static keys based on the discussion in:
> >>>
> >>>     https://lore.kernel.org/bpf/CAG48ez25mW+_oCxgCtbiGMX07g_ph79UOJa07h=o_6B6+Q-u5g@xxxxxxxxxxxxxx/
> >>>
> >>> * Since the code does not deal with security_hook_heads anymore, it goes
> >>>   from "being a BPF LSM" to "BPF program attachment to LSM hooks".
> >> I've finally been able to review the entire patch set.
> >> I can't imagine how it can make sense to add this much
> >> complexity to the LSM infrastructure in support of this
> >> feature. There is macro magic going on that is going to
> >> break, and soon. You are introducing dependencies on BPF
> >> into the infrastructure, and that's unnecessary and most
> >> likely harmful.
> > We will be happy to document each of the macros in detail. Do note a
> > few things here:
> >
> > * There is really nothing magical about them though,
> 
> 
> +#define LSM_HOOK_void(NAME, ...) \
> +	noinline void bpf_lsm_##NAME(__VA_ARGS__) {}
> +
> +#include <linux/lsm_hook_names.h>
> +#undef LSM_HOOK
> 
> I haven't seen anything this ... novel ... in a very long time.
> I see why you want to do this, but you're tying the two sets
> of code together unnaturally. When (not if) the two sets diverge
> you're going to be introducing another clever way to deal with
> the special case.

I really like this approach: it actually _simplifies_ the LSM piece in
that there is no need to keep the union and the hook lists in sync any
more: they're defined once now. (There were already 2 lists, and this
collapses the list into 1 place for all 3 users.) It's very visible in
the diffstat too (~300 lines removed):

 include/linux/lsm_hook_names.h | 353 +++++++++++++++++++
 include/linux/lsm_hooks.h      | 622 +--------------------------------
 2 files changed, 359 insertions(+), 616 deletions(-)

Also, there is no need to worry about divergence: the BPF will always
track the exposed LSM. Backward compat is (AIUI) explicitly a
non-feature.

I don't see why anything here is "harmful"?

-- 
Kees Cook