On 2/21/2020 4:22 PM, Kees Cook wrote:

On Fri, Feb 21, 2020 at 02:31:18PM -0800, Casey Schaufler wrote:

quoted

On 2/21/2020 11:41 AM, KP Singh wrote:

quoted

On 21-Feb 11:19, Casey Schaufler wrote:

quoted

On 2/20/2020 9:52 AM, KP Singh wrote:

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From: KP Singh <redacted>
# 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@mail.gmail.com/ (local)

* 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):

Erk. Too many smart people like this. I still don't, but it's possible
that I could learn to.

 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.

As written you're correct, it can't diverge. My concern is about
what happens when someone decides that they want the BPF and hook
to be different. I fear there will be a hideous solution.

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

Injecting large chucks of code via an #include does nothing
for readability. I've seen it fail disastrously many times,
usually after the original author has moved on and entrusted
the code to someone who missed some of the nuance.

I'll drop objection to this bit, but still object to making
BPF special in the infrastructure. It doesn't need to be and
it is exactly the kind of additional complexity we need to
avoid.