Re: [PATCH bpf-next v4 3/7] bpf: Introduce BPF_MODIFY_RETURN

On 05-Mär 14:43, Stephen Smalley wrote:

On Thu, Mar 5, 2020 at 10:54 AM KP Singh [off-list ref] wrote:

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On 05-Mar 08:51, Stephen Smalley wrote:

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IIUC you've switched from a model where the BPF program would be
invoked after the original function logic
and the BPF program is skipped if the original function logic returns
non-zero to a model where the BPF program is invoked first and
the original function logic is skipped if the BPF program returns
non-zero.  I'm not keen on that for userspace-loaded code attached

We do want to continue the KRSI series and the effort to implement a
proper BPF LSM. In the meantime, the tracing + error injection
solution helps us to:

  * Provide better debug capabilities.
  * And parallelize the effort to come up with the right helpers
    for our LSM work and work on sleepable BPF which is also essential
    for some of the helpers.

As you noted, in the KRSI v4 series, we mentioned that we would like
to have the user-space loaded BPF programs be unable to override the
decision made by the in-kernel logic/LSMs, but this got shot down:

   https://lore.kernel.org/bpf/00c216e1-bcfd-b7b1-5444-2a2dfa69190b@schaufler-ca.com (local)

I would like to continue this discussion when we post the v5 series
for KRSI as to what the correct precedence order should be for the
BPF_PROG_TYPE_LSM and would appreciate if you also bring it up there.

That's fine but I guess I don't see why you or anyone else would
bother with introducing a BPF_PROG_TYPE_LSM
if BPF_PROG_MODIFY_RETURN is accepted and is allowed to attach to the
LSM hooks.  What's the benefit to you
if you can achieve your goals directly with MODIFY_RETURN?

There is still value in being a proper LSM, as I had mentioned in KRSI
v3 that not all security_* wrappers simply call the attached hooks and
return their exit code.

It's also okay, taking into consideration Casey's objections and Kees'
suggestion, to be properly registered with the LSM framework (even if
it is with LSM_ORDER_LAST) and work towards improving some of the
performance bottle-necks in the framework. It would be a positive
outcome for all LSMs.

BPF_MODIFY_RETURN is just a first step which lays the foundation for
BPF_PROG_TYPE_LSM and facilitates us to build BPF infrastructure for
it.

- KP

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to LSM hooks; it means that userspace BPF programs can run even if
SELinux would have denied access and SELinux hooks get
skipped entirely if the BPF program returns an error.  I think Casey
may have wrongly pointed you in this direction on the grounds
it can already happen with the base DAC checking logic.  But that's

What we can do for this tracing/modify_ret series, is to remove
the special casing for "security_" functions in the BPF code and add
ALLOW_ERROR_INJECTION calls to the security hooks. This way, if
someone needs to disable the BPF programs being able to modify
security hooks, they can disable error injection. If that's okay, we
can send a patch.

Realistically distros tend to enable lots of developer-friendly
options including error injection, and most users don't build their
own kernels
and distros won't support them when they do. So telling users they can
just rebuild their kernel without error injection if they care about
BPF programs being able to modify security hooks isn't really viable.
The security modules need a way to veto it based on their policies.
That's why I suggested a security hook here.