[PATCH bpf-next 0/4] Reduce overhead of LSMs with static calls

[Casey Schaufler]

On 2/10/2023 12:03 PM, Paul Moore wrote:
> On Thu, Feb 9, 2023 at 11:56 AM Kees Cook <keescook at chromium.org> wrote:
>> On Fri, Jan 27, 2023 at 03:16:38PM -0500, Paul Moore wrote:
>>> On Thu, Jan 19, 2023 at 6:10 PM KP Singh <kpsingh at kernel.org> wrote:
>>>> # Background
>>>>
>>>> LSM hooks (callbacks) are currently invoked as indirect function calls. These
>>>> callbacks are registered into a linked list at boot time as the order of the
>>>> LSMs can be configured on the kernel command line with the "lsm=" command line
>>>> parameter.
>>> Thanks for sending this KP.  I had hoped to make a proper pass through
>>> this patchset this week but I ended up getting stuck trying to wrap my
>>> head around some network segmentation offload issues and didn't quite
>>> make it here.  Rest assured it is still in my review queue.
>>>
>>> However, I did manage to take a quick look at the patches and one of
>>> the first things that jumped out at me is it *looks* like this
>>> patchset is attempting two things: fix a problem where one LSM could
>>> trample another (especially problematic with the BPF LSM due to its
>>> nature), and reduce the overhead of making LSM calls.  I realize that
>>> in this patchset the fix and the optimization are heavily
>>> intermingled, but I wonder what it would take to develop a standalone
>>> fix using the existing indirect call approach?  I'm guessing that is
>>> going to potentially be a pretty significant patch, but if we could
>>> add a little standardization to the LSM hooks without adding too much
>>> in the way of code complexity or execution overhead I think that might
>>> be a win independent of any changes to how we call the hooks.
>>>
>>> Of course this could be crazy too, but I'm the guy who has to ask
>>> these questions :)
>> Hm, I am expecting this patch series to _not_ change any semantics of
>> the LSM "stack". I would agree: nothing should change in this series, as
>> it should be strictly a mechanical change from "iterate a list of
>> indirect calls" to "make a series of direct calls". Perhaps I missed
>> a logical change?
> I might be missing something too, but I'm thinking of patch 4/4 in
> this series that starts with this sentence:
>
>  "BPF LSM hooks have side-effects (even when a default value is
>   returned), as some hooks end up behaving differently due to
>   the very presence of the hook."

My understanding of the current "agreement" is that we keep BPF
hooks at the end for this very reason. 

> Ignoring the static call changes for a moment, I'm curious what it
> would look like to have a better mechanism for handling things like
> this.  What would it look like if we expanded the individual LSM error
> reporting back to the LSM layer to have a bit more information, e.g.
> "this LSM erred, but it is safe to continue evaluating other LSMs" and
> "this LSM erred, and it was too severe to continue evaluating other
> LSMs"?  Similarly, would we want to expand the hook registration to
> have more info, e.g. "run this hook even when other LSMs have failed"
> and "if other LSMs have failed, do not run this hook"?

I really don't want another LSM to have sway over Smack enforcement.
I would hate to see, for example, an LSM decide that because it has
initialized an inode no other LSM should be allowed to, even in an
error situation. There are really only two options Call all the hooks
every time and either succeed on all or report the most important
error. Or, "bail on fail", and acknowledge that following hooks may
not be called. Really, does "I failed, but it's not that important"
make sense as a return value?

If the return isn't important, make it a void hook.

> I realize that loading a BPF LSM is a privileged operation so we've
> largely mitigated the risk there, but with stacking on it's way to
> being more full featured, and IMA slowly working its way to proper LSM
> status, it seems to me like having a richer, and proper way to handle
> individual LSM failures would be a good thing.  I feel like patch 4/4
> definitely hints at this, but I could be mistaken.

We have bigger issues with BPF. There's nothing to prevent BPF from
implementing a secid_to_secctx() hook and making a system with SELinux
go cattywampus. BPF is stacked as if it isn't a "major" LSM, while
allowing it to do "major" LSM things. One reason we need full stacking
is to address this.

My $0.02. That and $1.98 will get you a beer on Tuesdays, 3-5pm.

>
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