[RFC][PATCH 4/4] security: Enforce limitations on return values from LSMs

[Roberto Sassu]

On Tue, 2022-11-15 at 21:35 -0500, Paul Moore wrote:
> On Tue, Nov 15, 2022 at 12:58 PM Roberto Sassu
> <roberto.sassu at huaweicloud.com> wrote:
> > From: Roberto Sassu <roberto.sassu at huawei.com>
> > 
> > LSMs should not be able to return arbitrary return values, as the callers
> > of the LSM infrastructure might not be ready to handle unexpected values
> > (e.g. positive values that are first converted to a pointer with ERR_PTR,
> > and then evaluated with IS_ERR()).
> > 
> > Modify call_int_hook() to call is_ret_value_allowed(), so that the return
> > value from each LSM for a given hook is checked. If for the interval the
> > return value falls into the corresponding flag is not set, change the
> > return value to the default value, just for the current LSM.
> > 
> > A misbehaving LSM would not have impact on the decision of other LSMs, as
> > the loop terminates whenever the return value is not zero.
> > 
> > Signed-off-by: Roberto Sassu <roberto.sassu at huawei.com>
> > ---
> >  security/security.c | 34 ++++++++++++++++++++++++++++++++++
> >  1 file changed, 34 insertions(+)
> 
> Casey touched on some of this in his reply to patch 0/4, but basically
> I see this as a BPF LSM specific problem and not a generalized LSM
> issue that should be addressed at the LSM layer.  Especially if the
> solution involves incurring additional processing for every LSM hook
> instantiation, regardless if a BPF LSM is present.  Reading your
> overall patchset description I believe that you understand this too.

Yes, I had this concern too. Thanks Paul and Casey for taking the time
to reply.

I liked the fact that the fix is extremely simple, but nevertheless it
should not impact the performance, if there are alternative ways. I
thought maybe we look at non-zero values, since the check is already
there. But it could be that there is an impact for it too (maybe for
audit_rule_match?).

> If you want to somehow instrument the LSM hook definitions (what I
> believe to be the motivation behind patch 3/4) to indicate valid
> return values for use by the BPF verifier, I think we could entertain
> that, or at least discuss it further, but I'm not inclined to support
> any runtime overhead at the LSM layer for a specific LSM.

Ok, yes. Patches 1-3 would help to keep in sync the LSM infrastructure
and eBPF, but it is not strictly needed. I could propose an eBPF-only
alternative to declare sets of functions per interval.

More or less, I developed an eBPF-based alternative also for patch 4.
It is just a proof of concept. Will propose it, to validate the idea.

Thanks

Roberto