BPF LSM and fexit [was: [PATCH bpf-next v3 04/10] bpf: lsm: Add mutable hooks list for the BPF LSM]

[Alexei Starovoitov]

On Tue, Feb 11, 2020 at 07:44:05PM +0100, Jann Horn wrote:
> On Tue, Feb 11, 2020 at 6:58 PM Alexei Starovoitov
> <alexei.starovoitov at gmail.com> wrote:
> > On Tue, Feb 11, 2020 at 01:43:34PM +0100, KP Singh wrote:
> [...]
> > > * When using the semantic provided by fexit, the BPF LSM program will
> > >   always be executed and will be able to override / clobber the
> > >   decision of LSMs which appear before it in the ordered list. This
> > >   semantic is very different from what we currently have (i.e. the BPF
> > >   LSM hook is only called if all the other LSMs allow the action) and
> > >   seems to be bypassing the LSM framework.
> >
> > It that's a concern it's trivial to add 'if (RC == 0)' check to fexit
> > trampoline generator specific to lsm progs.
> [...]
> > Using fexit mechanism and bpf_sk_storage generalization is
> > all that is needed. None of it should touch security/*.
> 
> If I understand your suggestion correctly, that seems like a terrible
> idea to me from the perspective of inspectability and debuggability.
> If at runtime, a function can branch off elsewhere to modify its
> decision, I want to see that in the source code. If someone e.g.
> changes the parameters or the locking rules around a security hook,
> how are they supposed to understand the implications if that happens
> through some magic fexit trampoline that is injected at runtime?

I'm not following the concern. There is error injection facility that is
heavily used with and without bpf. In this case there is really no difference
whether trampoline is used with direct call or indirect callback via function
pointer. Both will jump to bpf prog. The _source code_ of bpf program will
_always_ be available for humans to examine via "bpftool prog dump" since BTF
is required. So from inspectability and debuggability point of view lsm+bpf
stuff is way more visible than any builtin LSM. At any time people will be able
to see what exactly is running on the system. Assuming folks can read C code.