Re: [PATCH bpf-next v4 3/8] bpf: lsm: provide attachment points for BPF LSM programs

[KP Singh]

On 25-Feb 16:30, Casey Schaufler wrote:
> On 2/24/2020 9:41 PM, Alexei Starovoitov wrote:
> > On Mon, Feb 24, 2020 at 01:41:19PM -0800, Kees Cook wrote:
> >> But the LSM subsystem doesn't want special cases (Casey has worked very
> >> hard to generalize everything there for stacking). It is really hard to
> >> accept adding a new special case when there are still special cases yet
> >> to be worked out even in the LSM code itself[2].
> >> [2] Casey's work to generalize the LSM interfaces continues and it quite
> >> complex:
> >> https://lore.kernel.org/linux-security-module/20200214234203.7086-1-casey@xxxxxxxxxxxxxxxx/
> > I think the key mistake we made is that we classified KRSI as LSM.
> > LSM stacking, lsmblobs that the above set is trying to do are not necessary for KRSI.
> > I don't see anything in LSM infra that KRSI can reuse.
> > The only thing BPF needs is a function to attach to.
> > It can be a nop function or any other.
> > security_*() functions are interesting from that angle only.
> > Hence I propose to reconsider what I was suggesting earlier.
> > No changes to secruity/ directory.
> > Attach to security_*() funcs via bpf trampoline.
> > The key observation vs what I was saying earlier is KRSI and LSM are wrong names.
> > I think "security" is also loaded word that should be avoided.
> 
> No argument there.
> 
> > I'm proposing to rename BPF_PROG_TYPE_LSM into BPF_PROG_TYPE_OVERRIDE_RETURN.
> >
> >> So, unless James is going to take this over Casey's objections, the path
> >> forward I see here is:
> >>
> >> - land a "slow" KRSI (i.e. one that hooks every hook with a stub).
> >> - optimize calling for all LSMs
> > I'm very much surprised how 'slow' KRSI is an option at all.
> > 'slow' KRSI means that CONFIG_SECURITY_KRSI=y adds indirect calls to nop
> > functions for every place in the kernel that calls security_*().
> > This is not an acceptable overhead. Even w/o retpoline
> > this is not something datacenter servers can use.
> 
> In the universe I live in data centers will disable hyper-threading,
> reducing performance substantially, in the face of hypothetical security
> exploits. That's a massively greater performance impact than the handful
> of instructions required to do indirect calls. Not to mention the impact

Indirect calls have worse performance implications than just a few
instructions and are especially not suitable for hotpaths.

There have been multiple efforts to reduce their usage e.g.:

  - https://lwn.net/Articles/774743/
  - https://lwn.net/Articles/773985/

> of the BPF programs that have been included. Have you ever looked at what

  BPF programs are JIT'ed and optimized to native code.

> happens to system performance when polkitd is enabled?

However, let's discuss all this separately when we follow-up with
performance improvements after submitting the initial patch-set.

> 
> 
> >
> > Another option is to do this:
> > diff --git a/include/linux/security.h b/include/linux/security.h
> > index 64b19f050343..7887ce636fb1 100644
> > --- a/include/linux/security.h
> > +++ b/include/linux/security.h
> > @@ -240,7 +240,7 @@ static inline const char *kernel_load_data_id_str(enum kernel_load_data_id id)
> >         return kernel_load_data_str[id];
> >  }
> >
> > -#ifdef CONFIG_SECURITY
> > +#if defined(CONFIG_SECURITY) || defined(CONFIG_BPF_OVERRIDE_RETURN)
> >
> > Single line change to security.h and new file kernel/bpf/override_security.c
> > that will look like:
> > int security_binder_set_context_mgr(struct task_struct *mgr)
> > {
> >         return 0;
> > }
> >
> > int security_binder_transaction(struct task_struct *from,
> >                                 struct task_struct *to)
> > {
> >         return 0;
> > }
> > Essentially it will provide BPF side with a set of nop functions.
> > CONFIG_SECURITY is off. It may seem as a downside that it will force a choice
> > on kernel users. Either they build the kernel with CONFIG_SECURITY and their
> > choice of LSMs or build the kernel with CONFIG_BPF_OVERRIDE_RETURN and use
> > BPF_PROG_TYPE_OVERRIDE_RETURN programs to enforce any kind of policy. I think
> > it's a pro not a con.
> 
> Err, no. All distros use an LSM or two. Unless you can re-implement SELinux

The users mentioned here in this context are (I would assume) the more
performance sensitive users who would, potentially, disable
CONFIG_SECURITY because of the current performance characteristics.

We can also discuss this separately and only if we find that we need
it for the BPF_OVERRIDE_RET type attachment.

- KP

> in BPF (good luck with state transitions) you've built a warp drive without
> ever having mined dilithium crystals.
> 
>