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

[KP Singh]

On 24-Feb 21:41, 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@schaufler-ca.com/
> 
> 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.
> I'm proposing to rename BPF_PROG_TYPE_LSM into BPF_PROG_TYPE_OVERRIDE_RETURN.

The BPF_PROG_TYPE_OVERRIDE_RETURN seems to be useful in general as
well and we have the implementation already figured out as a part of
the LSM work. I will split that bit into a separate series.

- KP

> 
> > 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.
> 
> 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.