Re: [PATCH bpf-next v1 00/13] MAC and Audit policy using eBPF (KRSI)

[Alexei Starovoitov]

On Tue, Jan 14, 2020 at 12:42:22PM -0500, Stephen Smalley wrote:
> On 1/14/20 11:54 AM, Stephen Smalley wrote:
> > On 1/10/20 12:53 PM, Alexei Starovoitov wrote:
> > > On Fri, Jan 10, 2020 at 04:27:58PM +0100, KP Singh wrote:
> > > > On 09-Jan 14:47, Stephen Smalley wrote:
> > > > > On 1/9/20 2:43 PM, KP Singh wrote:
> > > > > > On 10-Jan 06:07, James Morris wrote:
> > > > > > > On Thu, 9 Jan 2020, Stephen Smalley wrote:
> > > > > > > 
> > > > > > > > On 1/9/20 1:11 PM, James Morris wrote:
> > > > > > > > > On Wed, 8 Jan 2020, Stephen Smalley wrote:
> > > > > > > > > 
> > > > > > > > > > The cover letter subject line and the
> > > > > > > > > > Kconfig help text refer to it as a
> > > > > > > > > > BPF-based "MAC and Audit policy".  It
> > > > > > > > > > has an enforce config option that
> > > > > > > > > > enables the bpf programs to deny access,
> > > > > > > > > > providing access control. IIRC,
> > > > > > > > > > in
> > > > > > > > > > the earlier discussion threads, the BPF
> > > > > > > > > > maintainers suggested that Smack
> > > > > > > > > > and
> > > > > > > > > > other LSMs could be entirely
> > > > > > > > > > re-implemented via it in the future, and
> > > > > > > > > > that
> > > > > > > > > > such an implementation would be more optimal.
> > > > > > > > > 
> > > > > > > > > In this case, the eBPF code is similar to a
> > > > > > > > > kernel module, rather than a
> > > > > > > > > loadable policy file.  It's a loadable
> > > > > > > > > mechanism, rather than a policy, in
> > > > > > > > > my view.
> > > > > > > > 
> > > > > > > > I thought you frowned on dynamically loadable
> > > > > > > > LSMs for both security and
> > > > > > > > correctness reasons?
> > > > > > 
> > > > > > Based on the feedback from the lists we've updated the design for v2.
> > > > > > 
> > > > > > In v2, LSM hook callbacks are allocated dynamically using BPF
> > > > > > trampolines, appended to a separate security_hook_heads and run
> > > > > > only after the statically allocated hooks.
> > > > > > 
> > > > > > The security_hook_heads for all the other LSMs (SELinux, AppArmor etc)
> > > > > > still remains __lsm_ro_after_init and cannot be modified. We are still
> > > > > > working on v2 (not ready for review yet) but the general idea can be
> > > > > > seen here:
> > > > > > 
> > > > > >      https://github.com/sinkap/linux-krsi/blob/patch/v1/trampoline_prototype/security/bpf/lsm.c
> > > > > > 
> > > > > > 
> > > > > > > 
> > > > > > > Evaluating the security impact of this is the next
> > > > > > > step. My understanding
> > > > > > > is that eBPF via BTF is constrained to read only access to hook
> > > > > > > parameters, and that its behavior would be entirely restrictive.
> > > > > > > 
> > > > > > > I'd like to understand the security impact more
> > > > > > > fully, though.  Can the
> > > > > > > eBPF code make arbitrary writes to the kernel, or
> > > > > > > read anything other than
> > > > > > > the correctly bounded LSM hook parameters?
> > > > > > > 
> > > > > > 
> > > > > > As mentioned, the BPF verifier does not allow writes to BTF types.
> > > > > > 
> > > > > > > > And a traditional security module would necessarily fall
> > > > > > > > under GPL; is the eBPF code required to be
> > > > > > > > likewise?  If not, KRSI is a
> > > > > > > > gateway for proprietary LSMs...
> > > > > > > 
> > > > > > > Right, we do not want this to be a GPL bypass.
> > > > > > 
> > > > > > This is not intended to be a GPL bypass and the BPF verifier checks
> > > > > > for license compatibility of the loaded program with GPL.
> > > > > 
> > > > > IIUC, it checks that the program is GPL compatible if it
> > > > > uses a function
> > > > > marked GPL-only.  But what specifically is marked GPL-only
> > > > > that is required
> > > > > for eBPF programs using KRSI?
> > > > 
> > > > Good point! If no-one objects, I can add it to the BPF_PROG_TYPE_LSM
> > > > specific verification for the v2 of the patch-set which would require
> > > > all BPF-LSM programs to be GPL.
> > > 
> > > I don't think it's a good idea to enforce license on the program.
> > > The kernel doesn't do it for modules.
> > > For years all of BPF tracing progs were GPL because they have to use
> > > GPL-ed helpers to do anything meaningful.
> > > So for KRSI just make sure that all helpers are GPL-ed as well.
> > 
> > IIUC, the example eBPF code included in this patch series showed a
> > program that used a GPL-only helper for the purpose of reporting event
> > output to userspace. But it could have just as easily omitted the use of
> > that helper and still implemented its own arbitrary access control model
> > on the LSM hooks to which it attached.  It seems like the question is
> > whether the kernel developers are ok with exposing the entire LSM hook
> > interface and all the associated data structures to non-GPLd code,
> > irrespective of what helpers it may or may not use.
> 
> Also, to be clear, while kernel modules aren't necessarily GPL, prior to
> this patch series, all Linux security modules were necessarily GPLd in order
> to use the LSM interface. 

Because they use securityfs_create_file() GPL-ed api, right?
but not because module license is enforced.

> So allowing non-GPL eBPF-based LSMs would be a
> change.

I don't see it this way. seccomp progs technically unlicensed. Yet they can
disallow any syscall. Primitive KRSI progs like
int bpf-prog(void*) { return REJECT; }
would be able to do selectively disable a syscall with an overhead acceptable
in production systems (unlike seccomp). I want this use case to be available to
people. It's a bait, because to do real progs people would need to GPL them.
Key helpers bpf_perf_event_output, bpf_ktime_get_ns, bpf_trace_printk are all
GPL-ed. It may look that most networking helpers are not-GPL, but real life is
different. To debug programs bpf_trace_printk() is necessary. To have
communication with user space bpf_perf_event_output() is necssary. To measure
anything or implement timestamps bpf_ktime_get_ns() is necessary. So today all
meaninful bpf programs are GPL. Those that are not GPL probably exist, but
they're toy programs. Hence I have zero concerns about GPL bypass coming from
tracing, networking, and, in the future, KRSI progs too.