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

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Wed, Jan 15, 2020 at 03:09:53PM +0100, Greg Kroah-Hartman wrote:
> On Wed, Jan 15, 2020 at 08:59:08AM -0500, Stephen Smalley wrote:
> > On 1/14/20 9:48 PM, Alexei Starovoitov wrote:
> > > 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.
> > 
> > No, securityfs was a later addition and is not required by all LSMs either.
> > Originally LSMs had to register their hooks via register_security(), which
> > was intentionally EXPORT_SYMBOL_GPL() to avoid exposing the LSM interface to
> > non-GPLd modules because there were significant concerns with doing so when
> > LSM was first merged.  Then in 20510f2f4e2dabb0ff6c13901807627ec9452f98
> > ("security: Convert LSM into a static interface"), the ability for loadable
> > modules to use register_security() at all was removed, limiting its use to
> > built-in modules.  In commit b1d9e6b0646d0e5ee5d9050bd236b6c65d66faef ("LSM:
> > Switch to lists of hooks"), register_security() was replaced by
> > security_add_hooks(), but this was likewise not exported for use by modules
> > and could only be used by built-in code.  The bpf LSM is providing a shim
> > that allows eBPF code to attach to these hooks that would otherwise not be
> > exposed to non-GPLd code, so if the bpf LSM does not require the eBPF
> > programs to also be GPLd, then that is a change from current practice.
> > 
> > > > 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.
> > 
> > You have more confidence than I do about that.  I would anticipate
> > developers of out-of-tree LSMs latching onto this bpf LSM and using it to
> > avoid GPL.  I don't see that any of those helpers are truly needed to
> > implement an access control model.
> 
> Yeah, I'm with Stephen here, this should be explicitly marked for
> GPL-only bpf code to prevent anyone from trying to route around the LSM
> apis we have today.  We have enough problem with companies trying to do
> that as-is, let's not give them any other ways to abuse our license.

Fine. Let's do per prog type check. We can undo it later when this early
concerns prove to be overblown.