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From:   KP Singh <kpsingh@chromium.org>
Date:   Thu, 9 Jan 2020 20:11:48 +0100
To:     James Morris <jmorris@namei.org>
Cc:     Stephen Smalley <sds@tycho.nsa.gov>,
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Subject: Re: [PATCH bpf-next v1 00/13] MAC and Audit policy using eBPF (KRSI)
Message-ID: <20200109191148.GA1894@chromium.org>
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On 10-Jan 05:11, 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.
> 
> This would be similar to the difference between iptables rules and 
> loadable eBPF networking code.  I'd be interested to know how the 
> eBPF networking scenarios are handled wrt kernel ABI.
> 
> 
> > Again, not arguing for or against, but wondering if people fully understand
> > the implications.  If it ends up being useful, people will build access
> > control systems with it, and it directly exposes a lot of kernel internals to
> > userspace.  There was a lot of concern originally about the LSM hook interface
> > becoming a stable ABI and/or about it being misused.  Exposing that interface
> > along with every kernel data structure exposed through it to userspace seems
> > like a major leap.
> 
> Agreed this is a leap, although I'm not sure I'd characterize it as 
> exposure to userspace -- it allows dynamic extension of the LSM API from 
> userland, but the code is executed in the kernel.
> 
> KP: One thing I'd like to understand better is the attack surface 
> introduced by this.  IIUC, the BTF fields are read only, so the eBPF code 
> should not be able to modify any LSM parameters, correct?
> 

That's correct, the verifier does not allow writes to BTF types:

from kernel/bpf/verifier.c:

        case PTR_TO_BTF_ID:
	if (type == BPF_WRITE) {
	        verbose(env, "Writes through BTF pointers are not allowed\n");
		return -EINVAL;
	}

We can also add additional checks on top of those added by the
verifier using the verifier_ops that each BPF program type can define. 

- KP

> 
> >  Even if the mainline kernel doesn't worry about any kind
> > of stable interface guarantees for it, the distros might be forced to provide
> > some kABI guarantees for it to appease ISVs and users...
> 
> How is this handled currently for other eBPF use-cases?
> 
> -- 
> James Morris
> <jmorris@namei.org>
>