[PATCH bpf-next] bpf, capabilities: introduce CAP_BPF

[Andy Lutomirski]

> On Aug 28, 2019, at 3:55 PM, Alexei Starovoitov <alexei.starovoitov at gmail.com> wrote:
> 
>> On Tue, Aug 27, 2019 at 11:12:29PM -0700, Andy Lutomirski wrote:
>>>> 
>>>> 
>>>> From the previous discussion, you want to make progress toward solving
>>>> a lot of problems with CAP_BPF.  One of them was making BPF
>>>> firewalling more generally useful. By making CAP_BPF grant the ability
>>>> to read kernel memory, you will make administrators much more nervous
>>>> to grant CAP_BPF.
>>> 
>>> Andy, were your email hacked?
>>> I explained several times that in this proposal
>>> CAP_BPF _and_ CAP_TRACING _both_ are necessary to read kernel memory.
>>> CAP_BPF alone is _not enough_.
>> 
>> You have indeed said this many times.  You've stated it as a matter of
>> fact as though it cannot possibly discussed.  I'm asking you to
>> justify it.
> 
> That's not how I see it.
> I kept stating that both CAP_BPF and CAP_TRACING are necessary to read
> kernel memory whereas you kept distorting my statement by dropping second
> part and then making claims that "CAP_BPF grant the ability to read
> kernel memory, you will make administrators much more nervous".

Mea culpa. CAP_BPF does, however, appear to permit breaking kASLR due to unsafe pointer conversions, and it allows reading and writing everyone’s maps.  I stand by my overall point.

> 
> Just s/CAP_BPF/CAP_BPF and CAP_TRACING/ in this above sentence.
> See that meaning suddenly changes?
> Now administrators would be worried about tasks that have both at once.
> They also would be worried about tasks that have CAP_TRACING alone,
> because that's what allows probe_kernel_read().

This is not all what I meant. Of course granting CAP_BPF+CAP_TRACING allows reading kernel memory. This is not at all a problem.  Here is a problem I see:

CAP_TRACING + CAP_BPF allows modification of other people’s maps and potentially other things that should not be implied by CAP_TRACING alone and that don’t need to be available to tracers. So CAP_TRACING, which is powerful but has somewhat limited scope, isn’t fully useful without CAP_BPF, and giving CAP_TRACING *and* CAP_BPF allows things that teachers shouldn’t be able to do. I think this would make the whole mechanism less useful to Android, for example.

(Also, I’m not sure quite what you mean by “CAP_TRACING ... allows probe_kernel_read()”. probe_kernel_read() is a kernel function that can’t be directly called by userspace. CAP_TRACING allows reading kernel memory in plenty of ways regardless.)

> 
>> It seems like you are specifically trying to add a new switch to turn
>> as much of BPF as possible on and off.  Why?
> 
> Didn't I explain it several times already with multiple examples
> from systemd, daemons, bpftrace ?
> 
> Let's try again.
> Take your laptop with linux distro.
> You're the only user there. I'm assuming you're not sharing it with
> partner and kids. This is my definition of 'single user system'.
> You can sudo on it at any time, but obviously prefer to run as many
> apps as possible without cap_sys_admin.
> Now you found some awesome open source app on the web that monitors
> the health of the kernel and will pop a nice message on a screen if
> something is wrong. Currently this app needs root. You hesitate,
> but the apps is so useful and it has strong upstream code review process
> that you keep running it 24/7.
> This is open source app. New versions come. You upgrade.
> You have enough trust in that app that you keep running it as root.
> But there is always a chance that new version doing accidentaly
> something stupid as 'kill -9 -1'. It's an open source app at the end.
> 
> Now I come with this CAP* proposal to make this app safer.
> I'm not making your system more secure and not making this app
> more secure. I can only make your laptop safer for day to day work
> by limiting the operations this app can do.
> This particular app monitros the kernel via bpf and tracing.
> Hence you can give it CAP_TRACING and CAP_BPF and drop the rest.

This won’t make me much more comfortable, since CAP_BPF lets it do an ever-growing set of nasty things. I’d much rather one or both of two things happen:

1. Give it CAP_TRACING only. It can leak my data, but it’s rather hard for it to crash my laptop, lose data, or cause other shenanigans.

2. Improve it a bit do all the privileged ops are wrapped by capset().

Does this make sense?  I’m a security person on occasion. I find vulnerabilities and exploit them deliberately and I break things by accident on a regular basis. In my considered opinion, CAP_TRACING alone, even extended to cover part of BPF as I’ve described, is decently safe. Getting root with just CAP_TRACING will be decently challenging, especially if I don’t get to read things like sshd’s memory, and improvements to mitigate even that could be added.  I am quite confident that attacks starting with CAP_TRACING will have clear audit signatures if auditing is on.  I am also confident that CAP_BPF *will* allow DoS and likely privilege escalation, and this will only get more likely as BPF gets more widely used. And, if BPF-based auditing ever becomes a thing, writing to the audit daemon’s maps will be a great way to cover one’s tracks.


> I think they have no choice but to do kernel.unprivileged_bpf_disabled=1.
> We, as a kernel community, are forcing the users into it.
> Hence I really do not see a value in any proposal today that expands
> unprivileged bpf usage.

I think you’re overemphasizing bpf’s role in the whole speculation mess. I realize that you’ve spent an insane amount of time on mitigations to stupid issues. I’ve spent a less insane amount of time on mitigating similar issues outside of bpf.  It’s a mess.  At the end of the day, the kernel does its best, and new bugs show up. New CPUs will be less buggy.