[PATCH v2 bpf-next 1/4] bpf: unprivileged BPF access via /dev/bpf

[Andy Lutomirski]

> On Aug 19, 2019, at 10:27 AM, Alexei Starovoitov <alexei.starovoitov at gmail.com> wrote:
> 
>> On Mon, Aug 19, 2019 at 11:15:11AM +0200, Thomas Gleixner wrote:
>> Alexei,
>> 
>>> On Sat, 17 Aug 2019, Alexei Starovoitov wrote:
>>>> On Fri, Aug 16, 2019 at 10:28:29PM +0200, Thomas Gleixner wrote:
>>>> On Fri, 16 Aug 2019, Alexei Starovoitov wrote:
>>>> While real usecases are helpful to understand a design decision, the design
>>>> needs to be usecase independent.
>>>> 
>>>> The kernel provides mechanisms, not policies. My impression of this whole
>>>> discussion is that it is policy driven. That's the wrong approach.
>>> 
>>> not sure what you mean by 'policy driven'.
>>> Proposed CAP_BPF is a policy?
>> 
>> I was referring to the discussion as a whole.
>> 
>>> Can kernel.unprivileged_bpf_disabled=1 be used now?
>>> Yes, but it will weaken overall system security because things that
>>> use unpriv to load bpf and CAP_NET_ADMIN to attach bpf would need
>>> to move to stronger CAP_SYS_ADMIN.
>>> 
>>> With CAP_BPF both load and attach would happen under CAP_BPF
>>> instead of CAP_SYS_ADMIN.
>> 
>> I'm not arguing against that.
>> 
>>>> So let's look at the mechanisms which we have at hand:
>>>> 
>>>> 1) Capabilities
>>>> 
>>>> 2) SUID and dropping priviledges
>>>> 
>>>> 3) Seccomp and LSM
>>>> 
>>>> Now the real interesting questions are:
>>>> 
>>>> A) What kind of restrictions does BPF allow? Is it a binary on/off or is
>>>>    there a more finegrained control of BPF functionality?
>>>> 
>>>>    TBH, I can't tell.
>>>> 
>>>> B) Depending on the answer to #A what is the control possibility for
>>>>    #1/#2/#3 ?
>>> 
>>> Can any of the mechanisms 1/2/3 address the concern in mds.rst?
>> 
>> Well, that depends. As with any other security policy which is implemented
>> via these mechanisms, the policy can be strict enough to prevent it by not
>> allowing certain operations. The more fine-grained the control is, it
>> allows the administrator who implements the policy to remove the
>> 'dangerous' parts from an untrusted user.
>> 
>> So really question #A is important for this. Is BPF just providing a binary
>> ON/OFF knob or does it allow to disable/enable certain aspects of BPF
>> functionality in a more fine grained way? If the latter, then it might be
>> possible to control functionality which might be abused for exploits of
>> some sorts (including MDS) in a way which allows other parts of BBF to be
>> exposed to less priviledged contexts.
> 
> I see. So the kernel.unprivileged_bpf_disabled knob is binary and I think it's
> the right mechanism to expose to users.
> Having N knobs for every map/prog type won't decrease attack surface.
> In the other email Andy's quoting seccomp man page...
> Today seccomp cannot really look into bpf_attr syscall args, but even
> if it could it won't secure the system.
> Examples:
> 1.
> spectre v2 is using bpf in-kernel interpreter in speculative way.
> The mere presence of interpreter as part of kernel .text makes the exploit
> easier to do. That was the reason to do CONFIG_BPF_JIT_ALWAYS_ON.
> For this case even kernel.unprivileged_bpf_disabled=1 was hopeless.
> 
> 2.
> var4 doing store hazard. It doesn't matter which program type is used.
> load/store instructions are the same across program types.
> 
> 3.
> prog_array was used as part of var1. I guess it was simply more
> convenient for Jann to do it this way :) All other map types
> have the same out-of-bounds speculation issue.
> 
> In general side channels are cpu bugs that are exploited via sequences
> of cpu instructions. In that sense bpf infra provides these instructions.
> So all program types and all maps have the same level of 'side channel risk'.
> 
>>> I believe Andy wants to expand the attack surface when
>>> kernel.unprivileged_bpf_disabled=0
>>> Before that happens I'd like the community to work on addressing the text above.
>> 
>> Well, that text above can be removed when the BPF wizards are entirely sure
>> that BPF cannot be abused to exploit stuff. 
> 
> Myself and Daniel looked at it in detail. I think we understood
> MDS mechanism well enough. Right now we're fairly confident that
> combination of existing mechanisms we did for var4 and
> verifier speculative analysis protect us from MDS.
> The thing is that every new cpu bug is looked at through the bpf lenses.
> Can it be exploited through bpf? Complexity of side channels
> is growing. Can the most recent swapgs be exploited ?
> What if we kprobe+bpf somewhere ?
> I don't think there is an issue, but we will never be 'entirely sure'.
> Even if myself and Daniel are sure the concern will stay.
> Unprivileged bpf as a whole is the concern due to side channels.
> The number of them are not yet disclosed. Who is going to analyze them?
> imo the only answer to that is kernel.unprivileged_bpf_disabled=1
> which together with CONFIG_BPF_JIT_ALWAYS_ON is secure enough.
> The other option is to sprinkle every bpf load/store with lfence
> which will make execution so slow that it will be unusable.
> Which is effectively the same as unprivileged_bpf_disabled=1.
> 
> There are other things we can do. Like kasan-style shadow memory
> for bpf execution. Auto re-JITing the code after it's running.
> We can do lfences everywhere for some time then re-JIT
> when kasan-ed shadow memory shows only clean memory accesses.
> The beauty of BPF that it is analyze-able and JIT-able instruction set.
> The verifier speculative analysis is an example that the kernel
> can analyze the speculative execution path that cpu will
> take before the code starts executing.
> Unprivileged bpf can made absolutely secure. It can be
> made more secure than the rest of the kernel.
> But today we should just go with unprivileged_bpf_disabled=1

I’m still okay with this.

> and CAP_BPF.
> 

I think this needs more design work.  I’m halfway through writing up an actual proposal. I’ll send it soon.