Re: [PATCH net-next 1/2] bpf: enable non-root eBPF programs

[Kees Cook]

On Mon, Oct 5, 2015 at 2:12 PM, Alexei Starovoitov <ast@xxxxxxxxxxxx> wrote:
> On 10/5/15 2:00 PM, Kees Cook wrote:
>>
>> On Mon, Oct 5, 2015 at 1:48 PM, Alexei Starovoitov<ast@xxxxxxxxxxxx>
>> wrote:
>>>
>>> >In order to let unprivileged users load and execute eBPF programs
>>> >teach verifier to prevent pointer leaks.
>>> >Verifier will prevent
>>> >- any arithmetic on pointers
>>> >   (except R10+Imm which is used to compute stack addresses)
>>> >- comparison of pointers
>>> >- passing pointers to helper functions
>>> >- indirectly passing pointers in stack to helper functions
>>> >- returning pointer from bpf program
>>> >- storing pointers into ctx or maps
>>
>> Does the arithmetic restriction include using a pointer as an index to
>> a maps-based tail call? I'm still worried about pointer-based
>> side-effects.
>
>
> the array maps that hold FDs (BPF_MAP_TYPE_PROG_ARRAY and
> BPF_MAP_TYPE_PERF_EVENT_ARRAY) don't have lookup/update accessors
> from the program side, so programs cannot see or manipulate
> those pointers.
> For the former only bpf_tail_call() is allowed that takes integer
> index and jumps to it. And the latter map accessed with

Okay, so I can't take a pointer, put it on the stack, take it back any
part of it as an integer and use it for a tail call?

Sounds like this is shaping up nicely! Thanks for adding all these checks.

-Kees

> bpf_perf_event_read() that also takes index only (this helper
> is not available to socket filters anyway).
> Also bpf_tail_call() can only jump to the program of the same type.
> So I'm quite certain it's safe.
>
> Yes, please ask questions and try to poke holes. Now it is time.
>



-- 
Kees Cook
Chrome OS Security
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