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

From: Alexei Starovoitov <[email-masked]>
Date: Mon, 5 Oct 2015 14:12:36 -0700

On 10/5/15 2:00 PM, Kees Cook wrote:
> On Mon, Oct 5, 2015 at 1:48 PM, Alexei Starovoitov<[email-masked]>  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
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.