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

On Mon, Oct 5, 2015 at 2:12 PM, Alexei Starovoitov [off-list ref] wrote:

On 10/5/15 2:00 PM, Kees Cook wrote:

quoted

On Mon, Oct 5, 2015 at 1:48 PM, Alexei Starovoitov[off-list ref]
wrote:

quoted

quoted

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