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

On 10/5/15 2:16 PM, Andy Lutomirski wrote:

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

quoted

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
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.

At some point there will be an unprivileged way to create a map,
though, and we don't want to let pointers get poked into the map.

yes. exactly. With these two patches non-root can create a map
against memlock user limit and have a program store bytes into it
(like data from network packet), but it cannot store pointers into it.
That's covered by test "unpriv: write pointer into map elem value"
I've added new tests for all cases that can 'leak pointer':
  unpriv: return pointer OK
  unpriv: add const to pointer OK
  unpriv: add pointer to pointer OK
  unpriv: neg pointer OK
  unpriv: cmp pointer with const OK
  unpriv: cmp pointer with pointer OK
  unpriv: pass pointer to printk OK
  unpriv: pass pointer to helper function OK
  unpriv: indirectly pass pointer on stack to helper function OK
  unpriv: mangle pointer on stack 1 OK
  unpriv: mangle pointer on stack 2 OK
  unpriv: read pointer from stack in small chunks OK
  unpriv: write pointer into ctx OK
  unpriv: write pointer into map elem value OK
  unpriv: partial copy of pointer OK

the most interesting one is 'indirectly pass pointer'.
It checks the case where user stores a pointer into a stack
and then uses that stack region either as a key for lookup or
as part of format string for printk.