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

[Alexei Starovoitov]

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

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