RE: [RFCv2 4/4] bpf: inhibit speculated out-of-bounds pointers

[Reshetova, Elena]

> On Fri, Jan 05, 2018 at 02:57:50PM +0000, Mark Rutland wrote:
> > Note: this patch is an *example* use of the nospec API. It is understood
> > that this is incomplete, etc.
> >
> > Under speculation, CPUs may mis-predict branches in bounds checks. Thus,
> > memory accesses under a bounds check may be speculated even if the
> > bounds check fails, providing a primitive for building a side channel.
> >
> > The EBPF map code has a number of such bounds-checks accesses in
> > map_lookup_elem implementations. This patch modifies these to use the
> > nospec helpers to inhibit such side channels.
> >
> > The JITted lookup_elem implementations remain potentially vulnerable,
> > and are disabled (with JITted code falling back to the C
> > implementations).
> >
> > Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
> > Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
> > Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > ---
> >  kernel/bpf/arraymap.c | 20 +++++++++++++-------
> >  kernel/bpf/cpumap.c   |  5 ++---
> >  kernel/bpf/devmap.c   |  3 ++-
> >  kernel/bpf/sockmap.c  |  3 ++-
> >  4 files changed, 19 insertions(+), 12 deletions(-)
> 
> Mark, did you see my email with this patch yesterday ?
> https://patchwork.ozlabs.org/patch/855911/
> 
> btw your patch does not fix the variant 1 exploit.
> 
> Also all of the pre-embargo patches from Elena that add lfence
> in the bpf interpreter and x64 JIT also do not fix it.
> 
> The exploit works via bpf_tail_call and not via map_lookup.

Could you please clarify this part? The actual jump
to the out-of-bounds index is indeed made by bpf_tail_call, 
but the "speculation" bypassing step happens when it does map_lookup_elem
on the out-of-bound index.  


Best Regards,
Elena.