linux-kernel - Re: [RFC PATCH 4/4] bpf: inhibit speculated out-of-bounds pointers

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Date:   Thu, 4 Jan 2018 00:45:29 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        Will Deacon <will.deacon@....com>
Subject: Re: [RFC PATCH 4/4] bpf: inhibit speculated out-of-bounds pointers

On Wed, Jan 03, 2018 at 10:38:27PM +0000, Mark Rutland wrote:
> 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).

Since this is now public, let me re-iterate that I don't particularly
like this approach. If you have to kill the JIT, could we please keep
that in the arch JIT implementation?