netdev - Re: [PATCH bpf] bpf: prevent out-of-bounds speculation

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Message-ID: <20180105175354.mmywt4litafjccix@lakrids.cambridge.arm.com>
Date:   Fri, 5 Jan 2018 17:53:55 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Alexei Starovoitov <ast@...com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jann Horn <jannh@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Alan Cox <alan@...ux.intel.com>, netdev@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH bpf] bpf: prevent out-of-bounds speculation

Hi Alexei,

On Thu, Jan 04, 2018 at 08:28:11PM -0800, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@...nel.org>
> 
> 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.
> 
> To avoid leaking kernel data round up array-based maps and mask the index
> after bounds check, so speculated load with out of bounds index will load
> either valid value from the array or zero from the padded area.

This is on my radar, and I'll try to review / test this next week.

Thanks,
Mark.