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

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Message-ID: <20180109150446.n3urfgzwcz4zenvt@lakrids.cambridge.arm.com>
Date:   Tue, 9 Jan 2018 15:04:46 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Alexei Starovoitov <ast@...com>,
        "David S . Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jann Horn <jannh@...gle.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Alan Cox <alan@...ux.intel.com>,
        Network Development <netdev@...r.kernel.org>,
        kernel-team <kernel-team@...com>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH bpf] bpf: prevent out-of-bounds speculation

On Mon, Jan 08, 2018 at 10:49:01AM -0800, Linus Torvalds wrote:
> On Mon, Jan 8, 2018 at 9:05 AM, Mark Rutland <mark.rutland@....com> wrote:
> >
> > I'm a little worried that in the presence of some CPU/compiler
> > optimisations, the masking may effectively be skipped under speculation.
> > So I'm not sure how robust this is going to be.
> 
> Honestly, I think the masking is a hell of a lot more robust than any
> of the "official" fixes.
> 
> More generic data speculation (as opposed to control speculation) is

>  (c) isn't actually done in any real CPU's today that I'm aware of
> (unless you want to call the return stack data speculation).

Maybe not generally, but the GPZ writeup claims that when a load that
misses in the cache, some CPUs speculate the value (as all-zeroes).

See "Variant 3: Rogue data cache load" in:

  https://googleprojectzero.blogspot.co.uk/2018/01/reading-privileged-memory-with-side.html

If a CPU speculates a load of a mask as all-zeroes, we're fine. If a CPU
can speculate the load of a mask as all-ones, the AND is effectively a
NOP.

I'll wait for Will to find out what's actually been built...

Thanks,
Mark.