From: Will Deacon <will.deacon@arm.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mark Rutland <mark.rutland@arm.com>,
Alexei Starovoitov <ast@fb.com>,
"David S . Miller" <davem@davemloft.net>,
Daniel Borkmann <daniel@iogearbox.net>,
Jann Horn <jannh@google.com>,
Dan Williams <dan.j.williams@intel.com>,
Peter Zijlstra <peterz@infradead.org>,
Elena Reshetova <elena.reshetova@intel.com>,
Alan Cox <alan@linux.intel.com>,
Network Development <netdev@vger.kernel.org>,
kernel-team <kernel-team@fb.com>
Subject: Re: [PATCH bpf] bpf: prevent out-of-bounds speculation
Date: Tue, 9 Jan 2018 10:21:30 +0000 [thread overview]
Message-ID: <20180109102129.GG4297@arm.com> (raw)
In-Reply-To: <CA+55aFz0tsreoa=5Ud2noFCpng-dizLBhT9WU9asyhpLfjdcYA@mail.gmail.com>
Hi Linus,
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@arm.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
>
> (a) mainly academic masturbation
>
> (b) hasn't even been shown to be a good idea even in _theory_ yet
> (except for the "completely unreal hardware" kind of theory where
> people assume some data oracle)
>
> (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).
>
> and the thing is, we should really not then worry about "... but maybe
> future CPU's will be more aggressive", which is the traditional worry
> in these kinds of cases.
>
> Why? Because quite honestly, any future CPU's that are more aggressive
> about speculating things like this are broken shit that we should call
> out as such, and tell people not to use.
>
> Seriously.
>
> In this particular case, we should be very much aware of future CPU's
> being more _constrained_, because CPU vendors had better start taking
> this thing into account.
>
> So the masking approach is FUNDAMENTALLY SAFER than the "let's try to
> limit control speculation".
>
> If somebody can point to a CPU that actually speculates across an
> address masking operation, I will be very surprised. And unless you
> can point to that, then stop trying to dismiss the masking approach.
Whilst I agree with your comments about future CPUs, this stuff is further
out of academia than you might think. We're definitely erring on the
belt-and-braces side of things at the moment, so let me go check what's
*actually* been built and I suspect we'll be able to make the masking work.
Stay tuned...
Will
next prev parent reply other threads:[~2018-01-09 10:21 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20180105042811.1590965-1-ast@fb.com>
2018-01-05 17:53 ` [PATCH bpf] bpf: prevent out-of-bounds speculation Mark Rutland
2018-01-08 17:05 ` Mark Rutland
2018-01-08 18:49 ` Linus Torvalds
2018-01-09 10:21 ` Will Deacon [this message]
2018-01-10 19:47 ` Will Deacon
2018-01-10 22:29 ` Alexei Starovoitov
2018-02-05 15:38 ` Will Deacon
2018-01-09 15:04 ` Mark Rutland
2018-01-17 14:47 ` Alan Cox
2018-01-08 23:20 ` Alexei Starovoitov
[not found] <20180105042242.1569490-1-ast@kernel.org>
2018-01-17 13:56 ` Alan Cox
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