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Re: [PATCH 0/5] x86: CVE-2017-5715, aka Spectre

On Thu, Jan 11, 2018 at 1:18 AM, Jeff Law <> wrote:
> On 01/10/2018 06:14 AM, Jakub Jelinek wrote:
>> On Wed, Jan 10, 2018 at 02:08:48PM +0100, Richard Biener wrote:
>>> On Wed, Jan 10, 2018 at 11:18 AM, Eric Botcazou <> wrote:
>>>>> It's really just a couple of new primitives to emit a jump as a call and
>>>>> one to slam in a new return address.  Given those I think you can do the
>>>>> entire implementation as RTL at expansion time and you've got a damn
>>>>> good shot at protecting most architectures from these kinds of attacks.
>>>> I think that you're a bit optimistic here and that implementing a generic and
>>>> robust framework at the RTL level might require some time.  Given the time and
>>>> (back-)portability constraints, it might be wiser to rush into architecture-
>>>> specific countermeasures than to rush into an half-backed RTL framework.
>>> Let me also say that while it might be nice to commonize code introducing these
>>> mitigations as late as possible to not disrupt optimization is important.  So I
>>> don't see a very strong motivation in trying very hard to make this more
>>> middle-endish, apart from maybe sharing helper functions where possible.
>> That and perhaps a common option to handle the cases that are common to
>> multiple backends (i.e. move some options from -m* namespace to -f*).
>> I'd say the decision about the options and ABI of what we emit is more
>> important than where we actually emit it, we can easily change where we do
>> that over time, but not the options nor the ABI.
> From a UI standpoint, I think the decision has already been made as LLVM
> has already thrown -mretpolines into their tree.   Sigh.

Well, given retpolines are largely kernel relevant right now we don't
need to care here.

> So I think the one thing we ought to seriously consider is at least
> reserving -mretpoline for this style of mitigation of spectre v2.  ALl
> target's don't have to implementation this style mitigation, but if they
> do, they use -mretpoline.

And I'd also like people not to bikeshed too much on this given we're
in the situation
of having exploitable kernels around for which we need a cooperating
compiler.  So
during the time we bikeshed this (rather than reviewing the actual
patches) we have
to "backport" the current non-upstream state anyway to deliver fixed
kernels to our

Yes, if this were a "normal feature" we could continue discussing and
trying to design
sth nice and shiny.  But this isn't a normal feature.

So please - I'd also like to get this into a released compiler (aka
7.3) as soon as possible
(given a RC for 7.3 was scheduled to be early this week).


> Jeff

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