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Re: [Xen-devel] [PATCH v3 2/7] xen/nospec: Use always_inline to fix code gen for evaluate_nospec


  • To: Norbert Manthey <nmanthey@xxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Mon, 28 Oct 2019 17:05:50 +0000
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  • Cc: Juergen Gross <jgross@xxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Wei Liu <wl@xxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Ian Jackson <ian.jackson@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Mon, 28 Oct 2019 17:05:59 +0000
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  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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On 25/10/2019 22:56, Norbert Manthey wrote:
> On 10/25/19 17:40, Jan Beulich wrote:
>> On 25.10.2019 17:27, Andrew Cooper wrote:
>>> On 25/10/2019 13:34, Jan Beulich wrote:
>>>> On 25.10.2019 14:10, Andrew Cooper wrote:
>>>>> The two choices to unblock 4.13 are this patch, or the previous version
>>>>> which made CONFIG_HARDEN_BRANCH depend on BROKEN, which was even more
>>>>> disliked.
>>>> Option 3 is to have just the config option, for people to turn this
>>>> off if they feel like doing so.
>>> Yes, but no.  A facade of security is worse than no security, and I
>>> don't consider doing that an acceptable solution in this case.
>> But I thought we all agree that this is something that's presumably
>> going to remain incomplete (as in not provably complete) altogether
>> anyway. It's just that without the change here it's far more
>> incomplete then with it.

This is inherently a best-effort approach, but without the
always_inline, it is tantamount to useless.

Only the grant table operations, and __mfn_valid() are correctly
protected with the code in its current form, where as the large changes
(in terms of absolute number of protected paths) comes from the predicates.

>>
>> In any event I think we should (also) have an opinion from the people
>> who had originally contributed this logic. You didn't Cc anyone of
>> them; I've added at least Norbert now.
> Thanks for adding me. I had a quick look into the discussion. Only
> making adding lfence statements around conditionals depending on config
> BROKEN does not help, as it would still need to be always_inline to work
> as expected, correct?

"depends on BROKEN" is a way to unconditionally compile out
functionality, which was one option considered to this problem.

> Hence, in my opinion, this patch does the right
> thing to benefit from the lfences that are placed after evaluation
> conditionals.

This patch is the alternative to compiling everything out.

I'm still holding out hope that we'll find a better compiler based
mitigation in the longer term, because I don't see either of these
options being viable strategies longterm.

~Andrew

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