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Re: [Xen-devel] [PATCH L1TF v10 2/8] nospec: introduce evaluate_nospec


  • To: Jan Beulich <JBeulich@xxxxxxxx>
  • From: Norbert Manthey <nmanthey@xxxxxxxxx>
  • Date: Thu, 14 Mar 2019 14:21:55 +0100
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  • Cc: Juergen Gross <jgross@xxxxxxxx>, Tim Deegan <tim@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>, Dario Faggioli <dfaggioli@xxxxxxxx>, Martin Pohlack <mpohlack@xxxxxxxxx>, wipawel@xxxxxxxxx, Julien Grall <julien.grall@xxxxxxx>, David Woodhouse <dwmw@xxxxxxxxxxxx>, "Martin Mazein\(amazein\)" <amazein@xxxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Bjoern Doebel <doebel@xxxxxxxxx>
  • Delivery-date: Thu, 14 Mar 2019 13:22:16 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

On 3/14/19 14:19, Jan Beulich wrote:
>>>> On 14.03.19 at 13:50, <nmanthey@xxxxxxxxx> wrote:
>> Since the L1TF vulnerability of Intel CPUs, loading hypervisor data into
>> L1 cache is problematic, because when hyperthreading is used as well, a
>> guest running on the sibling core can leak this potentially secret data.
>>
>> To prevent these speculative accesses, we block speculation after
>> accessing the domain property field by adding lfence instructions. This
>> way, the CPU continues executing and loading data only once the condition
>> is actually evaluated.
>>
>> As this protection is typically used in if statements, the lfence has to
>> come in a compatible way. Therefore, a function that returns true after an
>> lfence instruction is introduced. To protect both branches after a
>> conditional, an lfence instruction has to be added for the two branches.
>> To be able to block speculation after several evaluations, the generic
>> barrier macro block_speculation is also introduced.
>>
>> As the L1TF vulnerability is only present on the x86 architecture, there is
>> no need to add protection for other architectures. Hence, the introduced
>> functions are defined but empty.
>>
>> On the x86 architecture, by default, the lfence instruction is not present
>> either. Only when a L1TF vulnerable platform is detected, the lfence
>> instruction is patched in via alternative patching. Similarly, PV guests
>> are protected wrt L1TF by default, so that the protection is furthermore
>> disabled in case HVM is exclueded via the build configuration.
>>
>> Introducing the lfence instructions catches a lot of potential leaks with
>> a simple unintrusive code change. During performance testing, we did not
>> notice performance effects.
>>
>> This is part of the speculative hardening effort.
>>
>> Signed-off-by: Norbert Manthey <nmanthey@xxxxxxxxx>
>> Acked-by: Julien Grall <julien.grall@xxxxxxx>
> I did give my ack on v9, and I see no indication of changes which
> may have invalidated it.

That is a miss on my side.

Norbert

>
> Jan
>
>



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