Re: [PATCH 0/29] arm meltdown fix backporting review for lts 4.9

[Ard Biesheuvel]

On 13 March 2018 at 10:04, Greg KH <greg@xxxxxxxxx> wrote:
> On Wed, Mar 07, 2018 at 06:24:09PM +0000, Ard Biesheuvel wrote:
>> On 2 March 2018 at 16:54, Greg KH <greg@xxxxxxxxx> wrote:
>> > On Fri, Mar 02, 2018 at 05:14:50PM +0800, Alex Shi wrote:
>> >>
>> >>
>> >> On 03/01/2018 11:24 PM, Greg KH wrote:
>> >> > On Wed, Feb 28, 2018 at 11:56:22AM +0800, Alex Shi wrote:
>> >> >> Hi All,
>> >> >>
>> >> >> This backport patchset fixed the meltdown issue, it's original branch:
>> >> >> https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/log/?h=kpti
>> >> >> A few dependency or fixingpatches are also picked up, if they are necessary
>> >> >>  and no functional changes.
>> >> >>
>> >> >> The patchset also on repository:
>> >> >>    git://git.linaro.org/kernel/linux-linaro-stable.git lts-4.9-spectrevv2
>> >> >>
>> >> >> No bug found yet from kernelci.org and lkft testing.
>> >> >
>> >> > No bugs is good, but does it actually fix the meltdown problem?  What
>> >> > did you test it on?
>> >>
>> >> Oh, I have no A73/A75 cpu, so I can not reproduce meltdown bug.
>> >
>> > Then why should I trust this backport at all?
>> >
>> > Please test on the hardware that is affected, otherwise you do not know
>> > if your patches do anything or not.
>> >
>>
>> I don't think it is feasible to test these backports by confirming
>> that they make the fundamental issue go away. We simply don't have the
>> code to reproduce all the variants, and we have to rely on the
>> information provided by ARM Ltd. regarding which cores are affected
>> and which aren't.
>
> You really don't have the reproducers?  Please work with ARM to resolve
> that, this should not be a non-tested set of patches.  That's really
> worse than no patches at all, as if they were applied, that would
> provide a false-sense of "all is fixed".
>

I know that on x86, the line between architecture and platform is
blurry. That is not the case on ARM, though.

Unlike platform firmware, the OS is built on top of an abstracted
platform which is described by ARM's Architecture Reference Manual. If
ARM Ltd. issues recommendations regarding what firmware PSCI methods
to call when doing a context switch, or which barrier instruction to
issue in certain circumstances, they do so because a certain class of
hardware may require it in some cases. It is really not up to me to go
find some exploit code on GitHub, run it before and after applying the
patch and conclude that the problem is fixed. Instead, what I should
do is confirm that the changes result in the recommended actions to be
taken at the appropriate times.