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Subject: Re: [PATCH v4 01/12] Documentation: Document arm64 kpti control
To:     Jeremy Linton <jeremy.linton@arm.com>
Cc:     linux-arm-kernel@lists.infradead.org, stefan.wahren@i2se.com,
        Jonathan Corbet <corbet@lwn.net>, mlangsdo@redhat.com,
        linux-doc@vger.kernel.org, suzuki.poulose@arm.com,
        marc.zyngier@arm.com, catalin.marinas@arm.com,
        julien.thierry@arm.com, will.deacon@arm.com,
        linux-kernel@vger.kernel.org, steven.price@arm.com,
        ykaukab@suse.de, dave.martin@arm.com, shankerd@codeaurora.org
References: <20190125180711.1970973-1-jeremy.linton@arm.com>
 <20190125180711.1970973-2-jeremy.linton@arm.com>
 <20190130180247.15ab0577@donnerap.cambridge.arm.com>
 <69fceac6-7e83-7b38-44fb-2c02245f51df@arm.com>
From:   =?UTF-8?Q?Andr=c3=a9_Przywara?= <andre.przywara@arm.com>
Organization: ARM Ltd.
Message-ID: <7bf0faad-1064-69cb-32fe-1782e6c742d2@arm.com>
Date:   Wed, 6 Feb 2019 21:06:30 +0000
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On 06/02/2019 19:24, Jeremy Linton wrote:
> Hi,
> 
> 
> I just realized I replied to this off-list.
> 
> On 01/30/2019 12:02 PM, Andre Przywara wrote:
>> On Fri, 25 Jan 2019 12:07:00 -0600
>> Jeremy Linton <jeremy.linton@arm.com> wrote:
>>
>> Hi,
>>
>>> For a while Arm64 has been capable of force enabling
>>> or disabling the kpti mitigations. Lets make sure the
>>> documentation reflects that.
>>>
>>> Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
>>> Cc: Jonathan Corbet <corbet@lwn.net>
>>> Cc: linux-doc@vger.kernel.org
>>> ---
>>>   Documentation/admin-guide/kernel-parameters.txt | 6 ++++++
>>>   1 file changed, 6 insertions(+)
>>>
>>> diff --git a/Documentation/admin-guide/kernel-parameters.txt
>>> b/Documentation/admin-guide/kernel-parameters.txt index
>>> b799bcf67d7b..9475f02c79da 100644 ---
>>> a/Documentation/admin-guide/kernel-parameters.txt +++
>>> b/Documentation/admin-guide/kernel-parameters.txt @@ -1982,6 +1982,12
>>> @@ Built with CONFIG_DEBUG_KMEMLEAK_DEFAULT_OFF=y,
>>>               the default is off.
>>>   +    kpti=        [ARM64] Control page table isolation of
>>> user
>>> +            and kernel address spaces.
>>> +            Default: enabled on cores which need
>>> mitigation.
>>
>> Would this be a good place to mention that we enable it when
>> CONFIG_RANDOMIZE_BASE is enabled and we have a valid kaslr_offset? I
>> found this somewhat surprising, also it's unrelated to the
>> vulnerability.
> 
> Maybe, but I tend to think since this command line forces it on/off
> regardless of RANDOMIZE_BASE, that a better place to mention that
> RANDOMIZE_BASE forces kpti on is the Kconfig option.

True, kpti= takes precedence, in both ways. Disregard my comment then,
this is indeed not the right place to mention RANDOMIZE_BASE.

Cheers,
Andre.

> 
> BTW: Thanks for reviewing this.
> 
> 
>>
>> Cheers,
>> Andre
>>
>>> +            0: force disabled
>>> +            1: force enabled
>>> +
>>>       kvm.ignore_msrs=[KVM] Ignore guest accesses to unhandled
>>> MSRs. Default is 0 (don't ignore, but inject #GP)
>>>   
>>
>