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Subject: Re: [PATCH v4 01/12] Documentation: Document arm64 kpti control
To:     Andre Przywara <andre.przywara@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>
From:   Jeremy Linton <jeremy.linton@arm.com>
Message-ID: <69fceac6-7e83-7b38-44fb-2c02245f51df@arm.com>
Date:   Wed, 6 Feb 2019 13:24:17 -0600
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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.

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)
>>   
>