linux-kernel - Re: [PATCH] x86/speculation: Fix incorrect MDS/TAA mitigation status

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Date:   Thu, 14 Nov 2019 14:11:34 -0500
From:   Waiman Long <longman@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Josh Poimboeuf <jpoimboe@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Mark Gross <mgross@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>
Subject: Re: [PATCH] x86/speculation: Fix incorrect MDS/TAA mitigation status

On 11/14/19 1:48 PM, Thomas Gleixner wrote:
> On Thu, 14 Nov 2019, Waiman Long wrote:
>> On 11/14/19 12:45 PM, Borislav Petkov wrote:
>>>> -	/* TAA mitigation is turned off on the cmdline (tsx_async_abort=off) */
>>>> -	if (taa_mitigation == TAA_MITIGATION_OFF)
>>>> +	/*
>>>> +	 * TAA mitigation via VERW is turned off if both
>>>> +	 * tsx_async_abort=off and mds=off are specified.
>>>> +	 */
>>> So this changes the dependency of switches so if anything, it should be
>>> properly documented first in all three:
>>>
>>> Documentation/admin-guide/hw-vuln/tsx_async_abort.rst
>>> Documentation/x86/tsx_async_abort.rst
>>> Documentation/admin-guide/kernel-parameters.txt
>>>
>>> However, before we do that, we need to agree on functionality:
>> I agree that the documentation needs to be updated. I am going to do
>> that once we have a consensus of what is the right thing to do.
>>> Will the mitigations be disabled only with *both* =off supplied on the
>>> command line or should the mitigations be disabled when *any* of the two
>>> =off is supplied?
>> The mitigation is disabled only with BOTH =off supplied or
>> "mitigations=off". This is the current behavior. This patch is just to
>> make sure that vulnerabilities files reflect the actual behavior. Of
>> course, we can change it to disable mitigation with either =off if this
>> is what the consensus turn out to be.
> I think the current behaviour is correct. It's just a coincidence that both
> issues happen to use the same mitigation technology in the exactly same
> places. So if you leave one on then the other gets mitigated as a side
> effect and the sysfs file should reflect that.
>
> Thanks,
>
> 	tglx
>
Good to hear that. I will send a v2 patch with document update.

Cheers,
Longman