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

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Date:   Thu, 14 Nov 2019 19:48:39 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Waiman Long <longman@...hat.com>
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 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