linux-kernel - Re: [PATCH v2 1/2] x86/speculation: Fix incorrect MDS/TAA mitigation status

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Date:   Fri, 15 Nov 2019 20:35:54 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Boris Petkov <bp@...en8.de>
cc:     Waiman Long <longman@...hat.com>, 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 v2 1/2] x86/speculation: Fix incorrect MDS/TAA mitigation
 status

On Fri, 15 Nov 2019, Boris Petkov wrote:

> On November 15, 2019 5:14:44 PM GMT+01:00, Waiman Long <longman@...hat.com> wrote:
> >For MDS vulnerable processors with TSX support, enabling either MDS or
> >TAA mitigations will enable the use of VERW to flush internal processor
> >buffers at the right code path. IOW, they are either both mitigated
> >or both not. However, if the command line options are inconsistent,
> >the vulnerabilites sysfs files may not report the mitigation status
> >correctly.
> >
> >For example, with only the "mds=off" option:
> >
> >  vulnerabilities/mds:Vulnerable; SMT vulnerable
> >vulnerabilities/tsx_async_abort:Mitigation: Clear CPU buffers; SMT
> >vulnerable
> >
> >The mds vulnerabilities file has wrong status in this case. Similarly,
> >the taa vulnerability file will be wrong with mds mitigation on, but
> >taa off.
> >
> >Change taa_select_mitigation() to sync up the two mitigation status
> >and have them turned off if both "mds=off" and "tsx_async_abort=off"
> >are present.
> >
> >Both hw-vuln/mds.rst and hw-vuln/tsx_async_abort.rst are updated
> >to emphasize the fact that both "mds=off" and "tsx_async_abort=off"
> >have to be specified together for processors that are affected by both
> >TAA and MDS to be effective. As kernel-parameter.txt references both
> >documents above, it is not necessary to update it.
> 
> What about kernel-parameters.txt?

See the last sentence of the paragraph you replied to :)

But serioulsy, yes we should mention the interaction in
kernel-parameters.txt as well. Something like:

	off        - Unconditionally disable MDS mitigation.
+		     On TAA affected machines, mds=off can be prevented
+		     by an active TAA mitigation as both vulnerabilities
+		     are mitigated with the same mechanism.

and the other way round for TAA.

Thanks,

	tglx