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

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Date:   Thu, 14 Nov 2019 14:35:17 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Cc:     Waiman Long <longman@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        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, Nov 14, 2019 at 12:12:58PM -0800, Pawan Gupta wrote:
> On Wed, Nov 13, 2019 at 02:33:50PM -0500, Waiman Long 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 mitigated. 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.
> > 
> > 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.
> > 
> > Signed-off-by: Waiman Long <longman@...hat.com>
> > ---
> >  arch/x86/kernel/cpu/bugs.c | 17 +++++++++++++++--
> >  1 file changed, 15 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> > index 4c7b0fa15a19..418d41c1fd0d 100644
> > --- a/arch/x86/kernel/cpu/bugs.c
> > +++ b/arch/x86/kernel/cpu/bugs.c
> > @@ -304,8 +304,12 @@ static void __init taa_select_mitigation(void)
> >  		return;
> >  	}
> >  
> > -	/* 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.
> > +	 */
> > +	if (taa_mitigation == TAA_MITIGATION_OFF &&
> > +	    mds_mitigation == MDS_MITIGATION_OFF)
> >  		goto out;
> >  
> >  	if (boot_cpu_has(X86_FEATURE_MD_CLEAR))
> > @@ -339,6 +343,15 @@ static void __init taa_select_mitigation(void)
> >  	if (taa_nosmt || cpu_mitigations_auto_nosmt())
> >  		cpu_smt_disable(false);
> >  
> > +	/*
> > +	 * Update MDS mitigation, if necessary, as the mds_user_clear is
> > +	 * now enabled for TAA mitigation.
> > +	 */
> > +	if (mds_mitigation == MDS_MITIGATION_OFF &&
> > +	    boot_cpu_has_bug(X86_BUG_MDS)) {
> > +		mds_mitigation = MDS_MITIGATION_FULL;
> > +		mds_select_mitigation();
> 
> This will cause a confusing print in dmesg from previous and this call
> to mds_select_mitigation().
> 
> 	"MDS: Vulnerable"
> 	"MDS: Mitigation: Clear CPU buffers"
> 
> Maybe delay MDS mitigation print till TAA is evaluated.

Since they're so intertwined it might make sense to just combine the two
mitigations into a single function.

-- 
Josh