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SubjectRe: [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@redhat.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

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