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

[Pawan Gupta]

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@xxxxxxxxxx>
> ---
>  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.

Thanks,
Pawan