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

[Waiman Long]

On 11/14/19 3:35 PM, Josh Poimboeuf wrote:
> 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@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.
> Since they're so intertwined it might make sense to just combine the two
> mitigations into a single function.
>
They are intertwined mainly for non-MDS_NO processors with TSX. The
mds_select_mitigation() function is pretty simple. Merging the two
together will make the MDS part harder to read. Also the pr_fmt() macro
has to be different for MDS and TAA.

Cheers,
Longman