[MODERATED] Re: [PATCH v2 2/8] MDSv2 1

[Kanth Ghatraju <Kanth.Ghatraju@oracle.com>]

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> On Dec 10, 2018, at 5:49 PM, speck for Jiri Kosina <speck@linutronix.de> wrote:
> 
> On Mon, 10 Dec 2018, speck for Andi Kleen wrote:
> 
>> From: Andi Kleen <ak@linux.intel.com>
>> Subject:  x86/speculation/mds: Clear CPU buffers on kernel exit
>> for 64bit
> [ ... snip ... ]
>> +.macro EXIT_MDS
>> +	/* Clear CPU buffers that could leak. Instruction must be in memory form. */
>> +	ALTERNATIVE_2 "", __stringify(push $__USER_DS ; verw (% _ASM_SP ) ; add $8, % _ASM_SP ),\
>> +		X86_FEATURE_MB_CLEAR, \
>> +		"call do_clear_cpu", \
>> +		X86_BUG_MDS_CLEAR_CPU
> 
> The software clearing sequence is introduced later in the series, so this
> seems not to be bisectable.
> 
> Thanks,
> 
> --
> Jiri Kosina
> SUSE Labs

I am forwarding some additional comments from Jamie Iles:

- altinstructions are probably not what we want at all for any of these
mitigations.

If you have a patched kernel but old microcode and then do a late load
of the microcode, you have already thrown away the alternatives by that
point and you'll show as being mitigated but in fact have nops all over
the place.  A static branch would solve this.

Jamie


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