Re: [Xen-devel] [PATCH v6 05/10] Clear AC bit in RFLAGS to protect Xen itself by SMAP

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 08/05/2014 02:41, Wu, Feng wrote:
>
>> -----Original Message-----
>> From: Andrew Cooper [mailto:andrew.cooper3@xxxxxxxxxx]
>> Sent: Wednesday, May 07, 2014 7:54 PM
>> To: Jan Beulich
>> Cc: Wu, Feng; ian.campbell@xxxxxxxxxx; Dong, Eddie; Nakajima, Jun; Tian,
>> Kevin; xen-devel@xxxxxxxxxxxxx
>> Subject: Re: [PATCH v6 05/10] Clear AC bit in RFLAGS to protect Xen itself by
>> SMAP
>>
>> On 07/05/14 12:40, Jan Beulich wrote:
>>>>>> On 07.05.14 at 11:44, <andrew.cooper3@xxxxxxxxxx> wrote:
>>>> On 07/05/14 09:19, Feng Wu wrote:
>>>>> @@ -673,6 +675,7 @@ ENTRY(nmi_crash)
>>>>>          ud2
>>>>>
>>>>>  ENTRY(machine_check)
>>>>> +        ASM_CLAC
>>>> This is not needed.  the start of handle_ist_exception has a SAVE_ALL,
>>>> which also covers the nmi entry point.
>>>>
>>>> On the subject of IST exceptions, perhaps the double fault explicitly
>>>> wants a STAC to reduce the likelihood of taking a further fault while
>>>> trying to dump state. ?
>>> I agree. And perhaps along with do_double_fault(), fatal_trap()
>>> should then also get a stac() added?
>>>
>>> Jan
>>>
>> With doubt_fault: being sole caller of do_double_fault(), editing the
>> entry point in entry.S to "ASM_STAC; SAVE_ALL 0" is sufficient to avoid
>> stac() in do_doube_fault() itself.
> I think it's better to add "ASM_STAC" just before " call  do_double_fault".
> Do you think this is okay, Andrew? Thanks!

I am not fussed where exactly the STAC goes in the entry point, but
don't leave a CLAC in the SAVE_ALL.

~Andrew

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