Re: [Xen-devel] [PATCH 2/4] x86: suppress SMAP and SMEP while running 32-bit PV guest code

["Jan Beulich" <jbeulich@xxxxxxxx>]

>>>> Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 03/09/16 1:33 PM >>>
>On 09/03/16 12:27, Wu, Feng wrote:
>> Oh, thanks for the clarification! Do you know how "An NMI or #MC may occur
>> between clearing CR4.SMEP and CR4.SMAP in compat_restore_all_guest and
>> it actually returning to guest context, in which case the guest would run 
>> with
>> the two features enabled. " can happen? Especially how the guest can run
>> with the two features enabled?
>
>NMIs and MCEs can occur at any point, even if interrupts are disabled.
>
>The bad situation is this sequence:
>
>* Xen is returning to the guest and disables CR4.SMEP/SMAP
>* NMI occurs while still in Xen
>* NMI exit path sees it is returning to Xen and re-enabled CR4.SMEP/SMAP

Well, almost: Re-enabling happens on the NMI entry path. The NMI exit
path would, seeing it's returning to Xen context, simply not disable them
again.

Jan

>* Xen ends up returning to guest with CR4.SMEP/SMAP enabled.
>
>~Andrew



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