Re: [Xen-devel] [Patch] Enable SMEP CPU feature support for XEN itself

[Keir Fraser <keir.xen@xxxxxxxxx>]

On 01/06/2011 17:15, "Li, Xin" <xin.li@xxxxxxxxx> wrote:

>>> This patch enables SMEP in Xen to protect Xen hypervisor from executing pv
>>> guest code,
>> 
>> Well not really. In the case that *Xen* execution triggers SMEP, you should
>> crash.
> 
> You don't expect Xen can trigger SMEP? somehow I agree, but in case there is
> any null pointer in Xen, an evil pv guest can easily get control of the
> system.

Of course. I don't disagree there can be bugs in Xen. :-)

>> 
>>> and kills a pv guest triggering SMEP fault.
>> 
>> Should only occur when the guest kernel triggers the SMEP.
> 
> According to code base size, it's much easier for malicious applications to
> explore
> security holes in kernel.  But unluckily SMEP doesn't apply to the ring 3
> where
> x86_64 pv kernel runs on.  It's wiser to use HVM :)

Yep, but 32-bit guests can still benefit.

>> Basically you need to pull your check out of spurious_page_fault() and into
>> the two callers, because their responses should differ (one crashes the
>> guest, the other crashes the hypervisor).
>> Please define an enumeration for the return codes from spurious_pf, rather
>> than using magic numbers.
> 
> Will do.

Thanks.

 - Keir



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