Re: [Xen-devel] Ping#2: Re: [PATCH 2/2] x86: don't allow clearing of TF_kernel_mode for other than 64-bit PV

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 04/12/17 10:15, Jan Beulich wrote:
>>>> On 03.07.17 at 16:56,  wrote:
>>>>> On 31.05.17 at 13:54,  wrote:
>>>>>> On 31.05.17 at 13:08, <andrew.cooper3@xxxxxxxxxx> wrote:
>>>> On 31/05/17 08:15, Jan Beulich wrote:
>>>>> The flag is really only meant for those, both HVM and 32-bit PV tell
>>>>> kernel from user mode based on CPL/RPL. Remove the all-question-marks
>>>>> comment and let's be on the safe side here and also suppress clearing
>>>>> for 32-bit PV (this isn't a fast path after all).
>>>>>
>>>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>>> Wouldn't it just be safer to disallow starting a 64bit PV guest in user
>>>> mode?
>>>>
>>>> No real kernel would do such a thing, and keeping the corner case around
>>>> is bad from an attack-surface point of view.
>>> If it really was "starting a guest", I would probably agree. But we're
>>> talking about starting a vCPU, and I could see uses for this (not the
>>> least in XTF). After all the operation allows for enough state to be
>>> set up such that further initialization inside the guest may not be
>>> necessary.
>> Any opinion here, or change of opinion on the original patch?
> I'd really like to get this off my list.

My opinion is unchanged.  This isn't a useful piece of functionality,
and it definitely doesn't warrant the attack surface it brings.

~Andrew

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