Re: [RFC PATCH 1/1] xen: Use a global mapping for runstate

[Julien Grall <julien@xen.org>]

Hi Jan,

On 29/05/2020 08:35, Jan Beulich wrote:
> On 28.05.2020 20:54, Julien Grall wrote:
>> On 28/05/2020 16:25, Bertrand Marquis wrote:
>>> At the moment on Arm, a Linux guest running with KTPI enabled will
>>> cause the following error when a context switch happens in user mode:
>>> (XEN) p2m.c:1890: d1v0: Failed to walk page-table va 0xffffff837ebe0cd0
>>>
>>> This patch is modifying runstate handling to map the area given by the
>>> guest inside Xen during the hypercall.
>>> This is removing the guest virtual to physical conversion during context
>>> switches which removes the bug
>>
>> It would be good to spell out that a virtual address is not stable. So
>> relying on it is wrong.
> 
> Guests at present are permitted to change the mapping underneath the
> virtual address provided (this may not be the best idea, but the
> interface is like it is).

Well yes, it could be point to data used by the userpsace. So you could 
corrupt a program. It is not very great.

So I would be ready to accept such restriction on Arm at least because 
KPTI use case is far more concerning that a kernel trying to change the 
location of the runstate in physical memory.

Cheers,

-- 
Julien Grall