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

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.


Julien Grall



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