[Xen-devel] update_runstate_area and Linux KPTI

[Julien Grall <julien.grall@xxxxxxx>]

Hi,

While I was looking at some unrelated problem with Xen ARM P2M code, I 
noticed that the function update_runstate_area is using guest virtual 
address to update the vCPU runstate. That function will be called when 
context switch to a vCPU. However, that vCPU may run in userspace 
context. When KPTI (kernel page table isolation) is used,
In the best case, that address is not mapped into the page-table 
currently used. Xen will not be able to update the region.
In the worst case, that address is mapped to a different region and Xen 
will corrupt some bits of the memory.
The code looks fundamentally wrong on Arm, I am entirely not sure about x86.

It look like to me that Xen should always use the guest physical address 
and therefore translate the virtual address to a physical one in 
VCPUOP_register_runstate_memory_area. So only the physical address will 
be used in update_runstate_area making the function much safer.
Any opinion on this approach?

Cheers,

--
Julien Grall

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