Re: [RFC PATCH v7 00/16] Add support for eXclusive Page Frame Ownership

[Khalid Aziz]

On 1/11/19 1:42 PM, Dave Hansen wrote:
>>> The second process could easily have the page's old TLB entry.  It could
>>> abuse that entry as long as that CPU doesn't context switch
>>> (switch_mm_irqs_off()) or otherwise flush the TLB entry.
>>
>> That is an interesting scenario. Working through this scenario, physmap
>> TLB entry for a page is flushed on the local processor when the page is
>> allocated to userspace, in xpfo_alloc_pages(). When the userspace passes
>> page back into kernel, that page is mapped into kernel space using a va
>> from kmap pool in xpfo_kmap() which can be different for each new
>> mapping of the same page. The physical page is unmapped from kernel on
>> the way back from kernel to userspace by xpfo_kunmap(). So two processes
>> on different CPUs sharing same physical page might not be seeing the
>> same virtual address for that page while they are in the kernel, as long
>> as it is an address from kmap pool. ret2dir attack relies upon being
>> able to craft a predictable virtual address in the kernel physmap for a
>> physical page and redirect execution to that address. Does that sound right?
> 
> All processes share one set of kernel page tables.  Or, did your patches
> change that somehow that I missed?
> 
> Since they share the page tables, they implicitly share kmap*()
> mappings.  kmap_atomic() is not *used* by more than one CPU, but the
> mapping is accessible and at least exists for all processors.
> 
> I'm basically assuming that any entry mapped in a shared page table is
> exploitable on any CPU regardless of where we logically *want* it to be
> used.
> 
> 

Ah, I see what you are saying. Virtual address on one processor is
visible on the other processor as well and one process could communicate
that va to the other process in some way so it could be exploited by the
other process. This va is exploitable only between the kmap and matching
kunmap but the window exists. I am trying to understand your scenario,
so I can address it right.

--
Khalid

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