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

[Dave Hansen]

>> 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.