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

[Andy Lutomirski <luto@kernel.org>]

On Fri, Jan 11, 2019 at 12:42 PM Dave Hansen <dave.hansen@intel.com> 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.
>
>

We can, very easily, have kernel mappings that are private to a given
mm.  Maybe this is useful here.