Re: [Xen-devel] [PATCH v2 7/9] x86/shadow: Use the pagewalk reserved bits helpers

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 16.03.17 at 17:31, <andrew.cooper3@xxxxxxxxxx> wrote:
> The shadow logic should not create a valid/present shadow of a guest PTE which
> contains reserved bits from the guests point of view.  It is not guaranteed
> that the hardware pagewalk will come to the same conclusion, and raise a
> pagefault.
> 
> Shadows created on demand from the pagefault handler are fine because the
> pagewalk over the guest tables will have injected the fault into the guest
> rather than creating a shadow.
> 
> However, shadows created by sh_resync_l1() and sh_prefetch() haven't undergone
> a pagewalk and need to account for reserved bits before creating the shadow.
> 
> In practice, this means a 3-level guest could previously cause PTEs with bits
> 63:52 set to be shadowed (and discarded).  This PTE should cause #PF[RSVD]
> when encountered by hardware, but the installed shadow is valid and hardware
> doesn't fault.
> 
> Reuse the pagewalk reserved bits helpers, and assert in
> l?e_propagate_from_guest() that shadows are not attempted to be created with
> reserved bits set.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

This looks fine to me, but I can't claim to know the shadow code well
enough to be certain all places where changes are needed are
actually covered. So I'll defer to Tim and George.

Jan


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