x86/mm: disallow page stealing from HVM domains

The operation's success can't be controlled by the guest, as the device
model may have an active mapping of the page. If we nevertheless
permitted this operation, we'd have to add further TLB flushing to
prevent scenarios like

"Domains A (HVM), B (PV), C (PV); B->target==A
 Steps:
 1. B maps page X from A as writable
 2. B unmaps page X without a TLB flush
 3. A sends page X to C via GNTTABOP_transfer
 4. C maps page X as pagetable (potentially causing a TLB flush in C,
 but not in B)

 At this point, X would be mapped as a pagetable in C while being
 writable through a stale TLB entry in B."

A similar scenario could be constructed for A using XENMEM_exchange and
some arbitrary PV domain C then having this page allocated.

This is XSA-217.

Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index 101ab33193935b8054c16f3bb21a80b3d4be2268..5dfb876e54d65884145d2793a52c67052e220bbe 100644 (file)
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -4418,6 +4418,9 @@ int steal_page(
     bool_t drop_dom_ref = 0;
     const struct domain *owner = dom_xen;
 
+    if ( paging_mode_external(d) )
+        return -1;
+
     spin_lock(&d->page_alloc_lock);
 
     if ( is_xen_heap_page(page) || ((owner = page_get_owner(page)) != d) )