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>
master commit: fae7d5be8bb8b7a5b7005c4f3b812a47661a721e
master date: 2017-06-20 14:29:51 +0200

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index ca7c92974a534de596b61e6877b6c5e280fdd9e1..8fe4e853190109e73fcef8f1334eec88d8ad1efc 100644 (file)
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -4217,6 +4217,9 @@ int steal_page(
     unsigned long x, y;
     bool_t drop_dom_ref = 0;
 
+    if ( paging_mode_external(d) )
+        return -1;
+
     spin_lock(&d->page_alloc_lock);
 
     if ( is_xen_heap_page(page) || (page_get_owner(page) != d) )