x86: make new_guest_cr3() preemptible

... as it may take significant amounts of time.

This is part of CVE-2013-1918 / XSA-45.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Tim Deegan <tim@xen.org>

1 files changed, 13 insertions, 2 deletions

diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index d36eddd229..4de9313c74 100644
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -2322,12 +2322,23 @@ static int emulate_privileged_op(struct cpu_user_regs *regs)
             gfn = !is_pv_32on64_vcpu(v)
                 ? xen_cr3_to_pfn(*reg) : compat_cr3_to_pfn(*reg);
             page = get_page_from_gfn(v->domain, gfn, NULL, P2M_ALLOC);
-            rc = page ? new_guest_cr3(page_to_mfn(page)) : 0;
             if ( page )
+            {
+                rc = new_guest_cr3(page_to_mfn(page));
                 put_page(page);
+            }
+            else
+                rc = -EINVAL;
             domain_unlock(v->domain);
-            if ( rc == 0 ) /* not okay */
+            switch ( rc )
+            {
+            case 0:
+                break;
+            case -EAGAIN: /* retry after preemption */
+                goto skip;
+            default:      /* not okay */
                 goto fail;
+            }
             break;
         }