domctl: also pause domain for "extended" context updates

This is not just for consistency with "base" context updates, but
actually needed so that guest side accesses can't race with control
domain side updates.

This would have been a security issue if XSA-77 hadn't waived them on
the affected domctl operation.

While looking at the code I also spotted a redundant NULL check in the
"base" context update handling code, which is being removed.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/domctl.c
+++ b/xen/arch/x86/domctl.c
@@ -853,6 +853,8 @@ long arch_do_domctl(
         }
         else
         {
+            if ( d == current->domain ) /* no domain_pause() */
+                break;
             ret = -EINVAL;
             if ( evc->size < offsetof(typeof(*evc), vmce) )
                 break;
@@ -861,6 +863,7 @@ long arch_do_domctl(
                 if ( !is_canonical_address(evc->sysenter_callback_eip) ||
                      !is_canonical_address(evc->syscall32_callback_eip) )
                     break;
+                domain_pause(d);
                 fixup_guest_code_selector(d, evc->sysenter_callback_cs);
                 v->arch.pv_vcpu.sysenter_callback_cs      =
                     evc->sysenter_callback_cs;
@@ -881,6 +884,8 @@ long arch_do_domctl(
                       (evc->syscall32_callback_cs & ~3) ||
                       evc->syscall32_callback_eip )
                 break;
+            else
+                domain_pause(d);
 
             BUILD_BUG_ON(offsetof(struct xen_domctl_ext_vcpucontext,
                                   mcg_cap) !=
@@ -899,6 +904,8 @@ long arch_do_domctl(
             }
             else
                 ret = 0;
+
+            domain_unpause(d);
         }
     }
     break;
--- a/xen/common/domctl.c
+++ b/xen/common/domctl.c
@@ -334,10 +334,6 @@ long do_domctl(XEN_GUEST_HANDLE_PARAM(xe
         unsigned int vcpu = op->u.vcpucontext.vcpu;
         struct vcpu *v;
 
-        ret = -ESRCH;
-        if ( d == NULL )
-            break;
-
         ret = -EINVAL;
         if ( (d == current->domain) || /* no domain_pause() */
              (vcpu >= d->max_vcpus) || ((v = d->vcpu[vcpu]) == NULL) )